Response of five temperate deciduous tree species to water stress

Gas exchange, tissue water relations, and leaf/root dry weight ratios were compared among young, container-grown plants of five temperate-zone, deciduous tree species (Acer negundo L., Betula papyrifera Marsh, Malus baccata Borkh, Robinia pseudoacacia L., and Ulmus parvifolia Jacq.) under well-watered and water-stressed conditions. There was a small decrease (mean reduction of 0.22 MPa across species) in the water potential at which turgor was lost (Psi(tlp)) in response to water stress. The Psi(tlp) for water-stressed plants was -1.18, -1.34, -1.61, -1.70, and -2.12 MPa for B. papyrifera, A. negundo, U. parvifolia, R. pseudoacacia, and M. baccata, respectively. Variation in Psi(tlp) resulted primarily from differences in tissue osmotic potential and not tissue elasticity. Rates of net photosynthesis declined in response to water stress. However, despite differences in Psi(tlp), there were no differences in net photosynthesis among water-stressed plants under the conditions of water stress imposed. In A. negundo and M. baccata, water use efficiency (net photosynthesis/transpiration) increased significantly in response to water stress. Comparisons among water-stressed plants showed that water use efficiency for M. baccata was greater than for B. papyrifera or U. parvifolia. There were no significant differences in water use efficiency among B. papyrifera, U. parvifolia, A. negundo, and R. pseudoacacia. Under water-stressed conditions, leaf/root dry weight ratios (an index of transpiration to absorptive capacity) ranged from 0.77 in R. pseudoacacia to 1.05 in B. papyrifera.     

Light acclimation potential and carry-over effects vary among three evergreen tree species with contrasting patterns of leaf emergence and maturation

We compared light acclimation potential among three evergreen broadleaved species with contrasting patterns of shoot elongation, leaf emergence and leaf maturation. Understory saplings were transferred to a high-light environment before bud break, grown for 13 months, and then transferred back to the understory to observe subsequent carry-over effects. Acclimation potential was highest and sapling mortality was lowest for Cinnamomum japonicum Sieb. ex Nakai. Indeterminate growth and successive leaf emergence allowed this species to acclimate to both high and low light by adjusting leaf production as well as leaf properties. Sapling mortality occurred after both transfers for Camellia japonica L., which also has indeterminate growth and successive leaf emergence. In this species, carry-over effects were observed at the individual level, but leaf-level acclimation potential was high. Acclimation potential was lowest and sapling mortality occurred soon after the transfer to high light for Quercus glauca Thunb. ex Murray. Determinate growth and flush-type leaf emergence resulted in significant carry-over effects in this species. Indeterminate growth and successive leaf emergence increase whole-plant acclimation potential by extending the period of growth and architectural development during the growing season. Similarly, we inferred that delayed leaf maturation, observed in many evergreen species, increases the acclimation potential of current-year leaves by extending the period of leaf development. In evergreen species, the acclimation potential of preexisting leaves determines the role that leaf turnover plays in whole-plant light acclimation, resulting in diverse strategies for light acclimation among species, as observed in this study.     

Evaluation of leaf display of evergreen broadleaved tree species and deciduous tree species in warm temperate conifer plantations

We investigated the sapling leaf display in the shade among trees of various leaf lifespans co-occurring under the canopy of a warm-temperate conifer plantation. We measured leaf-area ratio (aLAR) and morphological traits of saplings of evergreen broadleaved tree species and a deciduous tree species. Although we found large interspecific and intraspecific differences in aLAR even among saplings of similar size in the homogeneous light environment, we did not find a consistent trend in aLAR with leaf lifespan among the species. While deciduous trees annually produced a large leaf area, some evergreen broadleaved trees retained their leaves across years and had aLAR values as high as those of deciduous trees. Among leaf-level, shoot-level, and individual-level morphological traits, aLAR was positively correlated with current-year shoots mass per aboveground biomass in deciduous trees, and with the area of old leaves per aboveground mass in evergreen broadleaved trees. Thus, tree-to-tree variation in the degrees of annual shoot production and the accumulation of old leaves were responsible for the interspecific and intraspecific variations in aLAR.     

Seasonal and temperature dependence of photosynthesis and respiration for two co-occurring broad-leaved tree species with contrasting leaf phenology

We measured the seasonal and temperature responses of leaf photosynthesis and respiration of two co-occurring native New Zealand tree species with contrasting leaf phenology: winter-deciduous fuchsia (Fuchsia excorticata J. R. Forst & G. Forst) and annual evergreen wineberry (Aristotelia serrata J. R. Forst & G. Forst). There was no difference in the amount of nitrogen per unit leaf area (Narea, range 40-160 mmol m-2, P = 0.18) or specific leaf area (S, range 8-27 m2 kg-1, P = 0.87) in summer leaves of wineberry or fuchsia. The amount of nitrogen per unit leaf area and S varied significantly with height of leaves in the canopy for both species (r2 range 0.61-0.87). Parameters describing the maximum rates of rubisco carboxylation (Vcmax) and electron transport (Jmax) were related significantly to Narea, and were 60% higher on average in spring and summer leaves than in autumn and winter leaves for both species. The seasonal effect remained significant (P < 0.001) when Narea was included in a regression model, indicating that seasonal changes were not only due to changes in Narea. Values for Vcmax and Jmax were 30% lower in wineberry leaves than in fuchsia leaves on average, although the difference ranged from 15% in summer leaves to 39% in autumn leaves. Activation energies describing the temperature dependence of Vcmax and Jmax in wineberry were 111 and 114% of corresponding values for fuchsia (Ea (Vcmax) = 39.1 kJ mol-1, Ea (Jmax) = 32.9 kJ mol-1). Respiration at night was the same (P = 0.34) at 10 degrees C for both species (R10 = 0.7 micromol m-2 s-1), although activation energies (E0) were higher in wineberry than in fuchsia (47.4 and 32.9 kJ mol-1 K-1, respectively). These results show that rates of photosynthesis are higher in winter-deciduous fuchsia than in annual evergreen wineberry.     

Direct effect of elevated CO(2) on nocturnal in situ leaf respiration in nine temperate deciduous tree species is small

Direct (i.e., short-term) effects of elevated CO(2) on nocturnal in situ leaf respiration rate were measured in nine deciduous tree species (seven genera) in 20 3.5-4.0-h experiments. During the experiments, CO(2) concentration was alternated between 400 and 800 ppm (approximately 40 and 80 Pa of CO(2)). Data analysis accounted for effects on respiration rate of the normal decline in temperature with time after sunset. The median response to a 40-Pa increase in CO(2) was a 1.5% decrease in respiration rate, with responses ranging from a 5.6% inhibition to a 0.4% stimulation. Direct effects of elevated CO(2) on respiration were similar among the species. Thus, the response of nocturnal leaf respiration rate to a short-term CO(2) increase was small, and of little practical importance to the accuracy of measurements of respiration involving similar changes in CO(2) concentration during measurement. These direct respiratory responses of leaves to elevated CO(2) would translate into only slight, if any, effects on the carbon balance of temperate deciduous forests in a future atmosphere containing as much as 80 Pa CO(2).     

Interspecific and environmentally induced variation in foliar dark respiration among eighteen southeastern deciduous tree species

We measured variations in leaf dark respiration rate (Rd) and leaf nitrogen (N) across species, canopy light environment, and elevation for 18 co-occurring deciduous hardwood species in the southern Appalachian mountains of western North Carolina. Our overall objective was to estimate leaf respiration rates under typical conditions and to determine how they varied within and among species. Mean dark respiration rate at 20 degrees C (Rd,mass, micromol CO2 per kg leaf dry mass per s) for all 18 species was 7.31 micromol per kg per s. Mean Rd,mass of individual species varied from 5.17 micromol per kg per s for Quercus coccinea Muenchh. to 8.25 micromol per kg per s for Liriodendron tulipifera L. Dark respiration rate varied by leaf canopy position and was higher in leaves collected from high-light environments. When expressed on an area basis, dark respiration rate (Rd,area, micromol CO2 per kg leaf dry area per s) showed a strong linear relationship with the predictor variables leaf nitrogen (Narea, g N per square m leaf area) and leaf structure (LMA, g leaf dry mass per square m leaf area) (r squared = 0.62). This covariance was largely a result of changes in leaf structure with canopy position; smaller thicker leaves occur at upper canopy positions in high-light environments. Mass-based expression of leaf nitrogen and dark respiration rate showed that nitrogen concentration (Nmass, mg N per g leaf dry mass) was only moderately predictive of variation in Rd,mass for all leaves pooled (r squared = 0.11), within species, or among species. We found distinct elevational trends, with both Rd,mass and Nmass higher in trees originating from high-elevation, cooler growth environments. Consideration of interspecies differences, vertical gradients in canopy light environment, and elevation, may improve our ability to scale leaf respiration to the canopy in forest process models.     

Variation in the chemical composition of green leaves and leaf litters from three deciduous tree species growing on different soil types

The chemical composition of green leaves and leaf litters of sweet chestnut (Castanea sativa), oak (Quercus robur) and beech (Fagus sylvatica) were determined for 26 sites grouped into high fertility (HF) and low fertility (LF) soils according to base saturation and N-mineralization potentials. Measurements were made of total carbon, acid detergent fibre (ADF), Klason lignin, holo-cellulose, sugar constituents of hemicellulose and phenylpropanoid derivatives of lignin, and nutrient concentrations (N, Ca, P, Mg, K and Mn). Leaf and litter constituents varied within and between species according to soil groups, but beech showed contrasting responses to oak and chestnut. Beech leaves had lower ADF, lignin and cellulose on HF soils than LF soils, whereas oak and chestnut leaves had higher ADF, lignin and cellulose on HF than the LF soils. Conversely, the same constituents in beech leaf litter were higher on HF soils than LF soils, but lower in oak and chestnut leaf litter on HF soils than LF soils. The phenylpropanoid derivatives of lignin and sugar constituents of hemicellulose also showed similar variations in relation to soil groups with contrasting patterns for in leaves and litters. Re-absorption of N from leaves before litter fall was negatively correlated with soil N mineralization potential for beech (highest on LF soils) but showed an unexpected, positive relationship for oak and chestnut (highest on HF soils). These intra-specific differences of leaf and litter chemistry in relation to soil fertility status are unprecedented and largely unexplained. The observed patterns reflect phenotypic responses to soil type that result in continuum of litter quality, within and between tree species, that have been shown in related studies to significantly influence litter decomposition rates.     

Quantifying stomatal and non-stomatal limitations to carbon assimilation resulting from leaf aging and drought in mature deciduous tree species

Gas exchange techniques were used to investigate light-saturated carbon assimilation and its stomatal and non-stomatal limitations over two seasons in mature trees of five species in a closed deciduous forest. Stomatal and non-stomatal contributions to decreases in assimilation resulting from leaf age and drought were quantified relative to the maximum rates obtained early in the season at optimal soil water contents. Although carbon assimilation, stomatal conductance and photosynthetic capacity (V(cmax)) decreased with leaf age, decreases in V(cmax) accounted for about 75% of the leaf-age related reduction in light-saturated assimilation rates, with a secondary role for stomatal conductance (around 25%). However, when considered independently from leaf age, the drought response was dominated by stomatal limitations, accounting for about 75% of the total limitation. Some of the analytical difficulties associated with computing limitation partitioning are discussed, including path dependence, patchy stomatal closure and diffusion in the mesophyll. Although these considerations may introduce errors in our estimates, our analysis establishes some reasonable boundaries on relative limitations and shows differences between drought and non-drought years. Estimating seasonal limitations under natural conditions, as shown in this study, provides a useful basis for comparing limitation processes between years and species.     

Photosynthesis and water relations of savanna tree species differing in leaf phenology

Godmania macrocarpa Hemsley, a deciduous tree characteristic of fire protected areas of the savanna region of central Venezuela, was more drought tolerant, allocated more N to leaves and had consistently higher photosynthetic rates than Curatella americana L., a ubiquitous species growing successfully within the grasslands of tropical American savannas. Godmania macrocarpa maintained higher leaf conductance and photosynthesized at higher xylem water tensions than C. americana. As the dry season progressed, G. macrocarpa was more affected by water stress than C. americana, which may explain why G. macrocarpa shed its leaves before forming new leaves. For both species, leaf sap osmolality was strongly correlated with, but not completely accounted for by, soluble sugars. Integrated water-use efficiency, as measured by delta(13)C, was similar for both species, but young leaves were more efficient than old leaves. Water-use efficiency of adult leaves was similar in both species as a result of higher photosynthetic rates in G. macrocarpa and lower leaf conductances in C. americana. Compared to G. macrocarpa, instantaneous photosynthetic N-use efficiency was higher in C. americana despite its lower maximum photosynthetic rates. The absence of G. macrocarpa trees from open grasslands, despite their high productive capacity, is possibly the result of unfavorable conditions for germination, poor survival of seedlings, and lack of resistance against fire.     

Patterns of leaf conductance and water potential of five Himalayan tree species

We studied variations in water relations and drought response in five Himalayan tree species (Schima wallichii (DC.) Korth. (chilaune) and Castanopsis indica (Roxb.) Miq. (dhale katus) at an elevation of 1400 m, Quercus lanata Smith (banjh) and Rhododendron arboreum Smith (lali gurans) at 2020 m, and Quercus semecarpifolia Smith (khasru) at 2130 m) at Phulchowki Hill, Kathmandu, Nepal. Soil water potential at 15 (Psi(s15)) and 30 cm (Psi(s30)) depths, tree water potential at predawn (Psi(pd)) and midday (Psi(md)), and leaf conductance during the morning (g(wAM)) and afternoon (g(wPM)) were observed from December 1998 to April 2001, except during the monsoon months. There was significant variation among sites, species and months in Psi(pd), Psi(md), g(wAM) and g(wPM), and among months for all species for Psi(s15). Mean Psi(pd) and Psi(md) were lowest in Q. semecarpifolia (-0.40 and -1.18 MPa, respectively) and highest in S. wallichii (-0.20 and -0.63 MPa, respectively). The minimum Psi value for all species (-0.70 to -1.79 MPa) was observed in March 1999, after 4 months of unusually low rainfall. Some patterns of Psi(pd) were related to phenology and leaf damage. During leafing, Psi(pd) often increased. Mean g(wAM) and g(wPM) were highest in Q. semecarpifolia (172 and 190 mmol m(-2) s(-1), respectively) and lowest in C. indica (78 and 74 mmol m(-2) s(-1), respectively). Soil water potential (Psi) at 15 cm depth correlated with plant Psi in all species, but rarely with g(wAM) and not with g(wPM). Plant Psi declined with increasing elevation, whereas g(w) increased. As Psi(pd) declined, so did maximal g(w), but overall, g(w) was correlated with Psi(pd) only for R. arboreum. Schima wallichii maintained high Psi, with low stomatal conductance, as did Castanopsis indica, except that C. indica had low Psi during dry months. Rhododendron arboreum maintained high Psi(pd) and g(w), despite low soil Psi. Quercus lanata had low g(w) and low Psi(pd) in some months, but showed no correlation between tree Psi and g(w). Quercus semecarpifolia, which grows at the highest elevation, had low soil and plant Psi and high g(w).     

Responses of the seedlings of five dominant tree species in Changbai Mountain to soil water stress

Soil water stress was studied on the potted seedlings of five dominant tree species (Pinus koraienes Sieb.et Zucc.,Fraxinus mandshurica Rupr., Juglans mandshurica Maxim, Tilia amurensis Rupr. and Quercus mongolica Fisch.ex Turcz) from the broadleaved/Korean pine forest in Changbai Mountain. Leaf growth, water transpiration and photosynthesis were compared for each species under three soil moisture conditions: 85%-100% (high water, CK), 65%-85% (Medium water, MW) and 45%-65% (low water, LW) of 37.4% water-holding capacity in field. The results showed that the characteristic of typical drought-resistance of the leaves is significantly developed. The net photosynthetic rate and water use efficiency of Fraxinus mandshurica were higher in MW than those in CK. But for the other four species, the net photosynthetic rate and water use efficiency in CK were lower than those in MW and LW. The transpiration rate responding to soil moistures varied from species to soecies.     

Leaf litterfall concentrations and fluxes of elements in deciduous tree species

The influence of tree species and soil properties on leaf litterfall concentrations and fluxes of elements were studied in three mixed deciduous forest stands. Leaf litterfall fluxes of sixteen elements were measured during autumn defoliation in 100 to 150 yr old individuals of Fagus sylvatica L., Quercus robur L., Carpinus betulus L., Tilia cordata Mill, and Acer platanoides L. Compared to throughfall and precipitation, leaf litterfall dominated the flux of P, Ca, Mn, N and Mg (in decreasing order) to the soil below the tree canopies. For K and especially S and Na, however, the importance of leaf litterfall input to the total fluxes was small. Fluxes and especially concentrations of Ca, Mg, K, P and partly N were positively related to base saturation of the soil, while Mn was negatively related due to a higher mobilization rate and plant uptake of this element on acid soils. Litterfall fluxes for C. betulus were strongly positively influenced by base saturation and fluxes were usually higher than for F. sylvatica and Q. robur at the richest site. Quercus robur had often the lowest fluxes, especially of Ca and Mg. Tilia cordata had the highest leaf litter concentrations in twelve of the sixteen elements, and the highest fluxes particularly of N and K. Acer platanoides had the highest or among the highest concentrations and fluxes of Ca, Zn and Mn. The concentration of Al in C. betulus leaf litterfall, was about five times higher than in all other tree species at all sites.     

Reliance on stored water increases with tree size in three species in the Pacific Northwest

In tall old forests, limitations to water transport may limit maximum tree height and reduce photosynthesis and carbon sequestration. We evaluated the degree to which tall trees could potentially compensate for hydraulic limitations to water transport by increased use of water stored in xylem. Using sap flux measurements in three tree species of the Pacific Northwest, we showed that reliance on stored water increases with tree size and estimated that use of stored water increases photosynthesis. For Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), water stored in xylem accounted for 20 to 25% of total daily water use in 60-m trees, whereas stored water comprised 7% of daily water use in 15-m trees. For Oregon white oak (Quercus garryana Dougl. ex Hook.), water stored in xylem accounted for 10 to 23% of total daily water use in 25-m trees, whereas stored water comprised 9 to 13% of daily water use in 10-m trees. For ponderosa pine (Pinus ponderosa Dougl. ex Laws.), water stored in xylem accounted for 4 to 20% of total daily water use in 36-m trees, whereas stored water comprised 2 to 4% of daily water use in 12-m trees. In 60-m Douglas-fir trees, we estimated that use of stored water supported 18% more photosynthesis on a daily basis than would occur if no stored water were used, whereas 15-m Douglas-fir trees gained 10% greater daily photosynthesis from use of stored water. We conclude that water storage plays a significant role in the water and carbon economy of tall trees and old forests.     

Responses of Acer negundo genders to interannual differences in water availability determined from carbon isotope ratios of tree ring cellulose

Understanding the responses of riparian trees to water availability is critical for predicting the effects of changes in precipitation on riparian ecosystems. Dioecious Acer negundo L. (box elder) is a common riparian tree that is highly sensitive to water stress. Earlier studies indicated that the genders of A. negundo respond differently to water availability, with males being more conservative in their water use than females. To assess the potential effects of changes in precipitation on the sex ratio of riparian trees, we extended earlier studies of A. negundo by analyzing responses of male and female genotypes to interannual differences in water availability in a common garden. We measured growth of tree rings and used stable carbon isotope analysis of tree ring alpha-cellulose to integrate physiological responses to annual water treatments. During dry years, male and female trees exhibited similar growth and physiological responses. However, during wet years, females exhibited higher growth rates and lower carbon isotope ratios (indicating less conservative water use) than did males. Furthermore, we found that male trees exhibited similar stomatal behavior (inferred from integrated carbon isotope ratios) whether years were wet or dry, whereas females did not exhibit a consistent response to changes in water availability. We predict that with increasing precipitation and soil water availability, the representation of females will be favored because of shifts in the competitive interactions of the genders. Such changes may affect the reproductive output of these riparian trees and may influence overall water flux from riparian ecosystems. In addition, this study demonstrates the utility of carbon isotope analysis for assessing long-term responses of tree populations to shifts in water availability.     

Fine root respiration in northern hardwood forests in relation to temperature and nitrogen availability

We examined fine-root (< 2.0 mm diameter) respiration throughout one growing season in four northern hardwood stands dominated by sugar maple (Acer saccharum Marsh.), located along soil temperature and nitrogen (N) availability gradients. In each stand, we fertilized three 50 x 50 m plots with 30 kg NO(3) (-)-N ha(-1) year(-1) and an additional three plots received no N and served as controls. We predicted that root respiration rates would increase with increasing soil temperature and N availability. We reasoned that respiration would be greater for trees using NO(3) (-) as an N source than for trees using NH(4) (+) as an N source because of the greater carbon (C) costs associated with NO(3) (-) versus NH(4) (+) uptake and assimilation. Within stands, seasonal patterns of fine-root respiration rates followed temporal changes in soil temperature, ranging from a low of 2.1 micro mol O(2) kg(-1) s(-1) at 6 degrees C to a high of 7.0 micro mol O(2) kg(-1) s(-1) at 18 degrees C. Differences in respiration rates among stands at a given soil temperature were related to variability in total net N mineralized (48-90 micro g N g(-1)) throughout the growing season and associated changes in mean root tissue N concentration (1.18-1.36 mol N kg(-1)). The hypothesized increases in respiration in response to NO(3) (-) fertilization were not observed. The best-fit model describing patterns within and among stands had root respiration rates increasing exponentially with soil temperature and increasing linearly with increasing tissue N concentration: R = 1.347Ne(0.072T) (r(2) = 0.63, P < 0.01), where R is root respiration rate ( micro mol O(2) kg(-1) s(-1)), N is root tissue N concentration (mol N kg(-1)), and T is soil temperature ( degrees C). We conclude that, in northern hardwood forests dominated by sugar maple, root respiration is responsive to changes in both soil temperature and N availability, and that both factors should be considered in models of forest C dynamics.     

Changes in photosynthesis and leaf characteristics with tree height in five dipterocarp species in a tropical rain forest

Variations in leaf photosynthetic, morphological and biochemical properties with increasing plant height from seedlings to emergent trees were investigated in five dipterocarp species in a Malaysian tropical rain forest. Canopy openness increased significantly with tree height. Photosynthetic properties, such as photosynthetic capacity at light saturation, light compensation point, maximum rate of carboxylation and maximum rate of photosynthetic electron transport, all increased significantly with tree height. Leaf morphological and biochemical traits, such as leaf mass per area, palisade layer thickness, nitrogen concentration per unit area, chlorophyll concentration per unit dry mass and chlorophyll to nitrogen ratio, also changed significantly with tree height. Leaf properties had simple and significant relationships with tree height, with few intra- and interspecies differences. Our results therefore suggest that the photosynthetic capacity of dipterocarp trees depends on tree height, and that the trees adapt to the light environment by adjusting their leaf morphological and biochemical properties. These results should aid in developing models that can accurately estimate carbon dioxide flux and biomass production in tropical rain forests.     

Oomycota species associated with deciduous and coniferous seedlings in forest tree nurseries of Western Turkey

Occurrence and pathogenicity of Oomycota species causing root rot were investigated in 10 forest tree nurseries in western Turkey. Soil samples (129 in total) taken from the rhizosphere of symptomatic seedlings were baited for Oomycota using young leaves of Quercus suber, Rhododendron simsii and R. ponticum. Oomycota (178 isolates) were obtained by culturing on selective media, and identified using morphological methods followed by PCR and sequencing of the ITS rDNA and cox1 regions. Phytophthora aff. cactorum, P. citricola sensu lato, P. crassamura, P. syringae, Pythium aphanidermatum, Py. intermedium, Py. irregulare, Py. ultimum and Phytopythium vexans were common amongst the isolates. The highest diversity of Oomycota was found in the forest nurseries at Adapazar?‐Hendek and ?zmir‐Torbal?. Pathogenicity tests showed that the isolates caused lesions on a range of host plants. The importance of these nursery infections in transferring potentially damaging oomycete species to Turkish forests is discussed.     

Acclimatation of three co-occurring tree species to water stress and their role as site indicators in mixed pine-oak forests in the Sierra Madre Oriental,Mexico

Water availability and salt excess are limiting factors in Mexican mixed pine-oak forest. In order to characterise the acclimatation of native species to these stresses, leaf water (Ψ_w) and osmotic potentials (Ψ_s) of Juniperus flaccida, Pinus pseudostrobus and Quercus canbyi were measured under natural drought and non-drought conditions under two different aspects in the Sierra Madre Oriental. Factorial ANOVA revealed significant differences in Ψ_w and Ψ_s between two aspects, species and sampling dates. In general, all species showed high predawn and low midday values that declined progressively with increasing drought and soil–water loss. Seasonal and diurnal fluctuation of Ψ_w and Ψ_s were higher for J. flaccida and Q. canbyi than for P. pseudostrobus. Leaf Ψ_w and Ψ_s were mainly correlated with soil water content, while Ψ_s of P. pseudostrobus were hardly correlated with environmental variables. Thus, species have different strategies to withstand drought. P. pseudostrobus was identified as a species with isohydric water status regulation, while J. flaccida and Q. canbyi presented water potential patterns typical for anisohydric species. The type of water status regulation may be a critical factor for plant survival and mortality in the context of climate change. Nevertheless, for precise conclusions about the advantages and disadvantages of each type, further long-term investigations are required.     

Dark respiration rate increases with plant size in saplings of three temperate tree species despite decreasing tissue nitrogen and nonstructural carbohydrates

In shaded environments, minimizing dark respiration during growth could be an important aspect of maintaining a positive whole-plant net carbon balance. Changes with plant size in both biomass distribution to different tissue types and mass-specific respiration rates (R(d)) of those tissues would have an impact on whole-plant respiration. In this paper, we evaluated size-related variation in R(d), biomass distribution, and nitrogen (N) and total nonstructural carbohydrate (TNC) concentrations of leaves, stems and roots of three cold-temperate tree species (Abies balsamea (L.) Mill, Acer rubrum L. and Pinus strobus L.) in a forest understory. We sampled individuals varying in age (6 to 24 years old) and in size (from 2 to 500 g dry mass), and growing across a range of irradiances (from 1 to 13% of full sun) in northern Minnesota, USA. Within each species, we found small changes in R(d), N and TNC when comparing plants growing across this range of light availability. Consistent with our hypotheses, as plants grew larger, whole-plant N and TNC concentrations in all species declined as a result of a combination of changes in tissue N and shifts in biomass distribution patterns. However, contrary to our hypotheses, whole-plant and tissue R(d) increased with plant size in the three species.