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1.
Stöhr A  Lösch R 《Tree physiology》2004,24(2):169-180
We report on diurnal and seasonal variations in sap flow rate and stem water potential of Fraxinus excelsior L. saplings growing at the edge of a Fraxino-Aceretum forest in western Germany. Because of shallow soil, the trees were subjected to drought in summer. When soil water availability was not limiting, sap flow rate was related to changes in solar radiation and vapor pressure deficit. Maximum transpiration rates per leaf area were 3.5-7.4 mmol m-2 s-1, and maximum daily totals were 1.7-3.3 kg m-2 day-1. Under drought conditions, stem water potential dropped to midday minima of -2.6 to -3.5 MPa and sap flow rate was strongly related to this parameter. After the drought period, reduced apparent (whole-plant) hydraulic conductance was observed, which was attributed to a continued reduction in stomatal conductance after the drought stress had ceased. A model was developed that linked sap flow rate directly to climatic variables and stem water potential. Good correlation between measured and simulated sap flow rates allowed the model to be used for data interpretation.  相似文献   

2.
We estimated daily use of stored water by Scots pine (Pinus sylvestris L.) trees growing in a temperate climate with the ANAFORE model (ANAlysis of FORest Ecosystems) and compared the simulation results with sap flow measurements. The original model was expanded with a dynamic water flow and storage model that simulates sap flow dynamics in an individual tree. ANAFORE was able to accurately simulate diurnal patterns of measured sap flow under microclimatic conditions that differ from those of the calibration period. Strong relationships were found between stored water use and several tree characteristics (diameter at breast height, sapwood area, leaf area), but not with tree height. Relative to transpiration, stored water use varied over time (between < 1% and 44% of daily transpiration). On days when transpiration was high, trees were more dependent on stored water, indicating that the contribution of internal water to transpiration is not a constant in the water budget of trees.  相似文献   

3.
Perks MP  Irvine J  Grace J 《Tree physiology》2002,22(12):877-883
We investigated the effect of drought on canopy stomatal conductance (g(c)), and examined the hypothesis that g(c) is controlled by the chemical messenger abscisic acid (ABA) produced in roots. Beginning in November 1994, we subjected a mature stand of Scots pine (Pinus sylvestris L.) to an imposed 11-month drought. Control plots were maintained at average-season soil water content. Xylem sap was extracted from shoots at regular intervals from April to November 1995. Soil water, sap flow and leaf water potentials (predawn to dusk) were recorded at the same time. Canopy stomatal conductance was calculated from sap flow data and xylem sap ABA concentrations ([ABA(xyl)]) were measured by radioimmunoassay. Mean [ABA(xyl)] in control trees was 250 micromol m(-3). No diurnal variation in [ABA(xyl)] was detected. With soil drying, [ABA(xyl)] increased to a maximum in summer (600 micromol m(-3)), but decreased again toward autumn; however, no significant increase in ABA flux to the leaves occurred. A decline in g(c) was detected when volumetric soil water content declined below 0.12. The decline in g(c) could not have been mediated by increasing [ABA(xyl)] because stomatal closure appeared to precede any increase in [ABA(xyl)]. Peak sap flow velocity data were used to estimate delivery times for root-to-shoot signals in 15-m tall trees. Under normal field conditions, a signal would take 12 days to travel from the site of production (roots) to the presumed site of action (shoots). However, under drought conditions it may take a chemical signal in excess of 6 weeks. We conclude that a feedforward model of short-term stomatal response to soil drying, based solely on the action of a chemical messenger from the roots, is not applicable in mature conifer trees because signal transmission is too slow.  相似文献   

4.
We investigated the impact of drought on the physiology of 41-year-old Scots pine (Pinus sylvestris L.) in central Scotland. Measurements were made of the seasonal course of transpiration, canopy stomatal conductance, needle water potential, xylem water content, soil-to-needle hydraulic resistance, and growth. Comparison was made between drought-treated plots and those receiving average precipitation. In response to drought, transpiration rate declined once volumetric water content (VWC) over the top 20 cm of soil reached a threshold value of 12%. Thereafter, transpiration was a near linear function of soil water content. As the soil water deficit developed, the hydraulic resistance between soil and needles increased by a factor of three as predawn needle water potential declined from -0.54 to -0.71 MPa. A small but significant increase in xylem embolism was detected in 1-year-old shoots. Stomatal control of transpiration prevented needle water potential from declining below -1.5 MPa. Basal area, and shoot and needle growth were significantly reduced in the drought treatment. In the year following the drought, canopy stomatal conductance and soil-to-needle hydraulic resistance recovered. Current-year needle extension recovered, but a significant reduction in basal area increment was evident one year after the drought. The results suggest that, in response to soil water deficit, mature Scots pine closes its stomata sufficiently to prevent the development of substantial xylem embolism. Reduced growth in the year after a severe soil water deficit is most likely to be the result of reduced assimilation in the year of the drought, rather than to any residual embolism carried over from one year to the next.  相似文献   

5.
A dynamic model for simulating water flow in a Scots pine (Pinus sylvestris L.) tree was developed. The model is based on the cohesion theory and the assumption that fluctuating water tension driven by transpiration, together with the elasticity of wood tissue, causes variations in the diameter of a tree stem and branches. The change in xylem diameter can be linked to water tension in accordance with Hookea s law. The model was tested against field measurements of the diurnal xylem diameter change at different heights in a 37-year-old Scots pine at Hyyti?l?, southern Finland (61 degrees 51' N, 24 degrees 17' E, 181 m a.s.l.). Shoot transpiration and soil water potential were input data for the model. The biomechanical and hydraulic properties of wood and fine root hydraulic conductance were estimated from simulated and measured stem diameter changes during the course of 1 day. The estimated parameters attained values similar to literature values. The ratios of estimated parameters to literature values ranged from 0.5 to 0.9. The model predictions (stem diameters at several heights) were in close agreement with the measurements for a period of 6 days. The time lag between changes in transpiration rate and in sap flow rate at the base of the tree was about half an hour. The analysis showed that 40% of the resistance between the soil and the top of the tree was located in the rhizosphere. Modeling the water tension gradient and consequent woody diameter changes offer a convenient means of studying the link between wood hydraulic conductivity and control of transpiration.  相似文献   

6.
Brooks JR  Meinzer FC  Coulombe R  Gregg J 《Tree physiology》2002,22(15-16):1107-1117
The magnitude of hydraulic redistribution of soil water by roots and its impact on soil water balance were estimated by monitoring time courses of soil water status at multiple depths and root sap flow under drought conditions in a dry ponderosa pine (Pinus ponderosa Dougl. ex Laws) ecosystem and in a moist Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) ecosystem. The fate of deuterated water applied to small plots to create a strong horizontal soil water potential gradient was also monitored to assess the potential for horizontal redistribution of water and utilization of redistributed water by co-occurring shallow-rooted plants. In a 20-year-old Douglas-fir stand, approximately 28% of the water removed daily from the upper 2 m of soil was replaced by nocturnal hydraulic redistribution during late August. In an old-growth ponderosa pine stand, approximately 35% of the total daily water utilization from the upper 2 m of soil appeared to be replaced by hydraulic redistribution during July and August. By late September, hydraulic redistribution in the ponderosa pine stand was no longer apparent, even though total water use from the upper 2 m of soil was nearly identical to that observed earlier. Based on these results, hydraulic redistribution would allow 21 and 16 additional days of stored water to remain in the upper soil horizons in the ponderosa pine and Douglas-fir stands, respectively, after a 60-day drought. At both sites, localized applications of deuterated water induced strong reversal of root sap flow and caused soil water content to cease declining or even temporarily increase at locations too distant from the site of water application to have been influenced by movement of water through the soil without facilitation by roots. Xylem water deuterium values of ponderosa pine seedlings suggested utilization of redistributed water. Therefore, hydraulic redistribution may enhance seedling survival and maintain overstory transpiration during summer drought. These first approximations of the extent of hydraulic redistribution in these ecosystems suggest that it is likely to be an important process in both wet and dry forests of the Pacific Northwest.  相似文献   

7.
Reconstituted dikaryons of Pisolithus sp. (Pers.) Coker & Couch from South Africa influenced growth parameters (shoot length, shoot/root ratio and leaf area), nutrition and physiological indicators (transpiration rate, stomatal conductance and xylem water potential) of maritime pine (Pinus pinaster Ait.) seedlings during drought and recovery from drought. Seedlings colonized with certain dikaryons were more sensitive to water stress and showed less mycorrhiza formation under water stress than seedlings colonized with other dikaryons. Control (uninoculated) seedlings were significantly smaller than those inoculated with dikaryons. Transpiration rate, stomatal conductance and xylem water potential varied among mycorrhizal treatments during the water stress and recovery periods. After rewatering, the controls and seedlings inoculated with dikaryon 34 x 20 had a weaker recovery in transpiration rate, stomatal conductance and xylem water potential than the other treatments and appeared to have experienced damage due to the water stress. Concentrations of various elements differed in the shoots of Pinus pinaster colonized by the various dikaryons. It is suggested that breeding of ectomycorrhizal fungi could constitute a new tool for improving reforestation success in arid and semi-arid zones.  相似文献   

8.
To quantify the effects of crown thinning on the water balance and growth of the stand and to analyze the ecophysiological modifications induced by canopy opening on individual tree water relations, we conducted a thinning experiment in a 43-year-old Quercus petraea stand by removing trees from the upper canopy level. Soil water content, rainfall interception, sap flow, leaf water potential and stomatal conductance were monitored for two seasons following thinning. Seasonal time courses of leaf area index (LAI) and girth increment were also measured. Predawn leaf water potential was significantly higher in trees in the thinned stand than in the closed stand, as a consequence of higher relative extractable water in the soil. The improvement in water availability in the thinned stand resulted from decreases in both interception and transpiration. From Year 1 to Year 2, an increase in transpiration was observed in the thinned stand without any modification in LAI, whereas changes in transpiration in the closed stand were accompanied by variations in LAI. The different behaviors of the closed and open canopies were interpreted in terms of coupling to the atmosphere. Thinning increased inter-tree variability in sap flow density, which was closely related to a leaf area competition index. Stomatal conductance varied little inside the crown and differences in stomatal conductance between the treatments appeared only during a water shortage and affected mainly the closed stand. Thinning enhanced tree growth as a result of a longer growing period due to the absence of summer drought and higher rates of growth. Suppressed and dominant trees benefited more from thinning than trees in the codominant classes.  相似文献   

9.
Effects of soil and atmospheric drought on whole-tree transpiration (E(T)), leaf water potential (Ψ(L)) and whole-tree hydraulic conductance (K(T)) were investigated in mature rubber trees (Hevea brasiliensis, clone RRIM 600) during the full canopy stage in the rainy season in a drought-prone area of northeast Thailand. Under well-watered soil conditions, transpiration was tightly regulated in response to high evaporative demand, i.e., above reference evapotranspiration (ET(0)) ~2.2 mm day(-1) or maximum vapor pressure deficit ~1.8 kPa. When the trees experienced intermittent soil drought E(T) decreased sharply when relative extractable water in the top soil was?相似文献   

10.
The physiological responses to water deficits of Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) were studied under Mediterranean mountain climate. Minimum leaf water potentials were ?3.2 MPa for oak and ?2.1 MPa for pine, with higher predawn values for pubescent oak. Relative sap flow declined in both species when vapour pressure deficit (D) went above ca. 1.2 kPa, but stomatal control was stronger for pine during the 2003 summer drought. P. sylvestris plant hydraulic conductance on a half-total leaf area basis (k L,s?1) was 1.2–2.6 times higher than the values shown by Q. pubescens, and it showed a considerably steeper decrease during summer. Leaf-level gas exchange was positively related to k L,s?1 in both species. Scots pine was more vulnerable to xylem embolism and closed stomata to prevent substantial conductivity losses. The results of this study confirm that pubescent oak is more resistant to extreme drought events.  相似文献   

11.
As forests age, their structure and productivity change, yet in some cases, annual rates of water loss remain unchanged. To identify mechanisms that might explain such observations, and to determine if widely different age classes of forests differ functionally, we examined young (Y, approximately 25 years), mature (M, approximately 90 years) and old (O, approximately 250 years) ponderosa pine (Pinus ponderosa Dougl. ex P. Laws.) stands growing in a drought-prone region of central Oregon. Although the stands differed in tree leaf area index (LAIT) (Y = 0.9, M = 2.8, O = 2.1), cumulative tree transpiration measured by sap flow did not differ substantially during the growing season (100-112 mm). Yet when water was readily available, transpiration per unit leaf area of the youngest trees was about three times that of M trees and five times that of O trees. These patterns resulted from a nearly sixfold difference in leaf specific conductance (KL) between the youngest and oldest trees. At the time of maximum transpiration in the Y stand in May-June, gross carbon uptake (gross ecosystem production, GEP) was similar for Y and O stands despite an almost twofold difference in stand leaf area index (LAIS). However, the higher rate of water use by Y trees was not sustainable in the drought-prone environment, and between spring and late summer, KL of Y trees declined fivefold compared with a nearly twofold decline for M trees and a < 30% reduction in O trees. Because the Y stand contained a significant shrub understory and more exposed soil, there was no appreciable difference in mean daily latent energy fluxes between the Y stand and the older stands as measured by the eddy-covariance technique. These patterns resulted in 60 to 85% higher seasonal GEP and 55 to 65% higher water-use efficiency at the M and O stands compared with the Y stand.  相似文献   

12.
Repo T  Lehto T  Finér L 《Tree physiology》2008,28(10):1583-1591
In boreal regions, soil can remain frozen after the start of the growing season. We compared relationships between root characteristics and water relations in Scots pine (Pinus sylvestris L.) saplings subjected to soil frost treatments before and during the first week of the growing period in a controlled environment experiment. Delayed soil thawing delayed the onset of sap flow or totally blocked it if soil thawing lagged the start of the growing period by 7 days. This effect was reflected in the electrical impedance of needles and trunks and in the relative electrolyte leakage of needles. Prolonged soil frost reduced or completely inhibited root growth. In unfrozen soil, limited trunk sap flow was observed despite unfavorable aboveground growing conditions (low temperature, low irradiance, short photoperiod). Following the earliest soil thaw, sap flow varied during the growing season, depending on light and temperature conditions, phenological stage of the plant and the amount of live needles in the canopy. The results suggest that delayed soil thawing can reduce tree growth, and if prolonged, it can be lethal.  相似文献   

13.
We studied the response of stomatal conductance at leaf (gS) and canopy (GS) scales to increasing vapor pressure deficit (D) in mature Pinus palustris Mill. (longleaf pine) growing in a sandhill habitat in the coastal plain of the southeastern USA. Specifically, we determined if variation in the stomatal response to D was related to variation in hydraulic conductance along the soil-to-leaf pathway (KL) over the course of a growing season. Reductions in KL were associated with a severe growing season drought that significantly reduced soil water content (theta) in the upper 90-cm soil profile. Although KL recovered partially following the drought, it never reached pre-drought values. Stomatal sensitivity to D was well correlated with maximum gS at low D at both leaf and canopy scales, and KL appeared to influence this response by controlling maximum gS. Our results are consistent with the hypothesis that stomatal response to D occurs to regulate minimum leaf water potential, and that the sensitivity of this response is related to changes in whole-plant hydraulics.  相似文献   

14.
To clarify mechanisms underlying variation in transpiration rate among deciduous broad-leaved tree species, we measured diurnal changes in stomatal conductance (gs) and leaf water potential, and calculated the maximum transpiration rate (Emax), leaf-specific hydraulic conductance (K(s-l)) and difference between the soil water potential and the daily minimum leaf water potential (Psis - Psi(l,min)). Pressure-volume (P-V) measurements were made on leaves. Saplings of eight broad-leaved tree species that are common in Japanese cool temperate forests were studied. Maximum transpiration rate varied significantly among species. There was a statistically significant difference in Psis - Psi(l,min), but not in K(s-l). Species with large Emax also had large Psis - Psi(l,min) and gs. The results of the P-V analyses showed that species with a large Psis - Psi(l,min) maintained turgor even at low leaf water potentials. The similar daily minimum leaf pressure potentials (Psip) across all eight species indicate that Psip values below this minimum are critical. Based on these results, we suggest that the leaf cell capacity for turgor maintenance strongly affects Psis - Psi(l,min) and consequently Emax via stomatal regulation.  相似文献   

15.
To quantify the relationship between temporal and spatial variation in tree transpiration, we measured sap flow in 129 trees with constant-heat sap flow sensors in a subalpine forest in southern Wyoming, USA. The forest stand was located along a soil water gradient from a stream side to near the top of a ridge. The stand was dominated by Pinus contorta Dougl. ex Loud. with Picea engelmannii Parry ex Engelm and Abies lasiocarpa (Hook.) Nutt. present near the stream and scattered individuals of Populus tremuloides Michx. throughout the stand. We used a cyclic sampling design that maximized spatial information with a minimum number of samples for semivariogram analyses. All species exhibited previously established responses to environmental variables in which the dominant driver was a saturating response to vapor pressure deficit (D). This response to D is predictable from tree hydraulic theory in which stomatal conductance declines as D increases to prevent excessive cavitation. The degree to which stomatal conductance declines with D is dependent on both species and individual tree physiology and increases the variability in transpiration as D increases. We quantified this variability spatially by calculating the spatial autocorrelation within 0.2-kPa D bins. Across 11 bins of D, spatial autocorrelation in individual tree transpiration was inversely correlated to D and dropped from 45 to 20 m. Spatial autocorrelation was much less for transpiration per unit leaf area and not significant for transpiration per unit sapwood area suggesting that spatial autocorrelation within a particular D bin could be explained by tree size. Future research should focus on the mechanisms underlying tree size spatial variability, and the potentially broad applicability of the inverse relationship between D and spatial autocorrelation in tree transpiration.  相似文献   

16.
Sap flux density in branches, leaf transpiration, stomatal conductance and leaf water potentials were measured in 16-year-old Quercus suber L. trees growing in a plantation in southern Portugal to understand how evergreen Mediterranean trees regulate water loss during summer drought. Leaf specific hydraulic conductance and leaf gas exchange were monitored during the progressive summer drought to establish how changes along the hydraulic pathway influence shoot responses. As soil water became limiting, leaf water potential, stomatal conductance and leaf transpiration declined significantly. Predawn leaf water potential reflected soil water potential measured at 1-m depth in the rhizospheres of most trees. The lowest predawn leaf water potential recorded during this period was -1.8 MPa. Mean maximum stomatal conductance declined from 300 to 50 mmol m(-2) s(-1), reducing transpiration from 6 to 2 mmol m(-2) s(-1). Changes in leaf gas exchange were attributed to reduced soil water availability, increased resistances along the hydraulic pathway and, hence, reduced leaf water supply. There was a strong coupling between changes in soil water content and stomatal conductance as well as between stomatal conductance and leaf specific hydraulic conductance. Despite significant seasonal differences among trees in predawn leaf water potential, stomatal conductance, leaf transpiration and leaf specific hydraulic conductance, there were no differences in midday leaf water potentials. The strong regulation of changes in leaf water potential in Q. suber both diurnally and seasonally is achieved through stomatal closure, which is sensitive to changes in both liquid and vapor phase conductance. This sensitivity allows for optimization of carbon and water resource use without compromising the root-shoot hydraulic link.  相似文献   

17.
文章通过使用Li-6400对移植成活一年、二年、三年后的15a生樟子松大树的光合速率、蒸腾速率、气孔导度、胞间CO2浓度、叶片水势以及叶绿素含量的测定,进行了不同移植成活时间对樟子松大树光合特性影响的研究。结果表明:①蒸腾速率、光合速率、气孔导度以及胞间CO2浓度均先减小后增加;②移植成活二年时,水分利用效率最高;③叶片水势不断下降;④叶绿素含量先降低,后升高,基本维持在的7mg.g-1的水平。由于移植成活二年时最易受到干旱胁迫的影响,所以,有必要在樟子松大树移植后的两年内采取有效的措施促进其成活及树势恢复。  相似文献   

18.
Under climate change, severe and recurrent droughts can reduce forest production and cause widespread tree dieback. The response of different vegetation types to climate change can vary greatly and, therefore, must be individually assessed. This study was carried out in a Mediterranean oak forest (Quercus pyrenaica) subject to seasonal summer drought. To examine the response of the forest to the climate conditions predicted under climate change, a Soil–Vegetation–Atmosphere Transfer model [SPA, Williams, M., Rastetter, E.B., Fernandes, D.N., Goulden, M.L., Wofsy, S.C., Shaver, G.R., Melillo, J.M., Munger, J.W., Fan, S.M., Nadelhoffer, K.J. 1996. Modelling the soil-plant-atmosphere continuum in a Quercus–Acer stand at Harvard Forest: the regulation of stomatal conductance by light, nitrogen and soil/plant hydraulic properties. Plant, Cell, Environment 19, 911–927] was used. The model was parameterized using mostly local measurements (independent of the verification data) and tested against in situ sap flow measurements obtained during year 2007. The predictions of the model were broadly consistent with the observed dynamics of sap flow (the model explained 71% of the variance in daily transpiration and 75% of half-hourly sap flow), leaf water potentials and soil water content. Once the model had been validated, simulations were carried out under warmer and dryer conditions. Predicted warmer conditions (4 °C) caused a moderate increase in total simulated transpiration. Less frequent precipitation (40% longer dry periods between rainfall events) had very little effect on transpiration. In contrast, transpiration was reduced by 17% when the soil water reserves at the beginning of the summer were lower than in 2007, corresponding to those measured in a very dry year (2005). The reduction was exacerbated when changes in temperature and rainfall were also considered (up to 28% decline in transpiration). The higher atmospheric CO2 concentrations (712 ppm) simulated together with climate change, did not prevent the decline in tree water use or soil water storage at the end of the summer. All scenarios caused the soil water storage to reach extremely low values at the end of the dry season (a minimum of 25 mm). It is concluded that climate change is likely to have a negative impact on tree water use and soil water resources in the study area, increasing the water deficit by as much as 30%.  相似文献   

19.
In the Mediterranean evergreen oak woodlands of southern Portugal, the main tree species are Quercus ilex ssp. rotundifolia Lam. (holm oak) and Quercus suber L. (cork oak). We studied a savannah-type woodland where these species coexist, with the aim of better understanding the mechanisms of tree adaptation to seasonal drought. In both species, seasonal variations in transpiration and predawn leaf water potential showed a maximum in spring followed by a decline through the rainless summer and a recovery with autumn rainfall. Although the observed decrease in predawn leaf water potential in summer indicates soil water depletion, trees maintained transpiration rates above 0.7 mm day(-1) during the summer drought. By that time, more than 70% of the transpired water was being taken from groundwater sources. The daily fluctuations in soil water content suggest that some root uptake of groundwater was mediated through the upper soil layers by hydraulic lift. During the dry season, Q. ilex maintained higher predawn leaf water potentials, canopy conductances and transpiration rates than Q. suber. The higher water status of Q. ilex was likely associated with their deeper root systems compared with Q. suber. Whole-tree hydraulic conductance and minimum midday leaf water potential were lower in Q. ilex, indicating that Q. ilex was more tolerant to drought than Q. suber. Overall, Q. ilex seemed to have more effective drought avoidance and drought tolerance mechanisms than Q. suber.  相似文献   

20.
We investigated the impact of seasonal soil water deficit on the processes driving net ecosystem exchange of carbon (NEE) in old-growth and recently regenerating ponderosa pine (Pinus ponderosa Doug. ex Laws.) stands in Oregon. We measured seasonal patterns of transpiration, canopy conductance and NEE, as well as soil water, soil temperature and soil respiration. The old-growth stand (O) included two primary age classes (50 and 250 years), had a leaf area index (LAI) of 2.1 and had never been logged. The recently regenerating stand (Y) consisted predominantly of 14-year-old ponderosa pine with an LAI of 1.0. Both stands experienced similar meteorological conditions with moderately cold wet winters and hot dry summers. By August, soil volumetric water content within the upper 30 cm had declined to a seasonal minimum of 0.07 at both sites. Between April and June, both stands showed similar rates of transpiration peaking at 0.96 mm day(-1); thereafter, trees at the Y site showed increasing drought stress with canopy stomatal resistance increasing 6-fold by mid-August relative to values for trees at the O site. Over the same period, predawn water potential (psi(pd)) of trees at the Y site declined from -0.54 to -1.24 MPa, whereas psi(pd) of trees at the O site remained greater than -0.8 MPa throughout the season. Soil respiration at the O site showed a strong seasonal correlation with soil temperature with no discernible constraints imposed by declining soil water. In contrast, soil respiration at the Y site peaked before seasonal maximal soil temperatures and declined thereafter with declining soil water. No pronounced seasonal pattern in daytime NEE was observed at either site between April and September. At the Y site this behavior was driven by concurrent soil water limitations on soil respiration and assimilation, whereas there was no evidence of seasonal soil water limitations on either process at the O site.  相似文献   

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