Phenology of fine roots and leaves in forest and grassland (original) (raw)
Related papers
Differential physiological responses to environmental change promote woody shrub expansion
Ecology and Evolution, 2013
Direct and indirect effects of warming are increasingly modifying the carbonrich vegetation and soils of the Arctic tundra, with important implications for the terrestrial carbon cycle. Understanding the biological and environmental influences on the processes that regulate foliar carbon cycling in tundra species is essential for predicting the future terrestrial carbon balance in this region. To determine the effect of climate change impacts on gas exchange in tundra, we quantified foliar photosynthesis (A net ), respiration in the dark and light (R D and R L , determined using the Kok method), photorespiration (PR), carbon gain efficiency (CGE, the ratio of photosynthetic CO 2 uptake to total CO 2 exchange of photosynthesis, PR, and respiration), and leaf traits of three dominant species -Betula nana, a woody shrub; Eriophorum vaginatum, a graminoid; and Rubus chamaemorus, a forbgrown under long-term warming and fertilization treatments since 1989 at Toolik Lake, Alaska. Under warming, B. nana exhibited the highest rates of A net and strongest light inhibition of respiration, increasing CGE nearly 50% compared with leaves grown in ambient conditions, which corresponded to a 52% increase in relative abundance. Gas exchange did not shift under fertilization in B. nana despite increases in leaf N and P and near-complete dominance at the community scale, suggesting a morphological rather than physiological response. Rubus chamaemorus, exhibited minimal shifts in foliar gas exchange, and responded similarly to B. nana under treatment conditions. By contrast, E. vaginatum, did not significantly alter its gas exchange physiology under treatments and exhibited dramatic decreases in relative cover (warming: À19.7%; fertilization: À79.7%; warming with fertilization: À91.1%). Our findings suggest a foliar physiological advantage in the woody shrub B. nana that is further mediated by warming and increased soil nutrient availability, which may facilitate shrub expansion and in turn alter the terrestrial carbon cycle in future tundra environments.
1. Many regions of the globe are experiencing a simultaneous change in the dominant plant functional type and regional climatology. We explored how atmospheric temperature and precipitation control leaf-and ecosystem-scale carbon fluxes within a pair of semi-arid shrublands, one upland and one riparian, that have undergone woody plant expansion. 2. Through a combination of leaf-level measurements on individual bunchgrasses and mesquites shrubs and ecosystem-scale monitoring using eddy covariance techniques, we sought to quantify rates of net carbon dioxide (CO 2 ) flux, CO 2 flux temperature sensitivity and the responsiveness of these parameters to seasonal rains and periods of soil dry-down. 3. We found significant differences in physiological acclimation between the two plant functional types, in that the shrubs consistently conducted photosynthesis across a broader temperature range than co-occurring grasses during dry periods, yet maximum photosynthetic rates in grasses were twice that of mesquites during the wetter monsoon season. Landscape position modulated these temperature sensitivities, as the range of functional temperatures and maximum rates of photosynthesis were two to three times greater within the riparian shrubland in dry times. 4. Also, it was unexpected that ecosystem-scale CO 2 uptake within both shrublands would become most temperature sensitive within the monsoon, when mesquites and grasses had their broadest range of function. This is probably explained by the changing contributions of component photosynthetic fluxes, in that the more temperature sensitive grasses, which had higher maximal rates of photosynthesis, became a larger component of the ecosystem flux. 5. Synthesis: Given projections of more variable precipitation and increased temperatures, it is important to understand differences in physiological activity between growth forms, as they are likely to drive patterns of ecosystem-scale CO 2 flux. As access to stable subsurface water declines with decreased precipitation, these differential patterns of temperature sensitivity among growth forms, which are dependent on connectivity to groundwater, will only become more important in determining ecosystem carbon source/sink status.
Conversion of grasslands to woodlands may alter the sensitivity of CO 2 exchange of individual plants and entire ecosystems to air temperature and precipitation. We combined leaf-level gas exchange and ecosystem-level eddy covariance measurements to quantify the effects of plant temperature sensitivity and ecosystem temperature responses within a grassland and mesquite woodland across seasonal precipitation periods. In so doing, we were able to estimate the role of moisture availability on ecosystem temperature sensitivity under large-scale vegetative shifts. Optimum temperatures (T opt ) for net photosynthetic assimilation (A) and net ecosystem productivity (NEP) were estimated from a function fitted to A and NEP plotted against air temperature. The convexities of these temperature responses were quantified by the range of temperatures over which a leaf or an ecosystem assimilated 50% of maximum NEP (Ω 50 ). Under dry pre-and postmonsoon conditions, leaf-level Ω 50 in C 3 shrubs were two-to-three times that of C 4 grasses, but under moist monsoon conditions, leaf-level Ω 50 was similar between growth forms. At the ecosystems-scale, grassland NEP was more sensitive to precipitation, as evidenced by a 104% increase in maximum NEP at monsoon onset, compared to a 57% increase in the woodland. Also, woodland NEP was greater across all temperatures experienced by both ecosystems in all seasons. By maintaining physiological function across a wider temperature range during water-limited periods, woody plants assimilated larger amounts of carbon. This higher carbon-assimilation capacity may have significant implications for ecosystem responses to projected climate change scenarios of higher temperatures and more variable precipitation, particularly as semiarid regions experience conversions from C 4 grasses to C 3 shrubs. As regional carbon models, CLM 4.0, are now able to incorporate functional type and photosynthetic pathway differences, this work highlights the need for a better integration of the interactive effects of growth form/functional type and photosynthetic pathway on water resource acquisition and temperature sensitivity.
Disentangling root responses to climate change in a semiarid grassland
Oecologia, 2014
mediated effects of elevated cO 2 , we included an irrigation treatment. We assessed root standing mass, morphology, residence time and seasonal appearance/disappearance of community-aggregated roots, as well as mass and n losses during decomposition of two dominant grass species (a c 3 and a c 4). In contrast to what is common in mesic grasslands, greater root standing mass under elevated cO 2 resulted from increased production, unmatched by disappearance. elevated cO 2 plus warming produced roots that were longer, thinner and had greater surface area, which, together with greater standing biomass, could potentially alter root function and dynamics. Decomposition increased under environmental conditions generated by elevated cO 2 , but not those generated by warming, likely due to soil desiccation with warming. elevated cO 2 , particularly under warming, slowed n release from c 4-but not c 3-roots, and consequently could indirectly affect n availability through treatment effects on species composition. elevated cO 2 and warming effects on root morphology and decomposition could offset increased c inputs from greater root biomass, thereby limiting future net c accrual in this semiarid grassland.
RESEARCH New Phytol. (2000), 147, 13–31 Global patterns of root turnover for terrestrial ecosystems
Root turnover is a critical component of ecosystem nutrient dynamics and carbon sequestration and is also an important sink for plant primary productivity. We tested global controls on root turnover across climatic gradients and for plant functional groups by using a database of 190 published studies. Root turnover rates increased exponentially with mean annual temperature for fine roots of grasslands (r# l 0.48) and forests (r# l 0.17) and for total root biomass in shrublands (r# l 0.55). On the basis of the best-fit exponential model, the Q "! for root turnover was 1.4 for forest small diameter roots (5 mm or less), 1.6 for grassland fine roots, and 1.9 for shrublands. Surprisingly, after accounting for temperature, there was no such global relationship between precipitation and root turnover. The slowest average turnover rates were observed for entire tree root systems (10% annually), followed by 34% for shrubland total roots, 53% for grassland fine roots, 55% for wetland fine roots, and 56% for forest fine roots. Root turnover decreased from tropical to high-latitude systems for all plant functional groups. To test whether global relationships can be used to predict interannual variability in root turnover, we evaluated 14 yr of published root turnover data from a shortgrass steppe site in northeastern Colorado, USA. At this site there was no correlation between interannual variability in mean annual temperature and root turnover. Rather, turnover was positively correlated with the ratio of growing season precipitation and maximum monthly temperature (r# l 0.61). We conclude that there are global patterns in rates of root turnover between plant groups and across climatic gradients but that these patterns cannot always be used for the successful prediction of the relationship of root turnover to climate change at a particular site. Aber JD, Melillo JM, Nadelhoffer KJ, McClaugherty CA, Pastor J. 1985. Fine root turnover in forest ecosystems in relation to quantity and form of nitrogen availability : a comparison of two methods. Oecologia 66 : 317-321. Aerts R, Bakker C, De Caluwe H. 1992. Root turnover as determinant of the cycling of C, N, and P in a dry heathland ecosystem. Biogeochemistry 15 : 175-190. Ares J. 1976. Dynamics of the root system of Blue Gramma. Journal of Range Management 29 : 208-213. Arneth A, Kelliher FM, McSeveny TM, Byers JN. 1998. Net ecosystem productivity, net primary productivity and ecosystem carbon sequestration in a Pinus radiata plantation subject to soil water deficit. Tree Physiology 18 : 785-793. Arthur MA, Fahey TJ. 1992. Biomass and nutrients in an Englemann spruce-subalpine fir forest in north central Colorado : pools, annual production, and internal cycling. Canadian Journal of Forest Research 22 : 315-325. Arunachalam A, Pandey HN, Tripathi RS, Maithani K.
Asynchronicity in root and shoot phenology in grasses and woody plants
Global Change …, 2010
Phenology is central to understanding vegetation response to climate change, as well as vegetation effects on plant resources, but most temporal production data is based on shoots, especially those of trees. In contrast, most production in temperate and colder regions is belowground, and is frequently dominated by grasses. We report root and shoot phenology in 7-year old monocultures of 10 dominant species (five woody species, five grasses) in southern Canada. Woody shoot production was greatest about 8 weeks before the peak of root production, whereas grass shoot maxima preceded root maxima by 2-4 weeks. Over the growing season, woody root, and grass root and shoot production increased significantly with soil temperature. In contrast, the timing of woody shoot production was not related to soil temperature (r 5 0.01). The duration of root production was significantly greater than that of shoot production (grasses: 22%, woody species: 54%). Woody species produced cooler and moister soils than grasses, but growth forms did not affect seasonal patterns of soil conditions. Although woody shoots are the current benchmark for phenology studies, the other three components examined here (woody plant roots, grass shoots and roots) differed greatly in peak production time, as well as production duration. These results highlight that shoot and root phenology is not coincident, and further, that major plant growth forms differ in their timing of above-and belowground production. Thus, considering total plant phenology instead of only tree shoot phenology should provide a better understanding of ecosystem response to climate change.
Earlier plant growth helps compensate for reduced carbon fixation after 13 years of warming
Functional Ecology, 2019
1. Drylands play a dominant role in global carbon cycling and are particularly vulnerable to increasing temperatures, but our understanding of how dryland ecosystems will respond to climatic change remains notably poor. Considering that the area of drylands is projected to increase by 11%-23% by 2,100, understanding the impacts of warming on the functions and services furnished by these arid and semi-arid ecosystems has numerous implications. 2. In a unique 13-year ecosystem warming experiment in a southwestern U.S. dry-land, we investigated the consequences of rising temperature on Achnatherum hymenoides, a widespread, keystone grass species on the Colorado Plateau. We tracked individual-and population-level responses to identify optimal strategies that may have been masked if considering only one level of plant response. 3. We found several factors combined to affect the timing and magnitude of plant responses during the 13th year of warming. These included large warming-induced biomass increases for individual plants, an 8.5-day advancement in the growing season and strong reductions in photosynthetic rates and population cover. 4. Importantly, we observed a lack of photosynthetic acclimation and, thus, a warming induced downregulation of photosynthetic rates. However, these physiological responses were concurrent with warmed-plant increases in growing season length and investment in photosynthetic surfaces, demonstrating the species' ability to balance carbon fixation limitations with warming. 5. These results, which bring together ecophysiological, phenological, reproductive and morphological assessments of plant responses to warming, suggest that the extent of change in A. hymenoides populations will be based upon numerous adaptive responses that vary in their direction and magnitude. Plant population responses to climatic warming remain poorly resolved, particularly for Earth's dry-lands, and our in situ experiment assessing multiple strategies offers a novel look into a warmer world.
Evidence of Physiological Decoupling from Grassland Ecosystem Drivers by an Encroaching Woody Shrub
PLoS ONE, 2013
Shrub encroachment of grasslands is a transformative ecological process by which native woody species increase in cover and frequency and replace the herbaceous community. Mechanisms of encroachment are typically assessed using temporal data or experimental manipulations, with few large spatial assessments of shrub physiology. In a mesic grassland in North America, we measured inter-and intra-annual variability in leaf d 13 C in Cornus drummondii across a grassland landscape with varying fire frequency, presence of large grazers and topographic variability. This assessment of changes in individual shrub physiology is the largest spatial and temporal assessment recorded to date. Despite a doubling of annual rainfall (in 2008 versus 2011), leaf d 13 C was statistically similar among and within years from 2008-11 (range of 228 to 227%). A topography*grazing interaction was present, with higher leaf d 13 C in locations that typically have more bare soil and higher sensible heat in the growing season (upland topographic positions and grazed grasslands). Leaf d 13 C from slopes varied among grazing contrasts, with upland and slope leaf d 13 C more similar in ungrazed locations, while slopes and lowlands were more similar in grazed locations. In 2011, canopy greenness (normalized difference vegetation index-NDVI) was assessed at the centroid of individual shrubs using high-resolution hyperspectral imagery. Canopy greenness was highest midsummer , likely reflecting temporal periods when C assimilation rates were highest. Similar to patterns seen in leaf d 13 C, NDVI was highest in locations that typically experience lowest sensible heat (lowlands and ungrazed). The ability of Cornus drummondii to decouple leaf physiological responses from climate variability and fire frequency is a likely contributor to the increase in cover and frequency of this shrub species in mesic grassland and may be generalizable to other grasslands undergoing woody encroachment.
Grassland root demography responses to multiple climate change drivers depend on root morphology
Plant and Soil, 2013
Aims We examine how root system demography and morphology are affected by air warming and multiple, simultaneous climate change drivers. Methods Using minirhizotrons, we studied root growth, morphology, median longevity, risk of mortality and standing root pool in the upper soil horizon of a temperate grassland ecosystem for 3 years. Grassland monoliths were subjected to four climate treatments in a replicated additive design: control (C); elevated temperature (T); combined T and summer precipitation reduction (TD); combined TD and elevated atmospheric CO 2 (TDCO 2 ).
Soil warming and CO 2 enrichment induce biomass shifts in alpine tree line vegetation
Global Change Biology, 2015
Responses of alpine treeline ecosystems to increasing atmospheric CO2 concentrations and global warming are poorly understood. We used an experiment at the Swiss treeline to investigate changes in vegetation biomass after 9 years of free air CO2 enrichment (+200 ppm; 2001-2009) and 6 years of soil warming (+4°C; 2007-2012). The study contained two key treeline species, Larix decidua and Pinus uncinata, both approximately 40 years old, growing in heath vegetation dominated by dwarf shrubs. In 2012, we harvested and measured biomass of all trees (including root systems), above-ground understorey vegetation and fine roots. Overall, soil warming had clearer effects on plant biomass than CO2 enrichment, and there were no interactive effects between treatments. Total plant biomass increased in warmed plots containing Pinus but not in those with Larix. This response was driven by changes in tree mass (+50%), which contributed an average of 84% (5.7 kg m-2) of total plant mass. Pinus coarse root mass was especially enhanced by warming (+100%), yielding an increased root mass fraction. Elevated CO2 led to an increased relative growth rate of Larix stem basal area but no change in the final biomass of either tree species. Total understory above-ground mass was not altered by soil warming or elevated CO2. However, Vaccinium myrtillus mass increased with both treatments, grass mass declined with warming, and forb and nonvascular plant (moss and lichen) mass decreased with both treatments. Fine roots showed a substantial reduction under soil warming (-40% for all roots <2 mm in diameter at 0-20 cm soil depth) but no change with CO2 enrichment. Our findings suggest that enhanced overall productivity and shifts in biomass allocation will occur at the treeline, particularly with global warming. However, individual species and functional groups will respond differently to these environmental changes, with consequences for ecosystem structure and functioning.