Dynamic response of stomata to changing irradiance in loblolly pine (Pinus taeda L.)

Dynamic changes in stomatal conductance and the rate of photosynthesis were measured as periods of shading (decrease in irradiance from 800 to 200 micro mol m(-2) s(-1)) between 5 and 60 min were imposed on needles of Pinus taeda L. trees under laboratory conditions. Shading induced a 39% decrease in stomatal conductance but the rate of change was slow. Average time constants (+/- standard error) were shorter for the decrease in stomatal conductance when shading was imposed for 30 min (14.8 +/- 1.3 min) than for the increase in stomatal conductance when shading was removed (25.5 +/- 3.4 min). The time constants for increasing stomatal conductance when shading was removed were linearly related to the length of the previous dark period. The rate of photosynthesis fell immediately by 58% when shading was imposed and increased more rapidly than the change in stomatal conductance when shading was removed. The increase in photosynthesis during the induction phase after shading was removed was limited by both stomatal and biochemical effects. The long time constants for stomatal response contributed to the poor correlations between stomatal conductance and instantaneous measurements of irradiance from field data. However, the slow response of stomatal conductance to changes in irradiance had little effect on total daily transpiration, carbon gain and water-use efficiency.     

Mean canopy stomatal conductance responses to water and nutrient availabilities in Picea abies and Pinus taeda

We compared sap-flux-scaled, mean, canopy stomatal conductance (GS) between Picea abies (L.) Karst. in Sweden and Pinus taeda (L.) in North Carolina, both growing on nutritionally poor soils. Stomatal conductance of Picea abies was approximately half that of Pinus taeda and the sensitivity of GS in Picea abies to vapor pressure deficit (D) was lower than in Pinus taeda. Optimal fertilization increased leaf area index (L) two- and threefold in Pinus taeda and Picea abies, respectively, regardless of whether irrigation was increased. Although it increased L, fertilization did not increase GS in Picea abies unless irrigation was also provided. In Pinus taeda growing on coarse, sandy soils, the doubling of L in response to fertilization reduced GS sharply unless irrigation was also provided. The reduction in GS with fertilization in the absence of irrigation resulted from the production of fine roots with low saturated hydraulic conductivity. When Pinus taeda received both fertilization and irrigation, the increase in L was accompanied by a large increase in GS. In Pinus taeda, a reference GS (defined as GS at D = 1 kPa; GSR) decreased in all treatments with decreasing volumetric soil water content (theta). In Picea abies, theta varied little within a treatment, but overall, GSR declined with theta, reaching lowest values when drought was imposed by the interception of precipitation. Despite the large difference in GS both between Picea abies and Pinus taeda and among treatments, stem growth was related to absorbed radiation, and stem growth response to treatment reflected mostly the changes in L.     

Canopy conductance of Pinus taeda,Liquidambar styraciflua and Quercus phellos under varying atmospheric and soil water conditions

Sap flow, and atmospheric and soil water data were collected in closed-top chambers under conditions of high soil water potential for saplings of Liquidambar styraciflua L., Quercus phellos L. and Pinus taeda L., three co-occurring species in the southeastern USA. Responses of canopy stomatal conductance (g(t)) to water stress induced by high atmospheric water vapor demand or transpiration rate were evaluated at two temporal scales. On a diurnal scale, the ratio of canopy stomatal conductance to maximum conductance (g(t)/g(t,max)) was related to vapor pressure deficit (D), and transpiration rate per unit leaf area (E(l)). High D or E(l) caused large reductions in g(t)/g(t,max) in L. styraciflua and P. taeda. The response of g(t)/g(t,max) to E(l) was light dependent in L. styraciflua, with higher g(t)/g(t,max) on sunny days than on cloudy days. In both L. styraciflua and Q. phellos, g(t)/g(t,max) decreased linearly with increasing D (indicative of a feed-forward mechanism of stomatal control), whereas g(t)/g(t,max) of P. taeda declined linearly with increasing E(l) (indicative of a feedback mechanism of stomatal control). Longer-term responses to depletion of soil water were observed as reductions in mean midday g(t)/g(t,max), but the reductions did not differ significantly between species. Thus, species that employ contrasting methods of stomatal control may show similar responses to soil water depletion in the long term.     

Responses of foliar photosynthetic electron transport, pigment stoichiometry, and stomatal conductance to interacting environmental factors in a mixed species forest canopy

We studied limitations caused by variations in leaf temperature and soil water availability on photosynthetic electron transport rates calculated from foliar chlorophyll fluorescence analysis (U) in a natural deciduous forest canopy composed of shade-intolerant Populus tremula L. and shade-tolerant Tilia cordata Mill. In both species, there was a positive linear relationship between light-saturated U (Umax) per unit leaf area and mean seasonal integrated daily quantum flux density (Ss, mol per square m per day). Acclimation of leaf dry mass per area and nitrogen per area to growth irradiance largely accounted for this positive scaling. However, the slopes of the Umax versus Ss relationships were greater on days when leaf temperature was high than on days when leaf temperature was low. Overall, Umax varied 2.5-fold across a temperature range of 20-30 degrees C. Maximum stomatal conductance (Gmax) also scaled positively with Ss. Although Gmax observed during daily time courses, and stomatal conductances during Umax measurements declined in response to seasonally decreasing soil water contents, was insensitive to prolonged water stress, and was not strongly correlated with stomatal conductances during its estimation. These results suggest that photorespiration was an important electron sink when intercellular CO2 concentration was low because of closed stomata. Given that xanthophyll cycle pool size (VAZ, sum of violaxanthin, antheraxanthin, and zeaxanthin) may play an important role in dissipation of excess excitation energy, the response of VAZ to fluctuating light and temperature provided another possible explanation for the stable Umax. Xanthophyll cycle carotenoids per total leaf chlorophyll (VAZ/Chl) scaled positively with integrated light and negatively with daily minimum air temperature, whereas the correlation between VAZ/Chl and irradiance was best with integrated light averaged over 3 days preceding foliar sampling. We conclude that the potential capacity for electron transport is determined by long-term acclimation of U to certain canopy light conditions, and that the rapid adjustment of the capacity for excitation energy dissipation plays a significant part in the stabilization of this potential capacity. Sustained high capacity of photosynthetic electron transport during stress periods provides an explanation for the instantaneous response of U to short-term weather fluctuations, but also indicates that U restricts potential carbon gain under conditions of water limitation less than does stomatal conductance.     

Effect of elevated ambient CO2 concentration on water use efficiency of Pinus sylvestriformis

IntroductionEvidencefrommanystudiesshowsthattheconcentrationofatmosphericCO,isdramaticallyraisingandadoubIingofthepresentCO,concentrationwouIdoccurduringthelasthaIfofthenextcentury.Therisingofatmospheric.CO,concentrationisstrongIycorrelatedwiththeincreaseinglobaIconsumptionoffossiIfueIsandaIsoaffectedsignificantIybytheclearingofforest(Bazzaz199o).PfantsgrowingatvariousambientconcentrationofCo,showgreatdifferences,includingtheprocessesofgrowthandphysiology.Photosynthesisandtran-spiration…     

Analysis of gas exchange of Merkus pine populations by the optimality approach

We hypothesized that northern and northeastern Thai populations of Merkus pine (Pinus merkusii Jungh. et de Vriese), which differ in the duration of their grass stage and which originate from slightly different climates in terms of water availability, differ in their gas exchange characteristics. We compared CO(2) exchange response to irradiance, diurnal regulation of leaf conductance within a 10-day period and structural properties among the populations. We used a model, which is based on the concept of optimal stomatal regulation, to analyze CO(2) exchange and transpiration rates. The two geographical groups did not differ in CO(2) exchange response to irradiance, diurnal transpiration, or water use. Mean stomatal length was significantly greater in the northeastern population than in the northern populations, but stomatal frequency did not differ among populations. First-year shoot growth and dry matter production, allocation of nitrogen to needles and root:shoot ratios were similar in the two geographical groups. Genotypic variation in the duration of the grass stage was not reflected in variation in gas exchange, indicating that the grass stage is an adaptation to more site-specific conditions. The modeled response of CO(2) exchange rates to irradiance fitted well the rates measured under laboratory conditions. The transpiration model, which utilized maximum leaf conductance and other parameters derived from the CO(2) exchange measurements, also fitted well the transpiration rates measured in a greenhouse under changing environmental conditions.     

Effect of elevated ambient CO2 concentration on water use efficiency ofPinus sylvestriformis

Pinus sylvestriformis is an important species as an indicator of global climate changes in Changbai Mountain, China. The water use efficiency (WUE) of this species (11-year old) was studied on response to elevated CO₂ concentration at 500±100 μL·L⁻¹ by directly injecting CO₂ into the canopy under natural condition in 1998–1999. The results showed that the elevated CO₂ concentration reduced averagely stomatal opening, stomatal conductance and stomatal density to 78%, 80% and 87% respectively, as compared to normal ambient. The elevated CO₂ reduced the transpiration and enhances the water use efficiency (WUE) of plant. The project was supported by Chinese Academy of Sciences Responsible editor: Chai Ruihai     

Analyses of assumptions and errors in the calculation of stomatal conductance from sap flux measurements

We analyzed assumptions and measurement errors in estimating canopy transpiration (E(L)) from sap flux (J(S)) measured with Granier-type sensors, and in calculating canopy stomatal conductance (G(S)) from E(L) and vapor pressure deficit (D). The study was performed in 12-year-old Pinus taeda L. stands with a wide range in leaf area index (L) and growth rate. No systematic differences in J(S) were found between the north and south sides of trees. However, J(S) in xylem between 20 and 40 mm from the cambium was 50 and 39% of J(S) in the outer 20-mm band of xylem in slow- and fast-growing trees, respectively. Sap flux measured in stems did not lag J(S) measured in branches, and time and frequency domain analyses of time series indicated that variability in J(S) in stems and branches is mostly explained by variation in D. Therefore, J(S) was used to estimate transpiration, after accounting for radial patterns. There was no difference between D and leaf-to-air vapor pressure gradient, and D did not have a vertical profile in stands of either low or high L suggesting a strong canopy-atmosphere coupling. Therefore, D estimated at one point in the canopy can be used to calculate G(S) in such stands. Given the uncertainties in J(S), relative humidity, and temperature measurements, to keep errors in G(S) estimates to less than 10%, estimates of G(S) should be limited to conditions in which D >/= 0.6 kPa.     

Seasonal CO(2) assimilation and stomatal limitations in a Pinus taeda canopy

Net CO(2) assimilation (A(net)) of canopy leaves is the principal process governing carbon storage from the atmosphere in forests, but it has rarely been measured over multiple seasons and multiple years. I measured midday A(net) in the upper canopy of maturing loblolly pine (Pinus taeda L.) trees in the piedmont region of the southeastern USA on 146 sunny days over 36 months. Concurrent data for leaf conductance and photosynthetic CO(2) response curves (A(net)-C(i) curves) were used to estimate the relative importance of stomatal limitations to CO(2) assimilation in the field. In fully expanded current-year and 1-year-old needles, midday light-saturated A(net) was constant over much of the growing season (5-6 &mgr;mol CO(2) m(-2) s(-1)), except during drought periods. During the winter season (November-March), midday A(net) of overwintering needles varied in proportion to leaf temperature. Net CO(2) assimilation at light saturation occurred when daytime air temperatures exceeded 5-6 degrees C, as happened on more than 90% of the sunny winter days. In both age classes of foliage, winter carbon assimilation accounted for approximately 15% of the daily carbon assimilation on sunny days throughout the year, and was relatively insensitive to year-to-year differences in temperature during this season. However, strong stomatal limitations to A(net) occurred as a result of water stress associated with freezing cycles in winter. During the growing season, drought-induced water stress produced the largest year-to-year differences in seasonal CO(2) assimilation on sunny days. Seasonal A(net) was more drought sensitive in current-year needles than in 1-year-old needles. Relative stomatal limitations to daily integrated A(net) were approximately 40% over the growing season, and summer drought rather than high temperatures had the largest impact on summer A(net) and integrated annual CO(2) uptake in the upper crown. Despite significant stomatal limitations, a long duration of near-peak A(net) in the upper crown, particularly in 1-year-old needles, conferred high seasonal and annual carbon gain.     

Measuring and modeling conductances of black spruce at three organizational scales: shoot,branch and canopy

To investigate the extent to which the energy balance of a globally important ecosystem is controlled by biological and environmental processes, measurements of water vapor flux were made on individual black spruce (Picea mariana [Mill.] B.S.P.) shoots, branches, and a whole canopy at the BOREAS Southern Study Area Old Black Spruce (SSA OBS) site. These measurements were used to estimate stomatal, branch boundary layer and canopy boundary layer conductances to water vapor. On a projected needle area basis, stomatal conductances varied between 14 and 92 mmol m(-2) s(-1), and total branch conductance varied seasonally between zero and about 35 mmol m(-2) s(-1). On a ground area basis, total canopy conductance varied between 24 and 105 mmol m(-2) s(-1). Total canopy conductance was partitioned into aerodynamic and physiological components by using shoot-scale measurements scaled by leaf area index. Good agreement was found with an independent estimate of aerodynamic conductance measured when the canopy was wet. Compared with most coniferous forests, the canopy was relatively uncoupled from the atmosphere, and at the ecosystem scale, the control of water vapor flux was approximately equally divided between physiological and abiotic conductances. Two widely used steady-state models of stomatal conductance were parameterized from the shoot and branch measurements. Parameters varied considerably throughout the growing season. A time-constant term was added to these static models to construct dynamic models of stomatal conductance under naturally varying environmental conditions. The dynamic versions of the models outperformed the static versions in explaining stomatal response to rapidly changing environmental conditions. The length of the time-constant term, derived using the dynamic models, suggested that stomata were slow to respond to changing environmental conditions, and that the speed of the response was strongly temperature-dependent.     

Effects of foliar nitrogen concentration on photosynthesis and water use efficiency in Douglas-fir

Leaf-level physiological processes were studied in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) to determine whether apparent increases in stand-level water use efficiency (WUE) observed in response to nitrogen (N) fertilization were attributable to foliar N effects on carbon fixation rates or on stomatal control of water loss. Photosynthesis and transpiration were measured at different light intensities and ambient CO(2) molar fractions and comparisons were made between current-year shoots with average foliar N concentrations of 1.58% (High-N) and 1.25% (Low-N). Photosynthetic rates and foliar N concentrations were positively correlated. In response to light, photosynthesis and stomatal conductance were closely coupled and a similar coupling was observed in response to different ambient CO(2) concentrations. Partitioning the photosynthetic responses into mesophyll and stomatal components indicated that foliar N altered mesophyll conductance but not stomatal control of water loss. High-N shoots had significantly greater rates of photosynthesis and transpiration than Low-N shoots and, as a result, instantaneous WUE did not differ significantly between High-N and Low-N shoots.     

Vertical gradients in photosynthetic gas exchange characteristics and refixation of respired CO(2) within boreal forest canopies

We compared vertical gradients in leaf gas exchange, CO(2) concentrations, and refixation of respired CO(2) in stands of Populus tremuloides Michx., Pinus banksiana Lamb. and Picea mariana (Mill.) B.S.P. at the northern and southern boundaries of the central Canadian boreal forest. Midsummer gas exchange rates in Populus tremuloides were over twice those of the two conifer species, and Pinus banksiana rates were greater than Picea mariana rates. Gas exchange differences among the species were attributed to variation in leaf nitrogen concentration. Despite these differences, ratios of intercellular CO(2) to ambient CO(2) (c(i)/c(a)) were similar among species, indicating a common balance between photosynthesis and stomatal conductance in boreal trees. At night, CO(2) concentrations were high and vertically stratified within the canopy, with maximum concentrations near the soil surface. Daytime CO(2) gradients were reduced and concentrations throughout the canopy were similar to the CO(2) concentration in the well-mixed atmosphere above the canopy space. Photosynthesis had a diurnal pattern opposite to the CO(2) profile, with the highest rates of photosynthesis occurring when CO(2) concentrations and gradients were lowest. After accounting for this diurnal interaction, we determined that photosynthesizing leaves in the understory experienced greater daily CO(2) concentrations than leaves at the top of the canopy. These elevated CO(2) concentrations were the result of plant and soil respiration. We estimated that understory leaves in the Picea mariana and Pinus banksiana stands gained approximately 5 to 6% of their carbon from respired CO(2).     

Plant size, not age, regulates growth and gas exchange in grafted Scots pine trees

We studied the effect of scion donor-tree age on the physiology and growth of 6- to 7-year-old grafted Scots pine (Pinus sylvestris L.) trees (4 and 5 years after grafting). Physiological measurements included photosynthethetic rate, stomatal conductance, transpiration, whole plant hydraulic conductance, needle nitrogen concentration and carbon isotope composition. Growth measurements included total and component biomasses, relative growth rates and growth efficiency. Scion donor trees ranged in age from 36 to 269 years at the time of grafting. Hydraulic conductance was measured gravimetrically, applying the Ohm's law analogy, and directly, with a high-pressure flow meter. We found no effect of scion donor-tree age on any of the variables measured. There was, however, great variation within scion donor-tree age groups, which was related to the size of the grafted trees. Differences in size may have been caused by variable initial grafting success, but there was no indication that grafting success and age were related. At the stem level, hydraulic conductance scaled with total leaf area so that total conductance per unit leaf area did not vary with crown size. However, leaf specific hydraulic conductance (gravimetric), transpiration, photosynthesis and stomatal conductance declined with increasing total tree leaf area and needle width. We hypothesize that needle width is inversely related to mesophyll conductance. We conclude that canopy and needle size and not scion donor-tree age determined gas exchange in our grafted trees.     

土壤水分胁迫对树木N2O排放速率的影响

采用封闭罩-气相色谱法观测研究了干旱胁迫对长白山阔叶红松林的几种优势树种-红松(Pinus koraiensis)、水曲柳(Fraxinus mandshurica)、胡桃楸(Juglans mandshurica)、椴树(Tilia amurensis)和蒙古栎(Quercus mongolica)叶片N2O排放。并同步测定5种树木叶片净光合速率、呼吸速率和气孔导度。结果表明：土壤水分胁迫明显降低树木叶片气孔导度、净光合速率和N2O排放速率，叶片气孔是树木N2O排放的主要通道。树木N2O排放以白天为主，在相同的水分条件下，不同的苗木有不同的N2O排放速率，同种苗木的N2O排放随干旱胁迫的加重而减少，在受到不同干旱胁迫时，针叶树红松N2O的排放速率降至正常水分条件下的34.43％和100．6％、阔叶树种N2O排放平均降至31．93％和86.35％。不同干旱胁迫的红松、水曲柳、胡逃楸、椴树和蒙古栎幼树叶片N2O排放速率为34．43、14．44、33．02、16.48和32．33ngN2O．g^-1DW．h^-1。图1表1参12。     

An ecophysiological evaluation of the suitability of Eucalyptus grandis for planting in the tropics

Water use by eucalypts has received a lot of attention in tropical countries during the past decade because of the large-scale introduction of these trees for afforestation. Eucalyptus grandis, widely used as a plantation item in tropical southern India, is the subject of a detailed ecophysiological study in this paper. A 4-year-old coppiced plantation was used for measurements. Microclimate data collected above the canopy were used along with stomatal conductance measurements to estimate the transpirational water loss by the Penman-Monteith equation assuming a two-layer canopy model. Leaf photosynthesis was measured diurnally and seasonally to understand the limitations in photosynthesis in the field. Results show that the water loss from the plantation ranges between 2.5 and 6.5 mm day⁻¹ depending on the season. When suitably extrapolated, this amounts to 1181 mm annually in the study location, where annual rainfall averages 1302 mm. The stomatal conductance measurements showed that the increase in atmospheric vapour pressure deficit induced stomatal closure. This was probably regulated by the leaf water potentials also. Based on the above results it is concluded that E. grandis need not be a high water consumer because of its good stomatal control of transpirational water loss, especially during the dry season when the atmospheric vapour pressure deficit is high. The photosynthesis measurements led to a conclusion that the dry period experienced in the study location does not seriously affect the photosynthetic rate of the trees on a leaf unit area basis.     

Response of Somatal Characteristics and its Plasticity to Different Light Intensities in Leaves of Seven Tropical Woody Seedlings

1 INTRODUCTIONAs an important structure in the leaves of terrestrialplants, stomata play a key role in the gas exchangebetween plants and atmosphere, and provide the mainchannels of water dissipation for plants. They also playa significant part in the regulation of photosynthesisand evaporation process. Researches have long beenconducted on stomata, and one of the most importantfields in plant physiological and ecological research isconcerned with the physiological and ecologicalcharacteri…     

Stomatal sensitivity to vapor pressure deficit and its relationship to hydraulic conductance in Pinus palustris

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.     

Interspecific variation in leaf water use associated with drought tolerance in four emergent dipterocarp species of a tropical rain forest in Borneo

By use of tree-tower and canopy-crane systems we studied variations in the water use, including transpiration, stomatal conductance, and leaf water potential, of the uppermost sun-exposed canopy leaves of four emergent dipterocarp species in an aseasonal tropical rain forest in Sarawak, Malaysia. Midday depression in stomatal conductance and leaf water potential was observed in all the species studied. Interspecific differences were clearly observed in the maxima of transpiration rates and stomatal conductance and the minima of leaf water potential among the four dipterocarp species. These interspecific variations were closely related to wood density and to factors affecting ecological patterns of distribution. Specifically, Shorea parvifolia and S. smithiana, both of which have a relatively low wood density for Dipterocarpaceae and are found on clay-rich soil, had a high transpiration rate in the daytime but had a large midday depression and a low leaf water potential. In contrast, Dryobalanops aromatica, which has a high wood density and is found in sandy soil areas, consumed less water even during the daytime. Dipterocarpus pachyphyllus, which has a high wood density and is found on clay-rich soil, stood intermediate between Shorea and D. aromatica in leaf water use. The two Shorea species had higher mortality than the others during the severe drought associated with El Ni?o in 1998, so daily pattern of leaf water use in each dipterocarp species might be correlated with its susceptibility to unusual drought events.     

Responses of foliar gas exchange to long-term elevated CO(2) concentrations in mature loblolly pine trees

Branches of field-grown mature loblolly pine (Pinus taeda L.) trees were exposed for 2 years (1992 and 1993) to ambient or elevated CO(2) concentrations (ambient + 165 micro mol mol(-1) or ambient + 330 micro mol mol(-1) CO(2)). Exposure to elevated CO(2) concentrations enhanced rates of net photosynthesis (P(n)) by 53-111% compared to P(n) of foliage exposed to ambient CO(2). At the same CO(2) measurement concentration, the ratio of intercellular to atmospheric CO(2) concentration (C(i)/C(a)) and stomatal conductance to water vapor did not differ among foliage grown in an ambient or enriched CO(2) concentration. Analysis of the relationship between P(n) and C(i) indicated no significant change in carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase/oxygenase during growth in elevated CO(2) concentrations. Based on estimates derived from P(n)/C(i) curves, there were no apparent treatment differences in dark respiration, CO(2) compensation point or P(n) at the mean C(i). In 1992, foliage in the three CO(2) treatments yielded similar estimates of CO(2)-saturated P(n) (P(max)), whereas in 1993, estimates of P(max) were higher for branches grown in elevated CO(2) than in ambient CO(2). We conclude that field-grown loblolly pine trees do not exhibit downward acclimation of leaf-level photosynthesis in their long-term response to elevated CO(2) concentrations.     

Effects of thinning on soil and tree water relations, transpiration and growth in an oak forest (Quercus petraea (Matt.) Liebl.)

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.