Stephen Pecot - Academia.edu (original) (raw)

Papers by Stephen Pecot

Research paper thumbnail of Light transmittance estimates in a longleaf pine woodland

Forest Science, 2003

While the importance of canopy structure in open woodlands and savannas on regulating the flow of... more While the importance of canopy structure in open woodlands and savannas on regulating the flow of energy and matter is well known, few studies have investigated how variation in overstory abundance influences canopy light transmission and the extent that estimates vary in their ability to characterize the light environment in these ecosystems. Canopy light transmittance (% photosynthetic photon flux density, or %PPFD) was measured with gallium arsenide phosphide (GaAsP) photodiodes and was monitored throughout the growing season in an open-canopy longleaf pine (Pinus palustris Mill.) woodland across an overstory abundance gradient. Several estimates of canopy light transmittance were also measured, including 10 minute averages of %PPFD under clear and overcast sky conditions during summer, fall, and winter, as well as light estimates derived from hemispherical photographs (gap fraction, GF: gap light index, GLI; and weighted canopy openness, WCO). Linear and curvilinear regression were used to analyze the relationship between (1) light measurements and canopy structure and (2) light estimates and growing-season % PPFD measured by light diodes. Of all light measurements in this study, ten-minute average % PPFD measurements during clear days were the most variable: only a small proportion in the 10minute average of light reaching the understory was correlated to either overstory structure or growing-season canopy transmittance. While measures on overcast days improved the correlations, they tended to overestimate transmittance. Gap fraction (GF) and gap light index (GLI) were strongly correlated with % PPFD and overstory structure, and the relationship between GLI and % PPFD was improved by using a direct light setting of 60%. In addition to strong correlation, hemispherical photographic estimates of light fell along a 1:1 line with % growing-season PPFD; thus, estimates were relatively unbiased. Measures of light in this ecosystem need to incorporate the large spatial and temporal variation in light transmittance that is characteristic of this ecosystem if investigators are to understand the relationship between open-canopy woodlands and their light environments. FOR. SCI. 49(5):752-762.

Research paper thumbnail of Adjustments in hydraulic architecture of Pinus palustris maintain similar stomatal conductance in xeric and mesic habitats

We investigated relationships between whole-tree hydraulic architecture and stomatal conductance ... more We investigated relationships between whole-tree hydraulic architecture and stomatal conductance in Pinus palustris Mill. (longleaf pine) across habitats that differed in soil properties and habitat structure. Trees occupying a xeric habitat (characterized by sandy, well-drained soils, higher nitrogen availability and lower overstory tree density) were shorter in stature and had lower sapwood-to-leaf area ratio ( A S : A L ) than trees in a mesic habitat. The soil-leaf water potential gradient ( Ψ Ψ Ψ Ψ S − − − − Ψ Ψ Ψ Ψ L ) and leaf-specific hydraulic conductance ( k L ) were similar between sites, as was tissuespecific hydraulic conductivity ( K S ) of roots. Leaf and canopy stomatal conductance ( g S and G S , respectively) were also similar between sites, and they tended to be somewhat higher at the xeric site during morning hours when vapour pressure deficit ( D ) was low. A hydraulic model incorporating tree height, A S : A L and Ψ Ψ Ψ Ψ S − − − − Ψ Ψ Ψ Ψ L accurately described the observed variation in individual tree G Sref ( G S at D = = = = 1 kPa) across sites and indicated that tree height was an important determinant of G Sref across sites. This, combined with a 42% higher root-to-leaf area ratio ( A R : A L ) at the xeric site, suggests that xeric site trees are hydraulically well equipped to realize equal -and sometimes higherpotential for conductance compared with trees on mesic sites. However, a slightly more sensitive stomatal closure response to increasing D observed in xeric site trees suggests that this potential for higher conductance may only be reached when D is low and when the capacity of the hydraulic system to supply water to foliage is not greatly challenged.

Research paper thumbnail of Competitive responses of seedlings and understory plants in longleaf pine woodlands: separating canopy influences above and below

A trenching study XVAS used to investigate above- and below-ground con~petitiotl ul a longleaf pi... more A trenching study XVAS used to investigate above- and below-ground con~petitiotl ul a longleaf pine (Pinus ynl- usiris P. Mill.) woodland. Trenchal and nontsenchtxi plots were replicated in the woodland matrix, at gap edges. md in gap centers representing a range. of overstfisy stocking. One-half of each plot received a herbicide treatment to remove the understory. We monitored pine survival

Research paper thumbnail of Fine root carbon allocation and fates in longleaf pine forests

The patterns and mechanistic controls of fine root production and mortality in a 19-year-old long... more The patterns and mechanistic controls of fine root production and mortality in a 19-year-old longleaf pine (Pinus palustris Miller) plantation were assessed using minirhizotrons in a complete factorial design experiment that manipulated root carbon source and sink strengths. Carbon source strength to roots was manipulated through foliar scorching, carbon sink strength in roots was altered via nitrogen fertilization, and carbon transfer from roots to the soil organic matter pool was impacted using a general soil pesticide to distinguish between root mortality via herbivory and senescence. Fertilization significantly increased fine root nitrogen concentrations and mortality rates in the nonscorched plots across the last four sample intervals. Scorching significantly reduced fine root biomass. The influence of soil herbivores on fine root carbon allocation and fates could not be assessed in this study because the soil pesticide had no measurable effect on herbivorous nematode and insec...

Research paper thumbnail of Silvicultural Alternatives In A Longleaf Pine/Wiregrass Woodland In Southwest Georgia: Understory Hardwood Response To Harvest-Created Gaps

Management of longleaf pine woodlands and savannas in areas that have multiple objectives includi... more Management of longleaf pine woodlands and savannas in areas that have multiple objectives including conservation of biodiversity is increasingly common on public and private lands, and various silvicultural approaches have been proposed to meet the diverse objectives. While considerable work has investigated how alternative silvicultural systems influence longleaf pine regeneration patterns, few studies document how competing understory hardwoods respond to the proposed silvicultural alternatives. We examined pine regeneration and understory hardwood response as part of a larger study in a mature longleaf pine forest with replicated blocks randomly assigned one of four silvicultural treatments: control (no cutting), single-tree selection, small-group selection, and large-group selection. Following harvest, understory woody (nonpine) plants increased their growth more than 3-fold due to decreased competition with the pine overstory in the gap-based approaches. This resulted in increa...

Research paper thumbnail of Adjustments in hydraulic architecture of Pinus palustris maintain similar stomatal conductance in xeric and mesic habitats

Plant, Cell and Environment, 2006

We investigated relationships between whole-tree hydraulic architecture and stomatal conductance ... more We investigated relationships between whole-tree hydraulic architecture and stomatal conductance in Pinus palustris Mill. (longleaf pine) across habitats that differed in soil properties and habitat structure. Trees occupying a xeric habitat (characterized by sandy, well-drained soils, higher nitrogen availability and lower overstory tree density) were shorter in stature and had lower sapwood-to-leaf area ratio ( A S : A L ) than trees in a mesic habitat. The soil-leaf water potential gradient ( Ψ Ψ Ψ Ψ S − − − − Ψ Ψ Ψ Ψ L ) and leaf-specific hydraulic conductance ( k L ) were similar between sites, as was tissuespecific hydraulic conductivity ( K S ) of roots. Leaf and canopy stomatal conductance ( g S and G S , respectively) were also similar between sites, and they tended to be somewhat higher at the xeric site during morning hours when vapour pressure deficit ( D ) was low. A hydraulic model incorporating tree height, A S : A L and Ψ Ψ Ψ Ψ S − − − − Ψ Ψ Ψ Ψ L accurately described the observed variation in individual tree G Sref ( G S at D = = = = 1 kPa) across sites and indicated that tree height was an important determinant of G Sref across sites. This, combined with a 42% higher root-to-leaf area ratio ( A R : A L ) at the xeric site, suggests that xeric site trees are hydraulically well equipped to realize equal -and sometimes higherpotential for conductance compared with trees on mesic sites. However, a slightly more sensitive stomatal closure response to increasing D observed in xeric site trees suggests that this potential for higher conductance may only be reached when D is low and when the capacity of the hydraulic system to supply water to foliage is not greatly challenged.

Research paper thumbnail of Controls of fine root dynamics across a gradient of gap sizes in a pine woodland

Oecologia, 2003

Controls of fine dynamics were investigated in a Pinus palustris Mill. (longleaf pine) woodland s... more Controls of fine dynamics were investigated in a Pinus palustris Mill. (longleaf pine) woodland subjected to two understory vegetation treatments (control versus removed) and four overstory treatments (no gap control, and canopy gaps of three sizes with constant total gap area per stand). Fine root (<2 mm diameter) dynamics were measured over 11 months using ingrowth cores (all treatments) and minirhizotrons (understory removed in no gap control and large gap treatments only). At the fine (microsite) spatial scale, pine and non-pine root mass production responded negatively to each other (P=0.033). Each life form was significantly (P£0.028) related to nearby overstory density, and pine root production compensated for reductions in non-pine roots if understory vegetation was removed. Soil moisture and NO 3 mineralization rate were negatively related to pine root mass production (ingrowth cores; P<0.001 and P=0.052) and positively related to pine root length production, mortality and turnover (minirhizotrons; P from <0.001 to 0.078). Temperature variance was negatively related to pine root lifespan (P<0.001) and positively related to pine root turnover (P=0.003). At the ecosystem scale, pattern of overstory disturbance (gap size and number) had no significant effect on non-pine, pine, or total root production. However, the presence of gaps (versus the no-gap control) increased non-pine root mass production (AN-OVA, P=0.055) in natural understory conditions, and reduced pine root mass production (P=0.035) where the understory was removed. Ecosystem-wide pine root length production, mortality and turnover were positively related to weekly soil temperature (P£0.02). In natural systems, fine root dynamics are highly variable and strongly affected by biotic factors. Roots quickly close belowground gaps because one life form (pine or nonpine) compensates for the absence of the other. When understory vegetation is removed, however, pine roots respond to the local abiotic environment, particularly moisture and NO 3 .

Research paper thumbnail of Measuring external mycelia production of ectomycorrhizal fungi in the field: the soil matrix matters

New Phytologist, 2006

Assessing mycorrhizal fungi production in field settings has been hindered by the inability to me... more Assessing mycorrhizal fungi production in field settings has been hindered by the inability to measure external mycelia. Recently, external mycelia production was measured in the field using a novel in-growth core technique with acid-washed sand as the in-growth matrix. Here, we tested the assumption that external mycelia production in acid-washed sand is representative of that in native soil. • External mycelia production was estimated as the difference in fungal growth between closed (allowing only saprotrophic fungal production) and open (allowing mycorrhizal and saprotrophic fungal production) cores using a factorial design of soil matrices (acid-washed sand vs native) and fertilization treatments (control vs nitrogen (N)) in a longleaf pine ( Pinus palustris ) plantation. • In native soils, the ectomycorrhizal to saprotrophic fungal biomass signal was strong and consistent facilitating the assessment of external mycelia production, which was 300% higher than corresponding rates in acid-washed sand and inversely correlated with soil N. • These results demonstrate the efficacy and importance of using native soil as the in-growth matrix to measure ectomycorrhizal fungi external mycelia production in field settings.

Research paper thumbnail of Nitrogen decreases and precipitation increases ectomycorrhizal extramatrical mycelia production in a longleaf pine forest

Mycorrhiza, 2007

The rates and controls of ectomycorrhizal fungal production were assessed in a 22-year-old longle... more The rates and controls of ectomycorrhizal fungal production were assessed in a 22-year-old longleaf pine (Pinus palustris Mill.) plantation using a complete factorial design that included two foliar scorching (control and 95% plus needle scorch) and two nitrogen (N) fertilization (control and 5 g N m −2 year −1 ) treatments during an annual assessment. Ectomycorrhizal fungi production comprised of extramatrical mycelia, Hartig nets and mantles on fine root tips, and sporocarps was estimated to be 49 g m −2 year −1 in the control treatment plots. Extramatrical mycelia accounted for approximately 95% of the total mycorrhizal production estimate. Mycorrhizal production rates did not vary significantly among sample periods throughout the annual assessment (p=0.1366). In addition, reduction in foliar leaf area via experimental scorching treatments did not influence mycorrhizal production (p=0.9374), suggesting that stored carbon (C) may decouple the linkage between current photosynthate production and ectomycorrhizal fungi dynamics in this forest type. Nitrogen fertilization had a negative effect, whereas precipitation had a positive effect on mycorrhizal fungi production (p=0.0292; r 2 =0.42). These results support the widely speculated but poorly documented supposition that mycorrhizal fungi are a large and dynamic component of C flow and nutrient cycling dynamics in forest ecosystems.

Research paper thumbnail of Assessing the patterns and controls of fine root dynamics: an empirical test and methodological review

Journal of Ecology, 2006

1 Elucidation of the patterns and controls of forest net primary production at ecosystem scales h... more 1 Elucidation of the patterns and controls of forest net primary production at ecosystem scales has been hindered by a poor understanding of fine root production, due largely to technical limitations. 2 Fine root ( ≤ 0.5 mm diameter) production was assessed using minirhizotron, soil core, ingrowth core, nitrogen budget and carbon budget techniques in three longleaf pinewiregrass forest ecosystem types (hydric, mesic and xeric) forming an edaphic resource availability and above-ground productivity gradient. 3 Fine root production estimates differed substantially in magnitude, e.g. values ranged from 0 to 4618 kg ha − 1 year − 1 for the soil core and minirhizotron techniques, respectively, in the hydric site. 4 Minirhizotron production estimates in the hydric, mesic and xeric sites were 4618, 1905 and 2295 kg ha − 1 year − 1 , respectively. 5 Soil core and ingrowth core root production estimates were on average 81 and 54% lower, respectively, than corresponding minirhizotron production estimates, and minirhizotron estimates were negatively related to soil core and ingrowth core estimates across the resource gradient. 6 The N budget method yielded unreliable root production estimates, presumably due to the underestimation of N availability for plant assimilation. 7 C budget estimates of total below-ground C allocation (6773, 5646 and 4647 kg C ha − 1 year − 1 ) were positively related to minirhizotron production estimates, but negatively related to soil core and ingrowth core production estimates. 8 Critical evaluations of the assumptions, potential errors and results for each method suggest that the minirhizotron technique yielded the most reliable root production estimates, and that the negative relationship between minirhizotron and core-based estimates may be attributed to the inherent deficiency of the core techniques in assessing root production when mortality and production occur simultaneously. 9 Minirhizotron root production estimates were positively related to foliage production estimates, supporting the hypothesis of constant proportional allocation of production to foliage, wood and fine roots across resource availability gradients in temperate forests. 10 These results suggest that fine root production is not negatively correlated with soil resource availability and foliage production as is commonly perceived in the ecological community and represented in ecosystem computer models.

Research paper thumbnail of The perpetual forest: using undesirable species to bridge restoration

Journal of Applied Ecology, 2007

Conversion of established forests of undesirable species composition or structure to a multi-age,... more Conversion of established forests of undesirable species composition or structure to a multi-age, native forest community is a common restoration goal. However, for some ecosystems, the complexity of multiple disturbances and biotic factors requires unique approaches to advance community development. We use the longleaf pine (Pinus palustris Miller) ecosystem as a model of such a restoration paradigm with an approach that utilizes the undesirable species as a functional or structural bridge to foster ecological processes.

Research paper thumbnail of Spatial Distribution of Overstory Retention Influences Resources and Growth of Longleaf Pine Seedlings

Ecological Applications, 2003

Increasingly, overstory retention is being used in forests traditionally managed for single-cohor... more Increasingly, overstory retention is being used in forests traditionally managed for single-cohort structure. One rationale for retention is that residual stand structure better resembles the complex structure of forests after natural disturbance, helping to perpetuate ecosystem functions dependent on that structure. The benefits of retention come at the cost of reduced survival and growth of regeneration because of competition with residual trees. We argue that inhibition of regeneration depends not only on the number and size of residual trees, but also on their spatial arrangement, which ranges from dispersed to aggregated. We use a model of competition at the scale of seedlings to hypothesize that maximum stand-level resource availability, seedling growth, and seedling survival occur with aggregate retention, rather than dispersed retention, even with constant residual basal area. We test our hypotheses with a silvicultural experiment in longleaf pine (Pinus palustris) in Georgia, USA. Replicated treatments included an uncut control, dispersed retention, small-aggregate retention, and large-aggregate retention. We measured light, soil nitrogen, soil moisture, and growth of longleaf pine seedlings across the full range of overstory conditions in each treatment. Postharvest basal areas in the cut treatments were similar. Gap light index increased from the control to large-aggregate retention, as did nitrogen availability, measured on exchange resins. Nitrogen mineralization did not differ among treatments, nor did soil moisture or temperature. Seedling biomass increment increased significantly from the control to large-aggregate retention. Survival did not differ among treatments. We argue that these results are a consequence of exponential relationships between overstory competition intensity, resource availability, and seedling growth. Given this relationship, resources and seedling growth are low across a wide range of decreasing overstory competitor abundance but increase exponentially only at very low competitor abundance. This seedling-scale model translates into maximum stand scale resource availability and seedling growth with large-aggregate retention, compared to dispersed retention, because the probability of a seedling occupying a site free of overstory competition is greater with the former. Our research shows that one can improve competitive environments for regeneration by manipulating spatial distribution of residual trees without sacrificing the ecological benefits of overstory retention.

Research paper thumbnail of Gaps in a gappy forest: plant resources, longleaf pine regeneration, and understory response to tree removal in longleaf pine savannas

Canadian Journal of Forest Research, 2001

Resource availability and planted longleaf pine (Pinus palustris Mill.) seedling and understory v... more Resource availability and planted longleaf pine (Pinus palustris Mill.) seedling and understory vegetation response within and among three sizes of experimentally created canopy gaps (0.11, 0.41, 1.63 ha) in a mature longleaf pine savanna were investigated for 2 years. Longleaf pine seedlings and understory vegetation showed increased growth in gaps created by tree removal. Longleaf pine seedling growth within gaps was maximized approximately 18 m from the uncut savanna. Increased longleaf pine seedling survival under the uncut savanna canopy observed after the first year suggests that the overstory may facilitate establishment of longleaf pine seedlings rather than reduce survival through competition. Despite the relative openness of the uncut longleaf pine forest, light quantity was increased by tree removal. Light was also the resource most strongly correlated with seedling and understory vegetation growth. Although net N mineralization was correlated to seedling response, the amount of variation explained was low relative to light. Belowground (root) gaps were not strong, in part because of non-pine understory roots increasing in biomass following tree removal. These results suggest that regeneration of longleaf pine may be maximized within gap sizes as small as approximately 0.10 ha, due largely to increases in light availability.

Research paper thumbnail of Competitive responses of seedlings and understory plants in longleaf pine woodlands: separating canopy influences above and below ground

Canadian Journal of Forest Research, 2007

A trenching study was used to investigate above-and below-ground competition in a longleaf pine (... more A trenching study was used to investigate above-and below-ground competition in a longleaf pine (Pinus palustris P. Mill.) woodland. Trenched and nontrenched plots were replicated in the woodland matrix, at gap edges, and in gap centers representing a range of overstory stocking. One-half of each plot received a herbicide treatment to remove the understory. We monitored pine survival and growth, understory productivity, light level (gap fraction), and soil resources. The overstory facilitated pine seedling survival. Pine seedling growth was reduced as overstory stocking increased. Reduced growth of seedlings was also observed in gaps when the understory was left intact. Understory plants competed with seedlings by filling the root gaps that developed as a result of overstory disturbance. Hardwood growth increased in gaps, owing to decreased belowground competition with adult pines, while growth of herbaceous plants and pine seedlings increased with light availability. Large overstory gaps are not required to initiate regeneration in longleaf pine woodlands. Retaining overstory dispersed throughout the stand but variable in density, through single-tree selection approaches, may be an alternative to gap-based approaches. This approach would allow for the fuel continuity needed to sustain the frequent fire required to maintain the diversity characteristic of this type of woodland.

Research paper thumbnail of The influence of canopy, sky condition, and solar angle on light quality in a longleaf pine woodland

Canadian Journal of Forest Research, 2005

Light transmittance estimates under open, heterogeneous woodland canopies such as those of longle... more Light transmittance estimates under open, heterogeneous woodland canopies such as those of longleaf pine (Pinus palustris Mill.) forests report high spatial and temporal variation in the quantity of the light environment. In addition, light quality, that is, the ratio of red to far-red light (R:FR), regulates important aspects of plant development including stem extension, specific leaf area, and seed germination. We conducted two experiments to document sources of variation in R:FR (using a LI-COR 1800 portable spectroradiometer with a cosine-corrected light sensor) in a 70-to 90-year-old natural longleaf pine woodland in southwest Georgia, USA. The first experiment compared instantaneous measurements of R:FR over a 3-day period (March) with annual estimates of canopy transmittance (using gallium arsenide phosphide photodiodes) across the range of observed overstory abundance. The second experiment examined the effect of wiregrass cover (above or below), sky condition (blue sky or overcast), and solar angle (four sampling periods between October and March) on R:FR using a multifactorial repeated measures design. We found that (1) R:FR was significantly (p < 0.0001) and strongly (R 2 = 0.72) related to annual estimates of canopy transmittance (percent photosynthetic photon flux density, %PPFD); (2) R:FR and %PPFD showed significant negative relationships with increasing overstory stocking (R 2 = 0.20, p = 0.028 for R:FR, and R 2 = 0.87, p < 0.0001 for %PPFD); and (3) R:FR decreased with increasing solar angle from maximum zenith for the study site under blue skies, was greater under overcast skies (0.84 blue sky vs. 1.18 overcast sky), and decreased under wiregrass (Aristida stricta Michx.) canopies (1.10 above vs. 0.98 below).

Research paper thumbnail of Patterns and controls of ecosystem function in longleaf pine - wiregrass savannas. II. Nitrogen dynamics

Canadian Journal of Forest Research, 1999

Longleaf pine -wiregrass (Pinus palustris Mill. -Aristida stricta Michx.) woodlands occupy sites ... more Longleaf pine -wiregrass (Pinus palustris Mill. -Aristida stricta Michx.) woodlands occupy sites ranging from deep, xeric sandhills to the edge of wetlands in the southeastern United States. Aboveground net primary productivity (ANPP) of the overstory and understory were determined for three replicate sites of three site types (xeric, intermediate, and wet-mesic) that span a wide environmental gradient. In addition, soil moisture (at 30 and 90 cm) and N mineralization (in situ buried bag incubations) were measured through an annual cycle. Longleaf pine -wiregrass ecosystems varied by nearly twofold in ANPP across complex gradients. Overstory and understory and total (overstory and understory) ANPP were positively correlated to soil moisture at 30 and 90 cm. The proportion of understory ANPP relative to the total ANPP did not increase across the environmental gradient as predicted by hypotheses that invoke niche differentiation in rooting habits of grasses and trees. Contrary to expectations, cumulative net N mineralization was negatively related to soil moisture. All ANPP estimates were significantly and negatively related to cumulative N-mineralization. Further work is needed to explore the mechanisms by which soil moisture regulates productivity across space, time, and for individual species. Additional experimentation through resource addition would allow for investigations into multiple resource limitations and how resource limitations vary depending on gradient position.

Research paper thumbnail of Light transmittance estimates in a longleaf pine woodland

While the importance of canopy structure in open woodlands and savannas on regulating the flow of... more While the importance of canopy structure in open woodlands and savannas on regulating the flow of energy and matter is well known, few studies have investigated how variation in overstory abundance influences canopy light transmission and the extent that estimates vary in their ability to characterize the light environment in these ecosystems. Canopy light transmittance (% photosynthetic photon flux density, or %PPFD) was measured with gallium arsenide phosphide (GaAsP) photodiodes and was monitored throughout the growing season in an open-canopy longleaf pine (Pinus palustris Mill.) woodland across an overstory abundance gradient. Several estimates of canopy light transmittance were also measured, including 10 minute averages of %PPFD under clear and overcast sky conditions during summer, fall, and winter, as well as light estimates derived from hemispherical photographs (gap fraction, GF: gap light index, GLI; and weighted canopy openness, WCO). Linear and curvilinear regression were used to analyze the relationship between (1) light measurements and canopy structure and (2) light estimates and growing-season % PPFD measured by light diodes. Of all light measurements in this study, ten-minute average % PPFD measurements during clear days were the most variable: only a small proportion in the 10minute average of light reaching the understory was correlated to either overstory structure or growing-season canopy transmittance. While measures on overcast days improved the correlations, they tended to overestimate transmittance. Gap fraction (GF) and gap light index (GLI) were strongly correlated with % PPFD and overstory structure, and the relationship between GLI and % PPFD was improved by using a direct light setting of 60%. In addition to strong correlation, hemispherical photographic estimates of light fell along a 1:1 line with % growing-season PPFD; thus, estimates were relatively unbiased. Measures of light in this ecosystem need to incorporate the large spatial and temporal variation in light transmittance that is characteristic of this ecosystem if investigators are to understand the relationship between open-canopy woodlands and their light environments. FOR. SCI. 49(5):752-762.

Research paper thumbnail of Ectomycorrhizal fungal mycelia turnover in a longleaf pine forest

Elucidation of the patterns and controls of carbon (C) flow and nitrogen (N) cycling in forests h... more Elucidation of the patterns and controls of carbon (C) flow and nitrogen (N) cycling in forests has been hindered by a poor understanding of ectomycorrhizal fungal mycelia (EFM) dynamics. In this study, EFM standing biomass (based on soil ergosterol concentrations), production (based on ergosterol accrual in ingrowth cores), and turnover rate (the quotient of annual production and average standing biomass estimates) were assessed in a 25-yr-old longleaf pine (Pinus palustris) plantation where C flow was manipulated by foliar scorching and N fertilization for 5 yr before study initiation. In the controls, EFM standing biomass was 30 AE 7 g m À2 , production was 279 AE 63 g m À2 yr À1 , and turnover rate was 10 AE 3 times yr À1. The scorched 9 fertilized treatment had significantly higher EFM standing biomass (38 AE 8 g m À2), significantly lower production (205 AE 28 g m À2 yr À1), and a trend of decreased turnover rate (6 AE 1 times yr À1). The EFM turnover estimates, which are among the first reported for natural systems, indicate that EFM are a dynamic component of ecosystems, and that conventional assessments have probably underestimated the role of EFM in C flow and nutrient cycling.

Research paper thumbnail of Ectomycorrhizal fungal mycelia turnover in a longleaf pine forest

Elucidation of the patterns and controls of carbon (C) flow and nitrogen (N) cycling in forests h... more Elucidation of the patterns and controls of carbon (C) flow and nitrogen (N) cycling in forests has been hindered by a poor understanding of ectomycorrhizal fungal mycelia (EFM) dynamics. In this study, EFM standing biomass (based on soil ergosterol concentrations), production (based on ergosterol accrual in ingrowth cores), and turnover rate (the quotient of annual production and average standing biomass estimates) were assessed in a 25-yr-old longleaf pine (Pinus palustris) plantation where C flow was manipulated by foliar scorching and N fertilization for 5 yr before study initiation. In the controls, EFM standing biomass was 30 AE 7 g m À2 , production was 279 AE 63 g m À2 yr À1 , and turnover rate was 10 AE 3 times yr À1. The scorched 9 fertilized treatment had significantly higher EFM standing biomass (38 AE 8 g m À2), significantly lower production (205 AE 28 g m À2 yr À1), and a trend of decreased turnover rate (6 AE 1 times yr À1). The EFM turnover estimates, which are among the first reported for natural systems, indicate that EFM are a dynamic component of ecosystems, and that conventional assessments have probably underestimated the role of EFM in C flow and nutrient cycling.

Research paper thumbnail of Light transmittance estimates in a longleaf pine woodland

Forest Science, 2003

While the importance of canopy structure in open woodlands and savannas on regulating the flow of... more While the importance of canopy structure in open woodlands and savannas on regulating the flow of energy and matter is well known, few studies have investigated how variation in overstory abundance influences canopy light transmission and the extent that estimates vary in their ability to characterize the light environment in these ecosystems. Canopy light transmittance (% photosynthetic photon flux density, or %PPFD) was measured with gallium arsenide phosphide (GaAsP) photodiodes and was monitored throughout the growing season in an open-canopy longleaf pine (Pinus palustris Mill.) woodland across an overstory abundance gradient. Several estimates of canopy light transmittance were also measured, including 10 minute averages of %PPFD under clear and overcast sky conditions during summer, fall, and winter, as well as light estimates derived from hemispherical photographs (gap fraction, GF: gap light index, GLI; and weighted canopy openness, WCO). Linear and curvilinear regression were used to analyze the relationship between (1) light measurements and canopy structure and (2) light estimates and growing-season % PPFD measured by light diodes. Of all light measurements in this study, ten-minute average % PPFD measurements during clear days were the most variable: only a small proportion in the 10minute average of light reaching the understory was correlated to either overstory structure or growing-season canopy transmittance. While measures on overcast days improved the correlations, they tended to overestimate transmittance. Gap fraction (GF) and gap light index (GLI) were strongly correlated with % PPFD and overstory structure, and the relationship between GLI and % PPFD was improved by using a direct light setting of 60%. In addition to strong correlation, hemispherical photographic estimates of light fell along a 1:1 line with % growing-season PPFD; thus, estimates were relatively unbiased. Measures of light in this ecosystem need to incorporate the large spatial and temporal variation in light transmittance that is characteristic of this ecosystem if investigators are to understand the relationship between open-canopy woodlands and their light environments. FOR. SCI. 49(5):752-762.

Research paper thumbnail of Adjustments in hydraulic architecture of Pinus palustris maintain similar stomatal conductance in xeric and mesic habitats

We investigated relationships between whole-tree hydraulic architecture and stomatal conductance ... more We investigated relationships between whole-tree hydraulic architecture and stomatal conductance in Pinus palustris Mill. (longleaf pine) across habitats that differed in soil properties and habitat structure. Trees occupying a xeric habitat (characterized by sandy, well-drained soils, higher nitrogen availability and lower overstory tree density) were shorter in stature and had lower sapwood-to-leaf area ratio ( A S : A L ) than trees in a mesic habitat. The soil-leaf water potential gradient ( Ψ Ψ Ψ Ψ S − − − − Ψ Ψ Ψ Ψ L ) and leaf-specific hydraulic conductance ( k L ) were similar between sites, as was tissuespecific hydraulic conductivity ( K S ) of roots. Leaf and canopy stomatal conductance ( g S and G S , respectively) were also similar between sites, and they tended to be somewhat higher at the xeric site during morning hours when vapour pressure deficit ( D ) was low. A hydraulic model incorporating tree height, A S : A L and Ψ Ψ Ψ Ψ S − − − − Ψ Ψ Ψ Ψ L accurately described the observed variation in individual tree G Sref ( G S at D = = = = 1 kPa) across sites and indicated that tree height was an important determinant of G Sref across sites. This, combined with a 42% higher root-to-leaf area ratio ( A R : A L ) at the xeric site, suggests that xeric site trees are hydraulically well equipped to realize equal -and sometimes higherpotential for conductance compared with trees on mesic sites. However, a slightly more sensitive stomatal closure response to increasing D observed in xeric site trees suggests that this potential for higher conductance may only be reached when D is low and when the capacity of the hydraulic system to supply water to foliage is not greatly challenged.

Research paper thumbnail of Competitive responses of seedlings and understory plants in longleaf pine woodlands: separating canopy influences above and below

A trenching study XVAS used to investigate above- and below-ground con~petitiotl ul a longleaf pi... more A trenching study XVAS used to investigate above- and below-ground con~petitiotl ul a longleaf pine (Pinus ynl- usiris P. Mill.) woodland. Trenchal and nontsenchtxi plots were replicated in the woodland matrix, at gap edges. md in gap centers representing a range. of overstfisy stocking. One-half of each plot received a herbicide treatment to remove the understory. We monitored pine survival

Research paper thumbnail of Fine root carbon allocation and fates in longleaf pine forests

The patterns and mechanistic controls of fine root production and mortality in a 19-year-old long... more The patterns and mechanistic controls of fine root production and mortality in a 19-year-old longleaf pine (Pinus palustris Miller) plantation were assessed using minirhizotrons in a complete factorial design experiment that manipulated root carbon source and sink strengths. Carbon source strength to roots was manipulated through foliar scorching, carbon sink strength in roots was altered via nitrogen fertilization, and carbon transfer from roots to the soil organic matter pool was impacted using a general soil pesticide to distinguish between root mortality via herbivory and senescence. Fertilization significantly increased fine root nitrogen concentrations and mortality rates in the nonscorched plots across the last four sample intervals. Scorching significantly reduced fine root biomass. The influence of soil herbivores on fine root carbon allocation and fates could not be assessed in this study because the soil pesticide had no measurable effect on herbivorous nematode and insec...

Research paper thumbnail of Silvicultural Alternatives In A Longleaf Pine/Wiregrass Woodland In Southwest Georgia: Understory Hardwood Response To Harvest-Created Gaps

Management of longleaf pine woodlands and savannas in areas that have multiple objectives includi... more Management of longleaf pine woodlands and savannas in areas that have multiple objectives including conservation of biodiversity is increasingly common on public and private lands, and various silvicultural approaches have been proposed to meet the diverse objectives. While considerable work has investigated how alternative silvicultural systems influence longleaf pine regeneration patterns, few studies document how competing understory hardwoods respond to the proposed silvicultural alternatives. We examined pine regeneration and understory hardwood response as part of a larger study in a mature longleaf pine forest with replicated blocks randomly assigned one of four silvicultural treatments: control (no cutting), single-tree selection, small-group selection, and large-group selection. Following harvest, understory woody (nonpine) plants increased their growth more than 3-fold due to decreased competition with the pine overstory in the gap-based approaches. This resulted in increa...

Research paper thumbnail of Adjustments in hydraulic architecture of Pinus palustris maintain similar stomatal conductance in xeric and mesic habitats

Plant, Cell and Environment, 2006

We investigated relationships between whole-tree hydraulic architecture and stomatal conductance ... more We investigated relationships between whole-tree hydraulic architecture and stomatal conductance in Pinus palustris Mill. (longleaf pine) across habitats that differed in soil properties and habitat structure. Trees occupying a xeric habitat (characterized by sandy, well-drained soils, higher nitrogen availability and lower overstory tree density) were shorter in stature and had lower sapwood-to-leaf area ratio ( A S : A L ) than trees in a mesic habitat. The soil-leaf water potential gradient ( Ψ Ψ Ψ Ψ S − − − − Ψ Ψ Ψ Ψ L ) and leaf-specific hydraulic conductance ( k L ) were similar between sites, as was tissuespecific hydraulic conductivity ( K S ) of roots. Leaf and canopy stomatal conductance ( g S and G S , respectively) were also similar between sites, and they tended to be somewhat higher at the xeric site during morning hours when vapour pressure deficit ( D ) was low. A hydraulic model incorporating tree height, A S : A L and Ψ Ψ Ψ Ψ S − − − − Ψ Ψ Ψ Ψ L accurately described the observed variation in individual tree G Sref ( G S at D = = = = 1 kPa) across sites and indicated that tree height was an important determinant of G Sref across sites. This, combined with a 42% higher root-to-leaf area ratio ( A R : A L ) at the xeric site, suggests that xeric site trees are hydraulically well equipped to realize equal -and sometimes higherpotential for conductance compared with trees on mesic sites. However, a slightly more sensitive stomatal closure response to increasing D observed in xeric site trees suggests that this potential for higher conductance may only be reached when D is low and when the capacity of the hydraulic system to supply water to foliage is not greatly challenged.

Research paper thumbnail of Controls of fine root dynamics across a gradient of gap sizes in a pine woodland

Oecologia, 2003

Controls of fine dynamics were investigated in a Pinus palustris Mill. (longleaf pine) woodland s... more Controls of fine dynamics were investigated in a Pinus palustris Mill. (longleaf pine) woodland subjected to two understory vegetation treatments (control versus removed) and four overstory treatments (no gap control, and canopy gaps of three sizes with constant total gap area per stand). Fine root (<2 mm diameter) dynamics were measured over 11 months using ingrowth cores (all treatments) and minirhizotrons (understory removed in no gap control and large gap treatments only). At the fine (microsite) spatial scale, pine and non-pine root mass production responded negatively to each other (P=0.033). Each life form was significantly (P£0.028) related to nearby overstory density, and pine root production compensated for reductions in non-pine roots if understory vegetation was removed. Soil moisture and NO 3 mineralization rate were negatively related to pine root mass production (ingrowth cores; P<0.001 and P=0.052) and positively related to pine root length production, mortality and turnover (minirhizotrons; P from <0.001 to 0.078). Temperature variance was negatively related to pine root lifespan (P<0.001) and positively related to pine root turnover (P=0.003). At the ecosystem scale, pattern of overstory disturbance (gap size and number) had no significant effect on non-pine, pine, or total root production. However, the presence of gaps (versus the no-gap control) increased non-pine root mass production (AN-OVA, P=0.055) in natural understory conditions, and reduced pine root mass production (P=0.035) where the understory was removed. Ecosystem-wide pine root length production, mortality and turnover were positively related to weekly soil temperature (P£0.02). In natural systems, fine root dynamics are highly variable and strongly affected by biotic factors. Roots quickly close belowground gaps because one life form (pine or nonpine) compensates for the absence of the other. When understory vegetation is removed, however, pine roots respond to the local abiotic environment, particularly moisture and NO 3 .

Research paper thumbnail of Measuring external mycelia production of ectomycorrhizal fungi in the field: the soil matrix matters

New Phytologist, 2006

Assessing mycorrhizal fungi production in field settings has been hindered by the inability to me... more Assessing mycorrhizal fungi production in field settings has been hindered by the inability to measure external mycelia. Recently, external mycelia production was measured in the field using a novel in-growth core technique with acid-washed sand as the in-growth matrix. Here, we tested the assumption that external mycelia production in acid-washed sand is representative of that in native soil. • External mycelia production was estimated as the difference in fungal growth between closed (allowing only saprotrophic fungal production) and open (allowing mycorrhizal and saprotrophic fungal production) cores using a factorial design of soil matrices (acid-washed sand vs native) and fertilization treatments (control vs nitrogen (N)) in a longleaf pine ( Pinus palustris ) plantation. • In native soils, the ectomycorrhizal to saprotrophic fungal biomass signal was strong and consistent facilitating the assessment of external mycelia production, which was 300% higher than corresponding rates in acid-washed sand and inversely correlated with soil N. • These results demonstrate the efficacy and importance of using native soil as the in-growth matrix to measure ectomycorrhizal fungi external mycelia production in field settings.

Research paper thumbnail of Nitrogen decreases and precipitation increases ectomycorrhizal extramatrical mycelia production in a longleaf pine forest

Mycorrhiza, 2007

The rates and controls of ectomycorrhizal fungal production were assessed in a 22-year-old longle... more The rates and controls of ectomycorrhizal fungal production were assessed in a 22-year-old longleaf pine (Pinus palustris Mill.) plantation using a complete factorial design that included two foliar scorching (control and 95% plus needle scorch) and two nitrogen (N) fertilization (control and 5 g N m −2 year −1 ) treatments during an annual assessment. Ectomycorrhizal fungi production comprised of extramatrical mycelia, Hartig nets and mantles on fine root tips, and sporocarps was estimated to be 49 g m −2 year −1 in the control treatment plots. Extramatrical mycelia accounted for approximately 95% of the total mycorrhizal production estimate. Mycorrhizal production rates did not vary significantly among sample periods throughout the annual assessment (p=0.1366). In addition, reduction in foliar leaf area via experimental scorching treatments did not influence mycorrhizal production (p=0.9374), suggesting that stored carbon (C) may decouple the linkage between current photosynthate production and ectomycorrhizal fungi dynamics in this forest type. Nitrogen fertilization had a negative effect, whereas precipitation had a positive effect on mycorrhizal fungi production (p=0.0292; r 2 =0.42). These results support the widely speculated but poorly documented supposition that mycorrhizal fungi are a large and dynamic component of C flow and nutrient cycling dynamics in forest ecosystems.

Research paper thumbnail of Assessing the patterns and controls of fine root dynamics: an empirical test and methodological review

Journal of Ecology, 2006

1 Elucidation of the patterns and controls of forest net primary production at ecosystem scales h... more 1 Elucidation of the patterns and controls of forest net primary production at ecosystem scales has been hindered by a poor understanding of fine root production, due largely to technical limitations. 2 Fine root ( ≤ 0.5 mm diameter) production was assessed using minirhizotron, soil core, ingrowth core, nitrogen budget and carbon budget techniques in three longleaf pinewiregrass forest ecosystem types (hydric, mesic and xeric) forming an edaphic resource availability and above-ground productivity gradient. 3 Fine root production estimates differed substantially in magnitude, e.g. values ranged from 0 to 4618 kg ha − 1 year − 1 for the soil core and minirhizotron techniques, respectively, in the hydric site. 4 Minirhizotron production estimates in the hydric, mesic and xeric sites were 4618, 1905 and 2295 kg ha − 1 year − 1 , respectively. 5 Soil core and ingrowth core root production estimates were on average 81 and 54% lower, respectively, than corresponding minirhizotron production estimates, and minirhizotron estimates were negatively related to soil core and ingrowth core estimates across the resource gradient. 6 The N budget method yielded unreliable root production estimates, presumably due to the underestimation of N availability for plant assimilation. 7 C budget estimates of total below-ground C allocation (6773, 5646 and 4647 kg C ha − 1 year − 1 ) were positively related to minirhizotron production estimates, but negatively related to soil core and ingrowth core production estimates. 8 Critical evaluations of the assumptions, potential errors and results for each method suggest that the minirhizotron technique yielded the most reliable root production estimates, and that the negative relationship between minirhizotron and core-based estimates may be attributed to the inherent deficiency of the core techniques in assessing root production when mortality and production occur simultaneously. 9 Minirhizotron root production estimates were positively related to foliage production estimates, supporting the hypothesis of constant proportional allocation of production to foliage, wood and fine roots across resource availability gradients in temperate forests. 10 These results suggest that fine root production is not negatively correlated with soil resource availability and foliage production as is commonly perceived in the ecological community and represented in ecosystem computer models.

Research paper thumbnail of The perpetual forest: using undesirable species to bridge restoration

Journal of Applied Ecology, 2007

Conversion of established forests of undesirable species composition or structure to a multi-age,... more Conversion of established forests of undesirable species composition or structure to a multi-age, native forest community is a common restoration goal. However, for some ecosystems, the complexity of multiple disturbances and biotic factors requires unique approaches to advance community development. We use the longleaf pine (Pinus palustris Miller) ecosystem as a model of such a restoration paradigm with an approach that utilizes the undesirable species as a functional or structural bridge to foster ecological processes.

Research paper thumbnail of Spatial Distribution of Overstory Retention Influences Resources and Growth of Longleaf Pine Seedlings

Ecological Applications, 2003

Increasingly, overstory retention is being used in forests traditionally managed for single-cohor... more Increasingly, overstory retention is being used in forests traditionally managed for single-cohort structure. One rationale for retention is that residual stand structure better resembles the complex structure of forests after natural disturbance, helping to perpetuate ecosystem functions dependent on that structure. The benefits of retention come at the cost of reduced survival and growth of regeneration because of competition with residual trees. We argue that inhibition of regeneration depends not only on the number and size of residual trees, but also on their spatial arrangement, which ranges from dispersed to aggregated. We use a model of competition at the scale of seedlings to hypothesize that maximum stand-level resource availability, seedling growth, and seedling survival occur with aggregate retention, rather than dispersed retention, even with constant residual basal area. We test our hypotheses with a silvicultural experiment in longleaf pine (Pinus palustris) in Georgia, USA. Replicated treatments included an uncut control, dispersed retention, small-aggregate retention, and large-aggregate retention. We measured light, soil nitrogen, soil moisture, and growth of longleaf pine seedlings across the full range of overstory conditions in each treatment. Postharvest basal areas in the cut treatments were similar. Gap light index increased from the control to large-aggregate retention, as did nitrogen availability, measured on exchange resins. Nitrogen mineralization did not differ among treatments, nor did soil moisture or temperature. Seedling biomass increment increased significantly from the control to large-aggregate retention. Survival did not differ among treatments. We argue that these results are a consequence of exponential relationships between overstory competition intensity, resource availability, and seedling growth. Given this relationship, resources and seedling growth are low across a wide range of decreasing overstory competitor abundance but increase exponentially only at very low competitor abundance. This seedling-scale model translates into maximum stand scale resource availability and seedling growth with large-aggregate retention, compared to dispersed retention, because the probability of a seedling occupying a site free of overstory competition is greater with the former. Our research shows that one can improve competitive environments for regeneration by manipulating spatial distribution of residual trees without sacrificing the ecological benefits of overstory retention.

Research paper thumbnail of Gaps in a gappy forest: plant resources, longleaf pine regeneration, and understory response to tree removal in longleaf pine savannas

Canadian Journal of Forest Research, 2001

Resource availability and planted longleaf pine (Pinus palustris Mill.) seedling and understory v... more Resource availability and planted longleaf pine (Pinus palustris Mill.) seedling and understory vegetation response within and among three sizes of experimentally created canopy gaps (0.11, 0.41, 1.63 ha) in a mature longleaf pine savanna were investigated for 2 years. Longleaf pine seedlings and understory vegetation showed increased growth in gaps created by tree removal. Longleaf pine seedling growth within gaps was maximized approximately 18 m from the uncut savanna. Increased longleaf pine seedling survival under the uncut savanna canopy observed after the first year suggests that the overstory may facilitate establishment of longleaf pine seedlings rather than reduce survival through competition. Despite the relative openness of the uncut longleaf pine forest, light quantity was increased by tree removal. Light was also the resource most strongly correlated with seedling and understory vegetation growth. Although net N mineralization was correlated to seedling response, the amount of variation explained was low relative to light. Belowground (root) gaps were not strong, in part because of non-pine understory roots increasing in biomass following tree removal. These results suggest that regeneration of longleaf pine may be maximized within gap sizes as small as approximately 0.10 ha, due largely to increases in light availability.

Research paper thumbnail of Competitive responses of seedlings and understory plants in longleaf pine woodlands: separating canopy influences above and below ground

Canadian Journal of Forest Research, 2007

A trenching study was used to investigate above-and below-ground competition in a longleaf pine (... more A trenching study was used to investigate above-and below-ground competition in a longleaf pine (Pinus palustris P. Mill.) woodland. Trenched and nontrenched plots were replicated in the woodland matrix, at gap edges, and in gap centers representing a range of overstory stocking. One-half of each plot received a herbicide treatment to remove the understory. We monitored pine survival and growth, understory productivity, light level (gap fraction), and soil resources. The overstory facilitated pine seedling survival. Pine seedling growth was reduced as overstory stocking increased. Reduced growth of seedlings was also observed in gaps when the understory was left intact. Understory plants competed with seedlings by filling the root gaps that developed as a result of overstory disturbance. Hardwood growth increased in gaps, owing to decreased belowground competition with adult pines, while growth of herbaceous plants and pine seedlings increased with light availability. Large overstory gaps are not required to initiate regeneration in longleaf pine woodlands. Retaining overstory dispersed throughout the stand but variable in density, through single-tree selection approaches, may be an alternative to gap-based approaches. This approach would allow for the fuel continuity needed to sustain the frequent fire required to maintain the diversity characteristic of this type of woodland.

Research paper thumbnail of The influence of canopy, sky condition, and solar angle on light quality in a longleaf pine woodland

Canadian Journal of Forest Research, 2005

Light transmittance estimates under open, heterogeneous woodland canopies such as those of longle... more Light transmittance estimates under open, heterogeneous woodland canopies such as those of longleaf pine (Pinus palustris Mill.) forests report high spatial and temporal variation in the quantity of the light environment. In addition, light quality, that is, the ratio of red to far-red light (R:FR), regulates important aspects of plant development including stem extension, specific leaf area, and seed germination. We conducted two experiments to document sources of variation in R:FR (using a LI-COR 1800 portable spectroradiometer with a cosine-corrected light sensor) in a 70-to 90-year-old natural longleaf pine woodland in southwest Georgia, USA. The first experiment compared instantaneous measurements of R:FR over a 3-day period (March) with annual estimates of canopy transmittance (using gallium arsenide phosphide photodiodes) across the range of observed overstory abundance. The second experiment examined the effect of wiregrass cover (above or below), sky condition (blue sky or overcast), and solar angle (four sampling periods between October and March) on R:FR using a multifactorial repeated measures design. We found that (1) R:FR was significantly (p < 0.0001) and strongly (R 2 = 0.72) related to annual estimates of canopy transmittance (percent photosynthetic photon flux density, %PPFD); (2) R:FR and %PPFD showed significant negative relationships with increasing overstory stocking (R 2 = 0.20, p = 0.028 for R:FR, and R 2 = 0.87, p < 0.0001 for %PPFD); and (3) R:FR decreased with increasing solar angle from maximum zenith for the study site under blue skies, was greater under overcast skies (0.84 blue sky vs. 1.18 overcast sky), and decreased under wiregrass (Aristida stricta Michx.) canopies (1.10 above vs. 0.98 below).

Research paper thumbnail of Patterns and controls of ecosystem function in longleaf pine - wiregrass savannas. II. Nitrogen dynamics

Canadian Journal of Forest Research, 1999

Longleaf pine -wiregrass (Pinus palustris Mill. -Aristida stricta Michx.) woodlands occupy sites ... more Longleaf pine -wiregrass (Pinus palustris Mill. -Aristida stricta Michx.) woodlands occupy sites ranging from deep, xeric sandhills to the edge of wetlands in the southeastern United States. Aboveground net primary productivity (ANPP) of the overstory and understory were determined for three replicate sites of three site types (xeric, intermediate, and wet-mesic) that span a wide environmental gradient. In addition, soil moisture (at 30 and 90 cm) and N mineralization (in situ buried bag incubations) were measured through an annual cycle. Longleaf pine -wiregrass ecosystems varied by nearly twofold in ANPP across complex gradients. Overstory and understory and total (overstory and understory) ANPP were positively correlated to soil moisture at 30 and 90 cm. The proportion of understory ANPP relative to the total ANPP did not increase across the environmental gradient as predicted by hypotheses that invoke niche differentiation in rooting habits of grasses and trees. Contrary to expectations, cumulative net N mineralization was negatively related to soil moisture. All ANPP estimates were significantly and negatively related to cumulative N-mineralization. Further work is needed to explore the mechanisms by which soil moisture regulates productivity across space, time, and for individual species. Additional experimentation through resource addition would allow for investigations into multiple resource limitations and how resource limitations vary depending on gradient position.

Research paper thumbnail of Light transmittance estimates in a longleaf pine woodland

While the importance of canopy structure in open woodlands and savannas on regulating the flow of... more While the importance of canopy structure in open woodlands and savannas on regulating the flow of energy and matter is well known, few studies have investigated how variation in overstory abundance influences canopy light transmission and the extent that estimates vary in their ability to characterize the light environment in these ecosystems. Canopy light transmittance (% photosynthetic photon flux density, or %PPFD) was measured with gallium arsenide phosphide (GaAsP) photodiodes and was monitored throughout the growing season in an open-canopy longleaf pine (Pinus palustris Mill.) woodland across an overstory abundance gradient. Several estimates of canopy light transmittance were also measured, including 10 minute averages of %PPFD under clear and overcast sky conditions during summer, fall, and winter, as well as light estimates derived from hemispherical photographs (gap fraction, GF: gap light index, GLI; and weighted canopy openness, WCO). Linear and curvilinear regression were used to analyze the relationship between (1) light measurements and canopy structure and (2) light estimates and growing-season % PPFD measured by light diodes. Of all light measurements in this study, ten-minute average % PPFD measurements during clear days were the most variable: only a small proportion in the 10minute average of light reaching the understory was correlated to either overstory structure or growing-season canopy transmittance. While measures on overcast days improved the correlations, they tended to overestimate transmittance. Gap fraction (GF) and gap light index (GLI) were strongly correlated with % PPFD and overstory structure, and the relationship between GLI and % PPFD was improved by using a direct light setting of 60%. In addition to strong correlation, hemispherical photographic estimates of light fell along a 1:1 line with % growing-season PPFD; thus, estimates were relatively unbiased. Measures of light in this ecosystem need to incorporate the large spatial and temporal variation in light transmittance that is characteristic of this ecosystem if investigators are to understand the relationship between open-canopy woodlands and their light environments. FOR. SCI. 49(5):752-762.

Research paper thumbnail of Ectomycorrhizal fungal mycelia turnover in a longleaf pine forest

Elucidation of the patterns and controls of carbon (C) flow and nitrogen (N) cycling in forests h... more Elucidation of the patterns and controls of carbon (C) flow and nitrogen (N) cycling in forests has been hindered by a poor understanding of ectomycorrhizal fungal mycelia (EFM) dynamics. In this study, EFM standing biomass (based on soil ergosterol concentrations), production (based on ergosterol accrual in ingrowth cores), and turnover rate (the quotient of annual production and average standing biomass estimates) were assessed in a 25-yr-old longleaf pine (Pinus palustris) plantation where C flow was manipulated by foliar scorching and N fertilization for 5 yr before study initiation. In the controls, EFM standing biomass was 30 AE 7 g m À2 , production was 279 AE 63 g m À2 yr À1 , and turnover rate was 10 AE 3 times yr À1. The scorched 9 fertilized treatment had significantly higher EFM standing biomass (38 AE 8 g m À2), significantly lower production (205 AE 28 g m À2 yr À1), and a trend of decreased turnover rate (6 AE 1 times yr À1). The EFM turnover estimates, which are among the first reported for natural systems, indicate that EFM are a dynamic component of ecosystems, and that conventional assessments have probably underestimated the role of EFM in C flow and nutrient cycling.

Research paper thumbnail of Ectomycorrhizal fungal mycelia turnover in a longleaf pine forest

Elucidation of the patterns and controls of carbon (C) flow and nitrogen (N) cycling in forests h... more Elucidation of the patterns and controls of carbon (C) flow and nitrogen (N) cycling in forests has been hindered by a poor understanding of ectomycorrhizal fungal mycelia (EFM) dynamics. In this study, EFM standing biomass (based on soil ergosterol concentrations), production (based on ergosterol accrual in ingrowth cores), and turnover rate (the quotient of annual production and average standing biomass estimates) were assessed in a 25-yr-old longleaf pine (Pinus palustris) plantation where C flow was manipulated by foliar scorching and N fertilization for 5 yr before study initiation. In the controls, EFM standing biomass was 30 AE 7 g m À2 , production was 279 AE 63 g m À2 yr À1 , and turnover rate was 10 AE 3 times yr À1. The scorched 9 fertilized treatment had significantly higher EFM standing biomass (38 AE 8 g m À2), significantly lower production (205 AE 28 g m À2 yr À1), and a trend of decreased turnover rate (6 AE 1 times yr À1). The EFM turnover estimates, which are among the first reported for natural systems, indicate that EFM are a dynamic component of ecosystems, and that conventional assessments have probably underestimated the role of EFM in C flow and nutrient cycling.