Spatial and temporal variations in photosynthetic capacity of a temperate deciduous-evergreen forest

Key message

The understory evergreen trees showed maximal photosynthetic capacity in winter, while the overstory deciduous trees showed this capacity in spring. The time lag in productive ecophysiologically active periods between deciduous overstory and evergreen understory trees in a common temperate forest was clearly related to the amount of overstory foliage.

Abstract

In temperate forests, where deciduous canopy trees and evergreen understory trees coexist, understory trees experience great variation in incident radiation corresponding to canopy dynamics represented by leaf-fall and leaf-out. It is generally thought that changes in the light environment affect understory plants’ ecophysiological traits. Thus, to project and estimate annual energy, water, and carbon exchange between forests and the atmosphere, it is necessary to investigate seasonal variation in the ecophysiological activities of both evergreen trees in the understory and deciduous trees that make up the canopy/overstory. We conducted leaf-scale gas-exchange measurements and nitrogen content analyses for six tree species along their heights throughout a complete year. Photosynthetic capacity as represented by the maximum carboxylation rate (V _cmax25) and photosynthetic nitrogen use efficiency (PNUE) of deciduous canopy trees peaked immediately after leaf-out in late May, declined and stabilised during the mid-growing season, and drastically decreased just before leaf-fall. On the other hand, the timing of lowest V _cmax25 and PNUE for evergreen understory trees coincided with that of the highest values for canopy trees. Furthermore, understory trees’ highest values appeared just before canopy tree leaf-out, when incident radiation in the understory was highest. This implies that failing to consider seasonal variation in leaf ecophysiological traits for both canopy and understory trees could lead to serious errors in estimating ecosystem productivity and energy balance for temperate forests.

     

Roles of dominant understory Sasa bamboo in carbon and nitrogen dynamics following canopy tree removal in a cool‐temperate forest in northern Japan

To clarify the role of dense understory vegetation in the stand structure, and in carbon (C) and nitrogen (N) dynamics of forest ecosystems with various conditions of overstory trees, we: (i) quantified the above‐ and below‐ground biomasses of understory dwarf bamboo (Sasa senanensis) at the old canopy‐gap area and the closed‐canopy area and compared the stand‐level biomasses of S. senanensis with that of overstory trees; (ii) determined the N leaching, soil respiration rates, fine‐root dynamics, plant area index (PAI) of S. senanensis, and soil temperature and moisture at the tree‐cut patches (cut) and the intact closed‐canopy patches (control). The biomass of S. senanensis in the canopy‐gap area was twice that at the closed‐canopy area. It equated to 12% of total biomass above ground but 41% below ground in the stand. The concentrations of NO₃^? and NH₄⁺ in the soil solution and soil respiration rates did not significantly change between cut and control plots, indicating that gap creation did not affect the C or N dynamics in the soil. Root‐length density and PAI of S. senanensis were significantly greater at the cut plots, suggesting the promotion of S. senanensis growth following tree cutting. The levels of soil temperature and soil moisture were not changed following tree cutting. These results show that S. senanensis is a key component species in this cool‐temperate forest ecosystem and plays significant roles in mitigating the loss of N and C from the soil following tree cutting by increasing its leaf and root biomass and stabilizing the soil environment.     

A spatial model of forest dynamics

Effects of spatial processes on temperate deciduous forest structure and dynamics were investigated with a spatial simulator derived from a forest gap model. The multi-species neighborhood model accounted for competitive interactions and endogenous disturbance in the form of small canopy gaps. Simulated and actual spatial pattern of old-growth stands were compared. The 400 yr simulations produced a pattern scale (0.07–0.2 ha patches) similar to that of an actual stand; simulated pattern intensity was greater than actual intensity, however. Distances to nearest neighbor were somewhat similar for trees in the simulated and actual stands; yet the frequency distributions of distance to nearest neighbor values differed substantially. The simulated stand patterns were generally less random than the actual patterns. Spatial pattern changed markedly during the course of simulated succession. Pattern approached a random dispersion in early succession. Intensity peaked at mid-succession (ca. 150 yr) with a hyperdispersed overstory and a strongly clumped understory. Pattern intensity diminished in late succession as a mixed size structure developed. Old-growth patch size was greater than the neighborhood (or gap) size, suggesting the gap-sized areas do not behave independently.     

Vertical stratification of a temperate forest caterpillar community in eastern North America

Vertical niche partitioning might be one of the main driving forces explaining the high diversity of forest ecosystems. However, the forest’s vertical dimension has received limited investigation, especially in temperate forests. Thus, our knowledge about how communities are vertically structured remains limited for temperate forest ecosystems. In this study, we investigated the vertical structuring of an arboreal caterpillar community in a temperate deciduous forest of eastern North America. Within a 0.2-ha forest stand, all deciduous trees ≥ 5 cm diameter at breast height (DBH) were felled and systematically searched for caterpillars. Sampled caterpillars were assigned to a specific stratum (i.e. understory, midstory, or canopy) depending on their vertical position and classified into feeding guild as either exposed feeders or shelter builders (i.e. leaf rollers, leaf tiers, webbers). In total, 3892 caterpillars representing 215 species of butterflies and moths were collected and identified. While stratum had no effect on caterpillar density, feeding guild composition changed significantly with shelter-building caterpillars becoming the dominant guild in the canopy. Species richness and diversity were found to be highest in the understory and midstory and declined strongly in the canopy. Family and species composition changed significantly among the strata; understory and canopy showed the lowest similarity. Food web analyses further revealed an increasing network specialization towards the canopy, caused by an increase in specialization of the caterpillar community. In summary, our study revealed a pronounced stratification of a temperate forest caterpillar community, unveiling a distinctly different assemblage of caterpillars dwelling in the canopy stratum.     

Heterogeneous dispersal of tree litterfall corresponding with patchy canopy structure in a temperate mixed forest

Spatial dispersal patterns of fine litter were analyzed in relation to forest structure within a temperate mixed stand, in northeastern Japan. The canopy layer was a mosaic of two types of patches, those of shade-tolerant Abies firma (estimated age being 81–146 yrs.) and early successional deciduous broad-leaved trees (about 45 yrs. of age). Litter input to the forest floor was observed for a period of three years using 38 traps set at a height of 1.2 m. The average annual total of fine litter was 6.233 (dw.) metric tons hectare^-1, 85.7% of the total litterfall. Fine litter was mostly leaves from the overstory (about 64%). Litter distribution maps were drawn, using the Davis's grid method, of the total fine litter and its two main elements, namely A. firma (a 36.1% share) and deciduous broad-leaved species (36.7%). The mosaic distribution of heterogeneous patches of litter was seem to corresponded with distribution of the canopy structure.     

Importance of the understory stratum to entomofaunal diversity in a temperate deciduous forest

The vertical stratification of lepidopteran and coleopteran communities in a cool-temperate deciduous forest in Japan was examined to evaluate the hypothesis of an expected uniform distribution of mobile flying insects between the canopy and understory of temperate forests. Lepidopteran and coleopteran insects were trapped using light traps at three sites in each of the canopy and understory for three consecutive nights each month from April to October 2001. For Lepidoptera, species richness, abundance, and family richness were significantly higher in the understory than in the canopy. For Coleoptera, only abundance was larger in the canopy relative to the understory; species and family richness did not differ between the strata. The beta diversity of the lepidopteran community was larger between the strata than among sites, but the coleopteran community showed an inverse pattern. These results imply the presence of vertical stratification within the lepidopteran community, but not within the coleopteran community, in the temperate forest. The understory contributes more than the canopy to lepidopteran diversity in the temperate forest, although this stratification may be relatively weak because, in contrast to the situation in tropical forests, the canopy and understory assemblages share many species.     

Plant biodiversity in a larch plantation from the view point of photosynthetic nitrogen use efficiency in northeast China

Forest floor of larch species often provides growth habitat for many kinds of understory species because of relatively sparse structure in a larch canopy. A rich flora of forest understory species may play an essential role in maintaining fertility of a larch stand. An attempt was made to evaluate photosynthetic nitrogen use efficiency (PNUE) of many understory and overstory species according to their Raunkiaer lifeform. By studying 72 perennial deciduous species in a larch plantation in northeast China, marked photosynthetic differences between phanerophytes (Ph) and other three lifeforms of chamaephytes (Ch), hemicryptophytes (He), and cryptophytes (Cr) were found, with marginal differences found among Ch, He, and Cr. Ph species had much lower PNUE, and much lower values of rate of nitrogen allocation to chlorophyll (Chl./N) and nitrogen allocation to carboxylation processes (V _cmax/N) were concurrently observed in Ph compared with the other three lifeforms. Ph had much lower leaf nitrogen per unit of projection area (N _area) and specific leaf area (SLA, cm² g^–1). At lower SLA, for Ph species the change of PNUE with SLA was small, but these changes became very large at higher SLA for Ch, He, and Cr species. Our findings indicate that leaf morphological change is important for clarifying photosynthesis differences among species with different lifeform.     

Microsite variation in light availability and photosynthesis in a cool-temperate deciduous broadleaf forest in central Japan

An empirical light simulation model was applied to estimate stand scale photosynthesis in a deciduous broadleaved forest in central Japan. Based on diurnal courses of photosynthetically active photon flux density (PPFD), we characterized the components of incoming light within the forest canopy, and found that the instantaneous relative PPFD (PPFD under the canopy relative to that above the canopy) under diffuse light condition was a reliable estimator of the intensity and duration of PPFD. We calculated the daily photosynthesis (A_day) for each PPFD class using photosynthesis–light response curves. Model simulated A_day were corroborated with the estimates obtained from the nearby CO₂ flux tower. The result demonstrated the potential of the light simulation model. The light use efficiency of two dominant species, Betula ermanii as overstory and Sasa senanensis as understory species, were then evaluated. At the forest understory, PPFD under 50 mol m^–2 s^–1 contributed to 77% of the sunshine duration on a completely clear day. Therefore, a higher apparent quantum yield for S. senanensis enhanced the utilization of low PPFD for photosynthesis. On the other hand, at the upper forest canopies, B. ermanii with a higher light-saturated photosynthetic rate used high PPFD efficiently. Consequently, potential of daily net photosynthesis for both B. ermanii and S. senanensis was high under each light condition. Such interspecific difference in the patterns of light utilization was suggested as one of factors allowing coexistence of the two species in the study forest.     

Overstory Community Composition and Elevated Atmospheric CO2 and O3 Modify Understory Biomass Production and Nitrogen Acquisition

Elevated atmospheric CO₂ and O₃ have the potential to affect the primary productivity of the forest overstory, but little attention has been given to potential responses of understory vegetation. Our objective was to document the effects of elevated atmospheric CO₂ and O₃ on understory species composition and biomass and to quantify nitrogen (N) acquisition by the understory vegetation. The research took place at the aspen free-air CO₂ and O₃ enrichment (FACE) experiment, which has four treatments (control, elevated CO₂, elevated O₃, and elevated CO₂+O₃) and three tree communities: aspen, aspen/birch, and aspen/maple. In June 2003, each FACE ring was uniformly labeled with 15N applied as NH₄Cl. Understory biomass was harvested in June of 2004 for productivity, N, and 15N measurements, and photosynthetically active radiation (PAR) was measured below the canopy. The understory was divided into five species groups, which dominate in this young aggrading forest: Taraxacum officinale (dandelion), Solidago sp. (goldenrod), Trifolium repens and T. pretense (clover), various species from the Poaceae family (grass), and composited minor components (CMC). Understory species composition, total and individual species biomass, N content, and 15N recovery showed overstory community effects, but the direct effects of treatments was masked by the high variability of these data. Total understory biomass increased with increasing light, and thus was greatest under the open canopy of the aspen/maple community, as well as the more open canopy of the elevated O₃ treatments. Species were different from one another in terms of 15N recovery, with virtually no 15N recovered in clover and the greatest amount recovered in dandelion. Thus, understory species composition and biomass appear to be driven by the structure of the overstory community, which is determined by the tree species present and their response to the treatments. However, N acquisition by the understory does not appear to be affected by either the overstory community or the treatments at this point.     

海南三亚滨海雨林、半落叶季雨林到落叶季雨林的植物群落多样性及种间联结研究

为探索三亚沿海地区(自东向西)从滨海雨林过渡到半落叶季雨林、落叶季雨林的物种多样性及种间关系的变化趋势.该文采用植物群落学调查方法对三种植被类型进行群落调查,进一步使用物种丰富度指数(dGL)、Shannon-Wiener指数(H)、Simpson指数(P)和Pielou均匀度指数(E)来表征各植被类型的物种多样性水平...     

伏牛山自然保护区森林冠层结构对林下植被特征的影响

在伏牛山自然保护区典型地段设立样方,测定了森林生态系统内几种典型群落类型的冠层结构、光环境特征,调查了林下植被的特征,分析了它们之间的相互关系.结果显示:各群落的冠层结构和光环境有一定的差异,单因素方差分析表明,部分群落间的差异性达到显著水平;各群落灌木层物种丰富度、多样性和均匀度均高于草本层,而优势度正相反;线性拟合的结果表明,草本层的物种丰富度、多样性与冠下光合量子通量密度间呈极显著负相关,优势度与冠下光合量子通量密度间呈显著正相关,灌木层各参数与冠层结构特征间相关性不显著.研究表明,冠层结构的变化对草本层(包括更新幼苗)的影响显著高于灌木层.林隙/林窗或林中空地的出现可能对草本物种或其他阳性及先锋物种具有促进作用,而对优势种幼苗的萌发和定植产生负效应.推测在典型的落叶阔叶林生态系统演替进程中,林下光照强度可能不是最主要的限制因素,优势种种子的扩散、萌发和定植限制可能更重要.     

Effects of fire on regional evapotranspiration in the central Canadian boreal forest

Changes in fire regimes are driving the carbon balance of much of the North American boreal forest, but few studies have examined fire‐driven changes in evapotranspiration (ET) at a regional scale. This study used a version of the Biome‐BGC process model with dynamic and competing vegetation types, and explicit spatial representation of a large (10⁶ km²) region, to simulate the effects of wildfire on ET and its components from 1948 to 2005 by comparing the fire dynamics of the 1948–1967 period with those of 1968–2005. Simulated ET averaged, over the entire temporal and spatial modeling domain, 323 mm yr⁻¹; simulation results indicated that changes in fire in recent decades decreased regional ET by 1.4% over the entire simulation, and by 3.9% in the last 10 years (1996–2005). Conifers dominated the transpiration (E_C) flux (120 mm yr⁻¹) but decreased by 18% relative to deciduous broadleaf trees in the last part of the 20th century, when increased fire resulted in increased soil evaporation, lower canopy evaporation, lower E_C, and a younger and more deciduous forest. Well‐ and poorly drained areas had similar rates of evaporation from the canopy and soil, but E_C was twice as high in the well‐drained areas. Mosses comprised a significant part of the evaporative flux to the atmosphere (22 mm yr⁻¹). Modeled annual ET was correlated with net primary production, but not with temperature or precipitation; ET and its components were consistent with previous field and modeling studies. Wildfire is driving significant changes in hydrological processes by affecting mean stand age, forest species, and energy balance. These changes, particularly in poorly drained areas, may control the future carbon balance of the boreal forest.     

The effect of dense understory dwarf bamboo (Sasa senanensis) on soil respiration before and after clearcutting of cool temperate deciduous broad-leaved forest

To evaluate the effect of understory dwarf bamboo (Sasa senanensis) on soil respiration in forest ecosystems, we compared soil respiration rates between four deciduous broad-leaved forest sites representing two levels of understory Sasa (with and without) and two levels of forest stand age (50-year-old stand and 1-year-old stand after clearcut). The understory Sasa enhances the soil respiration rate both before and after the clearcutting of deciduous broad-leaved forest. The Sasa sites had larger total belowground biomass compared with the non-Sasa sites, which could be attributed to Sasa presence. Our results also suggest that clearcutting decreases temperature-normalized soil respiration rates (R ₁₅) and temperature sensitivity (Q ₁₀) in both Sasa and non-Sasa ecosystems. Clearcutting significantly reduced the fine root biomass of trees and Sasa. The fine roots of trees and Sasa had high specific respiration rates compared with larger roots and rhizomes at Sasa and non-Sasa sites, respectively. Therefore, we hypothesize that the loss of fine roots after clearcutting is responsible for the reduction in soil respiration rate. A comparison with other studies revealed a positive linear relationship between total (tree and Sasa) fine root biomass and R ₁₅, suggesting that fine root biomass controls soil respiration at the landscape scale. The Q ₁₀ value is also likely to be related to fine root biomass, although the relationship was not significant. We conclude that understory Sasa increases belowground biomass, especially fine roots, and the spatial variation in soil respiration at the landscape scale.     

Transpiration of trees and forest stands: short and long-term monitoring using sapflow methods

We show that sapflow is a useful tool for studies of water fluxes in forest ecosystems, because (i) it gives access to the spatial variability within a forest stand, (ii) it can be used even on steep slopes, and (iii) when combined with eddy correlation measurements over forests, it allows separation of individual tree transpiration from the total water loss of the stand. Moreover, sapflow techniques are quite easy to implement. Four sapflow techniques currently coexist, all based on heat diffusion in the xylem. We found a good agreement between three of these techniques. Most results presented here were obtained using the radial flow meter (Granier 1985). Tree sapflow is computed as sap flux density times sapwood area. To scale up from trees to a stand, measurements have to be made on a representative sample of trees. Thus, a number of trees in each circumference class is selected according to the fraction of sapwood they represent in the total sapwood area of the stand. The variability of sap flux density among trees is usually low (CV. 10–15%) in close stands of temperate coniferous or deciduous forests, but is much higher (35–50%) in a tropical rain forest. It also increases after thinning or during a dry spell. A set of 5–10 sapflow sensors usually provides an accurate estimate of stand transpiration. Transpiration measured on two dense spruce stands in the Vosges mountains (France) and one Scot's pine plantation in the Rhine valley (Germany) showed that maximum rate was related to stand LAI and to local climate. Preliminary results comparing the sapflow of a stand of Pinus banksiana to the transpiration of large branches, as part of the BOREAS programme in Saskachewan, Canada showed a similar trend. For modelling purposes, tree canopy conductance (g_c) was calculated from Penman-Monteith equation. In most experiments, calculated canopy conductance was dependent on global radiation (positive effect) and on vapour pressure deficit (negative effect) in the absence of other limiting factors. A comparison of the vapour pressure deficit response curves of g_c for several tree species and sites showed only small differences among spruce, oak and pine forests when including understorey. Tropical rainforests exhibited a similar behaviour.     

Influence of stand structure on carbon-13 of vegetation, soils, and canopy air within deciduous and evergreen forests in Utah, United States

Carbon isotope ratios (δ¹³C) were studied in evergreen and deciduous forest ecosystems in semi-arid Utah (Pinus contorta, Populus tremuloides, Acer negundo and Acer grandidentatum). Measurements were taken in four to five stands of each forest ecosystem differing in overstory leaf area index (LAI) during two consecutive growing seasons. The δ¹³C_leaf (and carbon isotope discrimination) of understory vegetation in the evergreen stands (LAI 1.5–2.2) did not differ among canopies with increasing LAI, whereas understory in the deciduous stands (LAI 1.5–4.5) exhibited strongly decreasing δ¹³C_leaf values (increasing carbon isotope discrimination) with increasing LAI. The δ¹³C values of needles and leaves at the top of the canopy were relatively constant over the entire LAI range, indicating no change in intrinsic water-use efficiency with overstory LAI. In all canopies, δ¹³C_leaf decreased with decreasing height above the forest floor, primarily due to physiological changes affecting c _i/c _a (> 60%) and to a minor extent due to δ¹³C of canopy air (< 40%). This intra-canopy depletion of δ¹³C_leaf was lowest in the open stand (1‰) and greatest in the denser stands (4.5‰). Although overstory δ¹³C_leaf did not change with canopy LAI, δ¹³C of soil organic carbon increased with increasing LAI in Pinus contorta and Populus tremuloides ecosystems. In addition, δ¹³C of decomposing organic carbon became increasingly enriched over time (by 1.7–2.9‰) for all deciduous and evergreen dry temperate forests. The δ¹³C_canopy of CO₂ in canopy air varied temporally and spatially in all forest stands. Vertical canopy gradients of δ¹³C_canopy, and [CO₂]_canopy were larger in the deciduous Populus tremuloides than in the evergreen Pinu contorta stands of similar LAI. In a very wet and cool year, ecosystem discrimination (Δ_e) was similar for both deciduous Populus tremulodies (18.0 ± 0.7‰) and evergreen Pinus contorta (18.3 ± 0.9‰) stands. Gradients of δ¹³C_canopy and [CO₂]_canopy were larger in denser Acer spp. stands than those in the open stand. However, ¹³C enrichment above and photosynthetic draw-down of [CO₂]_canopy below tropospheric baseline values were larger in the open than in the dense stands, due to the presence of a vigorous understory vegetation. Seasonal patterns of the relationship δ¹³C_canopy versus 1/[CO₂]_canopy were strongly influenced by precipitation and air temperature during the growing season. Estimates of Δ_e for Acer spp. did not show a significant effect of stand structure, and averaged 16.8 ± 0.5‰ in 1933 and 17.4 ± 0.7‰ in 1994. However, Δ_e varied seasonally with small fluctuations for the open stand (2‰), but more pronounced changes for the dense stand (5‰). Received: 15 April 1996 / Accepted: 19 October 1996     

Up-scaling to stand transpiration of an Asian temperate mixed-deciduous forest from single tree sapflow measurements

Species diversity in mixed forest stands is one of the factors that complicate up-scaling of transpiration from individual trees to stand level, since tree species are architecturally and functionally different. In this study, thermal dissipation probes were used to measure sap flow in five different tree species in a mixed-deciduous mountain forest in South Korea. Easily measurable tree characteristics that could serve to define individual tree water use among the different species were employed to scale up transpiration from single trees to stand level. Tree water use (TWU) was derived from sap flux density (SFD) and sapwood area (SA). Canopy transpiration E was scaled from TWU while canopy conductance (g _c) was computed from E and VPD. SFD, TWU and g _c were correlated with tree diameter at breast height (DBH) for all the five measured species (SFD: R ² = 0.21, P = 0.036; TWU: R ² = 0.83, P < 0.001; g _c: R ² = 0.63, P < 0.001). Maximum stand transpiration (E) during June, before the onset of the Asian monsoon rains, was estimated at 0.97 ± 0.12 mm per day. There was a good (R ² = 0.94, P < 0.0001) agreement between measured and estimated E using the relationship between TWU and DBH. Our study shows that using functional models that employ converging traits among species could help in estimating water use in mixed forest stands. Compared to SA, DBH is a better scalar for water use of mixed forest stands since it is non-destructive and easily obtainable.     

Spider assemblages in the overstory, understory, and ground layers of managed stands in the western boreal mixedwood forest of Canada

Logging is the main human disturbance in the boreal forest; thus, understanding the effects of harvesting practices on biodiversity is essential for a more sustainable forestry. To assess changes in spider composition because of harvesting, samples were collected from three forest layers (overstory, understory, and ground) of deciduous and conifer dominated stands in the northwestern Canadian boreal mixedwood forest. Spider assemblages and feeding guild composition were compared between uncut controls and stands harvested to 20% retention. In total, 143 spider species were collected, 74 from the ground, 60 from the understory, and 71 from the overstory, and species composition of these three pools differed considerably among layers. Distinctive spider assemblages were collected from the canopy of each forest cover type but these were only slightly affected by harvesting. However, logging had a greater impact on the species composition in the understory and ground layers when compared with unharvested controls. Guild structure differed among layers, with wandering and sheet-weaving spiders dominant on the ground while orb-weaving and ambush spiders were better represented in the understory and overstory, respectively. Given the ecological importance of spiders and the expectation of faunal changes with increased harvesting, further efforts toward the understanding of species composition in higher strata of the boreal forest are needed.     

Sensitivity of canopy transpiration to altered precipitation in an upland oak forest: evidence from a long-term field manipulation study

Climate‐induced changes in regional precipitation could have important implications for the carbon, water, and nutrient cycles of forest ecosystems. However, few studies have examined the response of deciduous forests to increases or decreases in precipitation. Therefore, the throughfall displacement experiment (TDE) was established in 1993 near Oak Ridge, Tennessee to examine the sensitivity of an upland oak (Quercus spp.) forest to ambient, wet (+33%), and dry (?33%) precipitation regimes. Sap flux measurements on co‐occurring tree species were scaled using species‐specific estimates of stand sapwood area to derive daily and seasonal rates of canopy transpiration (E_C) from 2000 to 2003. With the exception of 2003, which was an extremely wet year, daily E_C in the dry plot, and occasionally during extended droughts in the ambient and wet plots, declined as water potential in the upper 0.35 m soil profile approached ‐3.0 MPa. Seasonal patterns of soil water potential and treatment‐specific differences in E_C were dependent on precipitation frequency and intensity. Supplemental precipitation added to the wet plot increased seasonal E_C on average by 9% (range ?1% to 19%), whereas extended periods of drought on the dry plot in 2000, 2001, and 2002 were sufficient to reduce seasonal E_C by 26–30% compared with the ambient plot. There was a strong correlation between seasonal E_C and the water stress integral, a cumulative index of drought severity and duration. A polynomial fitted to these data indicated that reductions in seasonal E_C on the order of 40% were possible given TDE‐imposed reductions in soil water potential. Application of this equation to all years of the TDE (1994–2003) revealed considerable interannual and treatment‐specific variation in canopy transpiration. In general, a 33% removal of throughfall on the dry plot during 1995, 1998, and 2002 resulted in a 23–32% reduction in seasonal E_C compared with the ambient plot. While droughts in deciduous forests are often limited in duration and tend to occur late in the growing season, soil water deficits of the magnitude observed in this study have the potential to impact local and regional forest water budgets.