Sari Palmroth - Profile on Academia.edu (original) (raw)

Papers by Sari Palmroth

Fertilization Affects Branching Pattern in Norway Spruce

ABSTRACT

Optimal Leaf Water Use Drives Ecosystem Water and Carbon Fluxes in a Changing Environment (Invited)

Canopy water and carbon exchange rates are controlled by leaf-level adjustment of stomatal apertu... more Canopy water and carbon exchange rates are controlled by leaf-level adjustment of stomatal aperture and photosynthetic capacity. Both leaf-level stomatal conductance and the leaf photosynthetic machinery respond nonlinearly to soil water availability, atmospheric CO2 concentration, and other environmental variables. Here we propose and test a canopy-scale coupled soil-vegetation-atmosphere model based on leaf-level optimality principles to describe leaf interactions with the immediate microclimate. Building on previous work, we assume that stomatal conductance is optimized so as to maximize the leaf-level net carbon gain, interpreted as photosynthesis minus the costs associated with water losses through leaf transpiration. This optimality principle is shown to describe a wealth of available leaf-level gas exchange data under different soil water availabilities and atmospheric conditions. The leaf-level optimality model is scaled up to the canopy-level by accounting for light and wind speed profiles within the canopy, as well as including a new model of soil-to-leaf water flow. The coupled model is applied to study the responses of a grassland ecosystem to different rainfall patterns and atmospheric CO2 concentrations.

Tree physiology, Jan 15, 2015

Manipulating tree belowground carbon (C) transport enables investigation of the ecological and ph... more Manipulating tree belowground carbon (C) transport enables investigation of the ecological and physiological roles of tree roots and their associated mycorrhizal fungi, as well as a range of other soil organisms and processes. Girdling remains the most reliable method for manipulating this flux and it has been used in numerous studies. However, girdling is destructive and irreversible. Belowground C transport is mediated by phloem tissue, pressurized through the high osmotic potential resulting from its high content of soluble sugars. We speculated that phloem transport may be reversibly blocked through the application of an external pressure on tree stems. Thus, we here introduce a technique based on compression of the phloem, which interrupts belowground flow of assimilates, but allows trees to recover when the external pressure is removed. Metal clamps were wrapped around the stems and tightened to achieve a pressure theoretically sufficient to collapse the phloem tissue, thereby...

Assessing cause and effect in ecological time series: An application of conditional Granger’s Spectral Causality Theory

Boreal forest and climate change – from processes and transport to trees, ecosystems and atmosphere

Silva Fennica, 2009

Forest Ecology and Management, 2015

Throughout the southern US, past forest management practices have replaced large areas of native ... more Throughout the southern US, past forest management practices have replaced large areas of native forests with loblolly pine plantations and have resulted in changes in forest response to extreme weather conditions. However, uncertainty remains about the response of planted versus natural species to drought across the geographical range of these forests. Taking advantage of a cluster of unmanaged stands (85-130 year-old hardwoods) and managed plantations (17-20 year-old loblolly pine) in coastal and Piedmont areas of North Carolina, tree water use, cavitation resistance, whole-tree hydraulic (K tree ) and stomatal (G s ) conductances were measured in four sites covering representative forests growing in the region. We also used a hydraulic model to predict the resilience of those sites to extreme soil drying. Our objectives were to determine: (1) if K tree and stomatal regulation in response to atmospheric and soil droughts differ between species and sites; (2) how ecosystem type, through tree water use, resistance to cavitation and rooting profiles, affects the water uptake limit that can be reached under drought; and (3) the influence of stand species composition on critical transpiration that sets a functional water uptake limit under drought conditions. The results show that across sites, water stress affected the coordination between K tree and G s . As soil water content dropped below 20% relative extractable water, K tree declined faster and thus explained the decrease in G s and in its sensitivity to vapor pressure deficit. Compared to branches, the capability of roots to resist high xylem tension has a great impact on tree-level water use and ultimately had important implications for pine plantations resistance to future summer droughts. Model simulations revealed that the decline in K tree due to xylem cavitation aggravated the effects of soil drying on tree transpiration. The critical transpiration rate (E crit ), which corresponds to the maximum rate at which transpiration begins to level off to prevent irreversible hydraulic failure, was higher in managed forest plantations than in their unmanaged counterparts. However, even with this higher E crit , the pine plantations operated very close to their critical leaf water potentials (i.e. to their permissible water potentials without total hydraulic failure), suggesting that intensively managed plantations are more drought-sensitive and can withstand less severe drought than natural forests.

Optimal plant water-use strategies under stochastic rainfall

Water Resources Research, 2014

ABSTRACT Plant hydraulic traits have been conjectured to be coordinated, thereby providing plants... more ABSTRACT Plant hydraulic traits have been conjectured to be coordinated, thereby providing plants with a balanced hydraulic system that protects them from cavitation while allowing an efficient transport of water necessary for photosynthesis. In particular, observations suggest correlations between the water potentials at which xylem cavitation impairs water movement and the one at stomatal closure, and between maximum xylem and stomatal conductances, begging the question as to whether such coordination emerges as an optimal water-use strategy under unpredictable rainfall. Here, mean transpiration is used as a proxy for long-term plant fitness and its variations as a function of the water potentials at 50% loss of stem conductivity due to cavitation and at 90% stomatal closure are explored. It is shown that coordination between these hydraulic traits is necessary to maximize , with rainfall patterns altering the optimal range of trait values. In contrast, coordination between ecosystem-level conductances appears not necessary to maximize . The optimal trait ranges are wider under drier than under mesic conditions, indicating that in semi-arid systems different water use strategies may be equally successful. Comparison with observations across species from a range of ecosystems confirms model predictions, indicating that the coordinated functioning of plant organs might indeed emerge from an optimal response to rainfall variability.

Silva Fennica, 1998

P. 1998. Utilizing a multipoint measuring system of photosynthetically active radiation in photos... more P. 1998. Utilizing a multipoint measuring system of photosynthetically active radiation in photosynthetic studies within canopies. Silva Fennica 32(4): 311-319.

New Phytologist, 2014

Models of forest energy, water and carbon cycles assume decreased stomatal conductance with eleva... more Models of forest energy, water and carbon cycles assume decreased stomatal conductance with elevated atmospheric CO 2 concentration ([CO 2 ]) based on leaf-scale measurements, a response not directly translatable to canopies. Where canopy-atmosphere are well-coupled, [CO 2 ]-induced structural changes, such as increasing leaf-area index (L D ), may cause, or compensate for, reduced mean canopy stomatal conductance (G S ), keeping transpiration (E C ) and, hence, runoff unaltered.

Tree Physiology, 2007

The spherical mean of the shoot silhouette-to-total leaf area ratio (STAR) and the shoot transmis... more The spherical mean of the shoot silhouette-to-total leaf area ratio (STAR) and the shoot transmission coefficient (c) are two key structural parameters in radiative transfer models for calculating canopy photosynthesis and leaf area index. The standard optical method for estimating these parameters might introduce errors in the estimates for species with flexible shoots and needles by changing shoot inclination relative to its inclination in situ. We devised and tested two methods to address this problem. First, we modified the standard optical method by designing an apparatus that allows shoots to be photographed in their original orientation. Second, we developed a faster, model-based approach to replace photography and tested the results against the established approach. We used shoots of three pine species, Pinus echinata Mill. (needle length~50 mm), P. taeda L. (~150 mm) and P. palustris Mill. (~300 mm). Values of the parameters simulated by the model were similar to those measured from the photographs. In our data, STAR varied about twofold among the pine species and was~40% higher in shade shoots than in sun shoots of P. taeda. The transmission coefficient for P. taeda shade shoots was alsõ 40% higher than that of sun shoots of all three species. We tested the versatility of the model by employing it on shoots of two other pine species (P. strobus L. and P. thumbergiana Parl.) as well as on shoots of Tsuga canadensis L. Carr. and Picea pungens Engelm. Regardless of shoot characteristics, the model generated values of shoot structural parameters similar to those estimated with the optical method. Although species-specific and vertical gradients in parameter values are best for modeling radiative transfer in conifer canopies, our results suggest that, in the absence of adequate data, STAR can be approximated as 0.16 for a wide range of shoot structures. For applications requiring angle-dependent parameterization, our new model facilitates rapid generation of these radiative transfer parameters.

Tree Physiology, 2002

We investigated effects of nutrient availability on shoot structure and light-interception effici... more We investigated effects of nutrient availability on shoot structure and light-interception efficiency based on data from control (C) and irrigated + fertilized (IL) trees of Norway spruce (Picea abies (L.) Karst.). The sampling of 1-year-old shoots was designed to cover the variation in canopy exposure within the live crown zone, where current-year shoots were still found. Canopy openness was used as a measure of light availability at the shoot's position. Openness values for the sample shoots ranged from 0.02 to 0.77 on the IL plot, and from 0.10 to 0.96 on the C plot.

Tree Physiology, 2001

We examined the effects of structural and physiological acclimation on the photosynthetic efficie... more We examined the effects of structural and physiological acclimation on the photosynthetic efficiency of Scots pine (Pinus sylvestris L.) shoots. We estimated daily light interception (DLI) and photosynthesis (DPHOT) of a number of sample shoots situated at different positions in the canopy. Photosynthetic efficiency (ε) was defined as the ratio of DPHOT to the potential daily light interception (DLI ref ) defined as the photosynthetically active radiation (PAR) intercepted per unit area of a sphere at the shoot location. To calculate DLI ref , DLI and DPHOT, the radiation field surrounding a shoot in the canopy was first modeled using simulated directional distributions of incoming PAR on a clear and an overcast day, and estimates of canopy gap fraction in different directions provided by hemispherical photographs. A model of shoot geometry and measured data on shoot structure and photosynthetic parameters were used to simulate the distribution of PAR irradiance on the needle surface area of the shoot.

Tree Physiology, 2013

In boreal forests, seedling establishment is limited by various factors including soil nitrogen (... more In boreal forests, seedling establishment is limited by various factors including soil nitrogen (N) availability. Seedlings may absorb N from soil in a variety of inorganic and organic forms; however, the energy and thus carbohydrate requirements for uptake and assimilation of N vary with N source. We studied the importance of current photoassimilates for the acquisition and allocation of different N sources by Scots pine (Pinus sylvestris (L.)) seedlings. Girdling was used as a tool to impair phloem transport of photoassimilates, and hence gradually deprive roots of carbohydrates. Seedlings were cultivated in a greenhouse on equimolar N concentrations of one of the N sources-arginine, ammonium or nitrate-and then girdled prior to a pulse-chase uptake experiment with isotopically labeled N sources. Girdling proved to be efficient in decreasing levels of soluble sugars and starch in the roots. Uptake rate of arginine N was highest, intermediate for ammonium N and lowest for nitrate N. Moreover, the uptake of arginine N was unaffected by girdling, while the uptake of the two inorganic N sources decreased to 45-56% of the ungirdled controls. In arginine-treated seedlings, 95-96% of the acquired arginine N resided in the roots, whereas a significant shift in the N distribution toward the shoot was evident in girdled seedlings treated with inorganic N. This spatial shift was especially pronounced in nitrate-treated seedlings suggesting that the reduction and following incorporation into roots was limited by the availability of current photoassimilates. These results suggest that there are energetic benefits for seedlings to utilize organic N sources, particularly under circumstances where carbohydrate supply is limited. Hence, these putative benefits might be of importance for the survival and growth of seedlings when carbohydrate reserves are depleted in early growing season, or in light-limited environments, such as those sustained by continuous cover forestry systems.

Tree Physiology, 2011

of crowns in stands, the clumping of leaves in shoots, the dimensions of leaves, shoots and crown... more of crowns in stands, the clumping of leaves in shoots, the dimensions of leaves, shoots and crowns, and the angle distribution of leaves . In turn, the prevailing Q at each location in the canopy affects leaf clumping and angles as well as leaf photosynthetic characteristics, including mesophyll properties and leaf biochemistry (Sprugel 1989, Niinemets and We analyzed the effect of simplifying assumptions in canopy representation of radiation transfer models, comparing modeled diffuse non-interceptance and photosynthetic photon flux density with measurements at different layers of complex pinebroadleaved canopy with large seasonal variation of leaf area index. The most detailed model included clumping of trees (i.e., stand density) and a vertical specification of leaf angle distribution and shoot clumping. A less detailed model replaced the vertically specified variables with their means. The most parsimonious model accounted for neither shoot clumping nor stand density. The vertical specification of shoot clumping and leaf angle distribution only slightly improved vertical and seasonal openness and light estimates over using mean values. Further simplification had little effect on total absorbed light but was more risky for estimates of the vertical distributions of openness and light absorbed by the canopy, which will affect photosynthesis estimates due to the non-linearity of photosynthetic light response. Including woody surfaces in winter, when leaf area was low, was essential for reproducing the measurements correctly. A sensitivity analysis showed that ignoring (i) shoot clumping could result in a substantial overestimation of total absorbed light with errors increasing with decreasing leaf area and (ii) stand density in sparse stands could lead to substantial overestimation of total absorbed light, and the effect is largely independent of leaf area. Also, (iii) the effect of changing leaf angle distribution increased with decreasing leaf area, and was larger and more persistent along the leaf area range with increasing shoot clumping. Overall, accounting for the effect of tree clumping on absorbed light is most important in stands composed of species where leaves are not very clumped (e.g., broadleaved). However, even in forests with highly clumped shoots (i.e., coniferous), an accurate estimation of absorbed light distribution in stands requires incorporation of stand density in the model.

Tree Physiology, 2010

Four-year-old seedlings of Picea abies [L.] Karst (Norway spruce) were grown in semi-controlled c... more Four-year-old seedlings of Picea abies [L.] Karst (Norway spruce) were grown in semi-controlled conditions with three watering regimes. The seedlings in the control group (c) were watered to prevent any dehydration effect. The two remaining groups were subjected to mild (ms) and severe water stress (ss), respectively. The following physiological variables were monitored until ss seedlings began to die: leaf water potential (ψ L ), stomatal conductance (g s ), CO 2 exchange (P N ), free proline content (Pro), total chlorophyll (a + b) concentration (Chl t ) and the maximal photochemical efficiency of photosystem II (F v /F m ). The results indicate that not all observed physiological parameters display the same degree of sensitivity to dehydration. After Day 12 of dehydration, ψ L of ss seedlings was already significantly lower than that of the two other groups. On Day 26, significant differences in ψ L were recorded among all treatments. Decreasing values of water potential were accompanied by early changes in P N , g s and Pro. A significant decrease in Chl t and F v /F m were only observed during the more advanced stages of dehydration. These results demonstrate that the drought response of P. abies seedlings include a number of parallel physiological and biochemical changes in concert, enhancing the capability of plants to survive and grow during drought periods, but only to a point.

Oecologia, 2014

P. sylvestris, individuals of the two shrubs did not increase their biomass or shift their alloca... more P. sylvestris, individuals of the two shrubs did not increase their biomass or shift their allocation between above-and belowground parts in response to n additions. altogether, our results indicate that the understory shrubs in these systems show little response to n additions in terms of photosynthetic physiology or growth and that changes in their performance are mostly associated with responses of the tree canopy.

Oecologia, 1999

We investigated the functional and structural responses of Scots pine to climate and estimated th... more We investigated the functional and structural responses of Scots pine to climate and estimated the importance of the genotype on the traits studied. We analysed 13 C isotope discrimination (∆ 13 C) of various provenances in a common garden experiment and gas exchange characteristics for provenances growing in their natural environment. No clear climatic trend was found in the foliar ∆ 13 C values of common garden trees. Similar results were obtained from estimation of λ (a largely VPD, temperature and light independent measure of intrinsic water use efficiency) from the gas exchange data. The ratio of needle mass to unit stem area and branch area to stem area increased towards south in both experiments and hence, seemed to be genetic. Trees from drier and warmer conditions seemed not to have either lower needle mass or higher intrinsic water use efficiency compared to northern latitudes.

New Phytologist, 2013

Soil and plant hydraulics constrain ecosystem productivity by setting physical limits to water tr... more Soil and plant hydraulics constrain ecosystem productivity by setting physical limits to water transport and hence carbon uptake by leaves. While more negative xylem water potentials provide a larger driving force for water transport, they also cause cavitation that limits hydraulic conductivity. An optimum balance between driving force and cavitation occurs at intermediate water potentials, thus defining the maximum transpiration rate the xylem can sustain (denoted as E(max)). The presence of this maximum raises the question as to whether plants regulate transpiration through stomata to function near E(max). To address this question, we calculated E(max) across plant functional types and climates using a hydraulic model and a global database of plant hydraulic traits. The predicted E(max) compared well with measured peak transpiration across plant sizes and growth conditions (R = 0.86, P &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; 0.001) and was relatively conserved among plant types (for a given plant size), while increasing across climates following the atmospheric evaporative demand. The fact that E(max) was roughly conserved across plant types and scales with the product of xylem saturated conductivity and water potential at 50% cavitation was used here to explain the safety-efficiency trade-off in plant xylem. Stomatal conductance allows maximum transpiration rates despite partial cavitation in the xylem thereby suggesting coordination between stomatal regulation and xylem hydraulic characteristics.

Observations of ultrafine aerosol particle formation and growth in boreal forest

Geophysical Research Letters, 1997

Number size distribution of ambient submicron and ultrafine aerosol particles have been measured ... more Number size distribution of ambient submicron and ultrafine aerosol particles have been measured on a continuous basis (every 10 minutes) for three quarters of the year 1996, at a forest site in Southern Finland. Continuous monitoring offers additional insight over the ...

Functional Ecology, 1997

1. The geographical aspects in photosynthetic light response and stomatal conductance in the shoo... more 1. The geographical aspects in photosynthetic light response and stomatal conductance in the shoots of Pinus sylvestris were studied together with structural properties of shoots and needles. Seven stands within the natural distribution area of P. sylvestris in Europe were chosen. CO 2 exchange, irradiance and stomatal conductance (g s ) for water vapour were measured and the maximum photosynthetic rate (P m ) was determined from the CO 2 exchange measurements.