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1.
The impact of nocturnal water loss and recharge of stem water storage on predawn disequilibrium between leaf (psiL) and soil (psiS) water potentials was studied in three dominant tropical savanna woody species in central Brazil (Cerrado). Sap flow continued throughout the night during the dry season and contributed from 13 to 28% of total daily transpiration. During the dry season, psiL was substantially less negative in covered transpiring leaves, throughout the day and night, than in exposed leaves. Before dawn, differences in psiL between covered and exposed leaves were about 0.4 MPa. When relationships between sap flow and psiL of exposed leaves were extrapolated to zero flow, the resulting values of psiL (a proxy of weighted mean soil water potential) in two of the species were similar to predawn values of covered leaves. Consistent with substantial nocturnal sap flow, stomatal conductance (gs) never dropped below 40 mmol m(-2) s(-1) at night, and in some cases, rose to as much as 100 mmol m(-2) s(-1) before the end of the dark period. Nocturnal gs decreased linearly with increasing air saturation deficit (D), but there were species-specific differences in the slopes of the relationships between nocturnal gs and D. Withdrawal and recharge of water from stem storage compartments were assessed by monitoring diel fluctuations of stem diameter with electronic dendrometers. Stem water storage compartments tended to recharge faster when nocturnal transpiration was reduced by covering the entire plant. Water potential of covered leaves did not stabilize in any of the plants before the end of the dark period, suggesting that, even in covered plants, water storage tissues were not fully rehydrated by dawn. Patterns of sap flow and expansion and contraction of stems reflected the dynamics of water movement during utilization and recharge of stem water storage tissues. This study showed that nighttime transpiration and recharge of internal water storage contribute to predawn disequilibrium in water potential between leaves and soil in neotropical savanna woody plants.  相似文献   

2.
We analyzed the partition of nocturnal sap flow into refilling of internal water storage and transpiration in Acacia mangium. Sap flow of trees was monitored continuously with Granier’s sensors for estimating the whole-tree transpiration. Possible night transpiration and stomatal conductance at the leaf level in the canopy were measured with a LI-6400 photosynthesis measuring system. For nocturnal leaf transpiration and stomatal conductance were weak, nocturnal sap flow of mature A. mangium trees was mainly associated with water recharge in the trunk. No significant change in night water recharge of the trunk was found at both seasonal and inter-annual scales. Morphological features of trees including diameter at the breast height (DBH), tree height, and canopy size could explain variances of night water recharge. Furthermore, although the contribution of nocturnal sap flow to the total transpiration varied among seasons and DBH classes, the error caused by night water recharge on wholetree transpiration was negligible. __________ Translated from Journal of Plant Ecology (Chinese Version), 2007, 31 (5): 777–786 [译自: 植物生态学报]  相似文献   

3.
Contrary to the conventional theory of optimal stomatal control, there is substantial transpiration at night in many tree species, but the functional significance of this phenomenon remains uncertain. To investigate the possible roles of nocturnal transpiration, we compared and contrasted the correlations of both nocturnal and diurnal sap flow with a range of traits in 21 temperate deciduous tree species. These traits included soil water affinity, shade tolerance, cold hardiness, nitrogen concentration of tissues, minimum transpiration rate of excised leaves, growth rate, photosynthetic capacity, stomatal length and density, and the water potential and relative water content of leaves at the wilting point. Nocturnal sap flow was higher in species with higher leaf nitrogen concentrations, higher rates of extension growth and lower shade tolerances. Diurnal sap flow was higher in species with higher leaf nitrogen concentrations and photosynthetic capacities on a leaf area basis. Because leaf metabolism and dark respiration, in particular, are strongly related to leaf nitrogen concentration, our findings suggest that nocturnal transpiration functions to sustain carbohydrate export and other processes driven by dark respiration, and that this function is most important in fast- growing shade-intolerant tree species.  相似文献   

4.
Stomatal conductance was quantified with sap flux sensors and whole-tree chambers in mature Norway spruce (Picea abies (L.) Karst.) trees after 3 years of exposure to elevated CO(2) concentration ([CO(2)]) in a 13-year nutrient optimization experiment. The long-term nutrient optimization treatment increased tree height by 3.7 m (67%) and basal diameter by 8 cm (68%); the short-term elevated [CO(2)] exposure had no effect on tree size or allometry. Nighttime transpiration was estimated as approximately 7% of daily transpiration in unchambered trees; accounting for the effect of nighttime flux on the processing of sap flux signals increased estimated daily water uptake by approximately 30%. Crown averaged stomatal conductance (g(s)) was described by a Jarvis-type model. The addition of a stomatal response time constant (tau) and total capacitance of stored water (C(tot)) improved the fit of the model. Model estimates for C(tot) scaled with sapwood volume of the bole in fertilized trees. Hydraulic support-defined as a lumped variable of leaf-specific hydraulic conductivity and water potential gradient (K(l)DeltaPsi) -was estimated from height, sapwood-to-leaf area ratio (A(s):A(l)) and changes in tracheid dimensions. Hydraulic support explained 55% of the variation in g(s) at reference conditions for trees across nutrient and [CO(2)] treatments. Removal of approximately 50% of A(l) from three trees yielded results suggesting that stomatal compensation (i.e., an increase in g(s)) after pruning scales inversely with K(l)DeltaPsi, indicating that the higher the potential hydraulic support after pruning, the less complete the stomatal compensation for the increase in A(s):A(l).  相似文献   

5.
Soil water potential (Psi(s)) is often estimated by measuring leaf water potential before dawn (Psi(pd)), based on the assumption that the plant water status has come into equilibrium with that of the soil. However, it has been documented for a number of plant species that stomata do not close completely at night, allowing for nocturnal transpiration and thus preventing nocturnal soil-plant water potential equilibration. The potential for nighttime transpiration necessitates testing the assumption of nocturnal equilibration before accepting Psi(pd) as a valid estimate of Psi(s). We determined the magnitude of disequilibrium between Psi(pd) and Psi(s) in four temperate conifer species across three height classes through a replicated study in northern Idaho. Based on both stomatal conductance and sap flux measurements, we confirmed that the combination of open stomata and high nocturnal atmospheric vapor pressure deficit (D) resulted in nocturnal transpiration in all four species. Nocturnal stomatal conductance (g(s-noc)) averaged about 33% of mid-morning conductance values. We used species-specific estimates of g(s-noc) and leaf specific conductance to correct Psi(pd) values for nocturnal transpiration at the time the samples were collected. Compared with the unadjusted values, corrected values reflected a significantly higher Psi(pd) (when D > 0.12 kPa). These results demonstrate that comparisons of Psi(pd) among species, canopy height classes and sites, and across growing seasons can be influenced by differential amounts of nocturnal transpiration, leading to flawed results. Consequently, it is important to account for the presence of nocturnal transpiration, either through a properly parameterized model or by making Psi(pd) measurements when D is sufficiently low that it cannot drive nocturnal transpiration. Violating these conditions will likely result in underestimation of Psi(s).  相似文献   

6.
At the leaf scale, it is a long-held assumption that stomata close at night in the absence of light, causing transpiration to decrease to zero. Energy balance models and evapotranspiration equations often rely on net radiation as an upper bound, and some models reduce evapotranspiration to zero at night when there is no solar radiation. Emerging research is showing, however, that transpiration can occur throughout the night in a variety of vegetation types and biomes. At the ecosystem scale, eddy covariance measurements have provided extensive data on latent heat flux for a multitude of ecosystem types globally. Nighttime eddy covariance measurements, however, are generally unreliable because of low turbulence. If significant nighttime water loss occurs, eddy flux towers may be missing key information on latent heat flux. We installed and measured rates of sap flow by the heat ratio method (Burgess et al. 2001) at two AmeriFlux (part of FLUXNET) sites in California. The heat ratio method allows measurement and quantification of low rates of sap flow, including negative rates (i.e., hydraulic lift). We measured sap flow in five Pinus ponderosa Dougl. ex Laws. trees and three Arctostaphylos manzanita Parry and two Ceanothus cordulatus A. Kellog shrubs in the Sierra Nevada Mountains, and in five Quercus douglasii Hook and Arn. trees at an oak savanna in the Central Valley of California. Nocturnal sap flow was observed in all species, and significant nighttime water loss was observed in both species of trees. Vapor pressure deficit and air temperature were both well correlated with nighttime transpiration; the influence of wind speed on nighttime transpiration was insignificant at both sites. We distinguished between storage-tissue refilling and water loss based on data from Year 2005, and calculated the percentage by which nighttime transpiration was underestimated by eddy covariance measurements at both sites.  相似文献   

7.
《林业研究》2021,32(5)
Minquartia guianensis Aubl.is a slow-growing species with several uses.In the juvenile state,it is well-adapted to low light conditions of the forest understory.However,it is still unknown how climate variability affects transpiration of this species,particularly under drought stress.In this study,we aimed to assess the effect of climatic variability on sap flow rates(SFR).SFR and radial growth were measured in six trees(14-50 cm diameter) in 2015 and 2016.Climate(precipitation,irradiance,relative humidity and temperature) and soil water content(SWC) data were also collected.SFR tended to increase in the dry season,with a negative relationship between SFR and SWC and precipitation(p 0.001),while there was a positive association between radial growth and monthly precipitation(p=0.004).Irradiance and temperature were the environmental factors more closely correlated with SFR during daytime(p0.001),whereas relative humidity and vapor pressure deficit were the most important factors at night(p0.001).Although negative SFR were sometimes recorded at night,the mean nocturnal sap flow was positive and across trees the nighttime sap flow accounted for 12.5%of the total daily sap flow.Increased transpiration during the dry season suggests that the root system of Minquartia was able to extract water from deep soil layers.These results widen our understanding of the ecophysiology of Amazonian trees under drought and provide further insight into the potential effect of the forecasted decline in precipitation in the Amazon region.  相似文献   

8.
Linking leaf and tree water use with an individual-tree model   总被引：2，自引：0，他引：2  
We tested the ability of a model to scale gas exchange from leaf level to whole-tree level by: (1) measuring leaf gas exchange in the canopy of 10 trees in a tall Eucalyptus delegatensis RT Baker forest in NSW, Australia; (2) monitoring sap flow of the same 10 trees during the measurement week; and (3) using an individual-tree-based model (MAESTRA) to link the two sets of measurements. Photosynthesis and stomatal conductance components of the model were parameterized with the leaf gas exchange data, and canopy structure was parameterized with crown heights, dimensions and leaf areas of each of the measurement trees and up to 45 neighboring trees. Transpiration of the measurement trees was predicted by the model and compared with sap flow data. Leaf gas exchange parameters were similar for all 10 trees, with the exception of two smaller trees that had relatively low stomatal conductances. We hypothesize that these trees may have experienced water stress as a result of competition from large neighboring trees. The model performed well, and in most cases, was able to replicate the time course of tree transpiration. Maximum rates of transpiration were higher than measured rates for some trees and lower than measured rates for others, which may have been a result of inaccuracy in estimating tree leaf area. There was a small lag (about 15-30 minutes) between sap flow and modeled transpiration for some trees in the morning, likely associated with use of water stored in stems. The model also captured patterns of variation in sap flow among trees. Overall, the study confirms the ability of models to estimate forest canopy transpiration from leaf-level measurements.  相似文献   

9.
Biologists have long been puzzled by the striking morphological and anatomical characteristics of Neotropical savanna trees which have large scleromorphic leaves, allocate more than half of their total biomass to belowground structures and produce new leaves during the peak of the dry season. Based on results of ongoing interdisciplinary projects in the savannas of central Brazil (cerrado), we reassessed the validity of six paradigms to account for the water economy of savanna vegetation. (1) All savanna woody species are similar in their ability to take up water from deep soil layers where its availability is relatively constant throughout the year. (2) There is no substantial competition between grasses and trees for water resources during the dry season because grasses exclusively explore upper soil layers, whereas trees access water in deeper soil layers. (3) Tree species have access to abundant groundwater, their stomatal control is weak and they tend to transpire freely. (4) Savanna trees experience increased water deficits during the dry season despite their access to deep soil water. (5) Stomatal conductance of savanna species is low at night to prevent nocturnal transpiration, particularly during the dry season. (6) Savanna tree species can be classified into functional groups according to leaf phenology. We evaluated each paradigm and found differences in the patterns of water uptake between deciduous and evergreen tree species, as well as among evergreen tree species, that have implications for regulation of tree water balance. The absence of resource interactions between herbaceous and woody plants is refuted by our observation that herbaceous plants use water from deep soil layers that is released by deep-rooted trees into the upper soil layer. We obtained evidence of strong stomatal control of transpiration and show that most species exhibit homeostasis in maximum water deficit, with midday water potentials being almost identical in the wet and dry seasons. Although stomatal control is strong during the day, nocturnal transpiration is high during the dry season. Our comparative studies showed that the grouping of species into functional categories is somewhat arbitrary and that ranking species along continuous functional axes better represents the ecological complexity of adaptations of cerrado woody species to their seasonal environment.  相似文献   

10.
Hydraulic redistribution of soil water by neotropical savanna trees   总被引：1，自引：0，他引：1  
The magnitude and direction of water transport by the roots of eight dominant Brazilian savanna (Cerrado) woody species were determined with a heat pulse system that allowed bidirectional measurements of sap flow. The patterns of sap flow observed during the dry season in species with dimorphic root systems were consistent with the occurrence of hydraulic redistribution of soil water, the movement of water from moist to drier regions of the soil profile via plant roots. In these species, shallow roots exhibited positive sap flow (from the soil into the plant) during the day and negative sap flow (from the plant into the soil) during the night. Sap flow in the taproots was positive throughout the 24-h period. Diel fluctuations in soil water potential, with maximum values occurring at night, provided evidence for partial rewetting of upper soil layers by water released from shallow roots. In other species, shallow roots exhibited negative sap flow during both the day and night, indicating that hydraulic redistribution was occurring continuously. A third sap flow pattern was observed at the end of the dry season after a heavy rainfall event when sap flow became negative in the taproot, and positive in the small roots, indicating movement of water from upper soil layers into shallow roots, and then into taproots and deeper soil layers. Experimental manipulations employed to evaluate the response of hydraulic redistribution to changes in plant and environmental conditions included watering the soil surface above shallow roots, decreasing transpiration by covering the plant and cutting roots where probes were inserted. Natural and manipulated patterns of sap flow in roots and stems were consistent with passive movement of water toward competing sinks in the soil and plant. Because dry shallow soil layers were often a stronger sink than the shoot, we suggest that the presence of a dimorphic root system in deciduous species may play a role in facilitating leaf expansion near the end of the dry season when the soil surrounding shallow lateral roots is still dry.  相似文献   

11.
The compensation heat pulse (CHP) method is widely used to estimate sap flow and transpiration in conducting organs of woody plants. Previous studies have reported a natural azimuthal variability in sap flow, which could have practical implications in locating the CHP probes and integrating their output. Sap flow of several olive trees (Olea europaea L. cv. 'Arbequina') previously grown under different irrigation treatments were monitored by the CHP method, and their xylem anatomical characteristics were analyzed from wood samples taken at the same location in which the probes were installed. A significant azimuthal variability in the sap flow was found in a well-irrigated olive tree monitored by eight CHP probes. The azimuthal variability was well related to crown architecture, but poorly to azimuthal differences in the xylem anatomical characteristics. Well-irrigated and deficit-irrigated olive trees showed similar xylem anatomical characteristics, but they differed in xylem growth and in the ratio of nocturnal-to-diurnal sap flow (N/D index). The results of this work indicate that transpiration cannot be accurately estimated by the CHP method in olive trees if a small number of sensors are employed and that the N/D index could be used as a sensitive water status indicator.  相似文献   

12.
We used 20-mm-long, Granier-type sensors to quantify the effects of tree size, azimuth and radial position in the xylem on the spatial variability in xylem sap flux in 64-year-old trees of Taxodium distichum L. Rich. growing in a flooded forest. This information was used to scale flux to the stand level to investigate variations in half-hourly and daily (24-hour) sums of sap flow, transpiration per unit of leaf area, and stand transpiration in relation to vapor pressure deficit (D) and photosynthetically active radiation (Q(o)). Measurements of xylem sap flux density (J(s)) indicated that: (1) J(s) in small diameter trees was 0.70 of that in medium and large diameter trees, but the relationship between stem diameter as a continuous variable and J(s) was not significant; (2) J(s) at 20-40 mm depth in the xylem was 0.40 of that at 0-20 mm depth; and (3) J(s) on the north side of trees was 0.64 of that in directions 120 degrees from the north. Daily transpiration was linearly related to daily daytime mean D, and reached a modest value of 1.3 mm day(-1), reflecting the low leaf area index (LAI = 2.2) of the stand. Because there was no soil water limitation, half-hourly water uptake was nearly linearly related to D at D < 0.6 kPa during both night and day, increasing to saturation during daytime at higher values of D. The positive effect of Q(o) on J(s) was significant, but relatively minor. Thus, a second-order polynomial with D explained 94% of the variation in J(s) and transpiration. An approximately 40% reduction in LAI by a hurricane resulted in decreases of about 18% in J(s) and stand transpiration, indicating partial stomatal compensation.  相似文献   

13.
Zweifel R  Böhm JP  Häsler R 《Tree physiology》2002,22(15-16):1125-1136
We used local microclimatic conditions and twig sap flow rates to interpret midday stomatal closure in the canopies of two 250-year-old Norway spruce (Picea abies (L.) Karst.) trees at a subalpine site in the Swiss Alps (1,650 m a.s.l.). Both trees showed midday stomatal closure on most clear summer days, despite the permanently wet soil. We used a modified Penman-Monteith formula to simulate potential transpiration of single twigs (ET(T)) based on high-resolution temporal and spatial microclimate data obtained both inside and outside the crowns. Comparison of calculated ET(T) values and measured twig sap flow rates enabled us to pinpoint the occurrence of midday stomatal closure and the microclimatic conditions present at that time. We found that vapor pressure deficit (and for upper-crown twigs, ET(T)) largely explained the timing of initial midday stomatal closure but gave no explanation for the different patterns of stomatal behavior after initial closure in upper- and lower-crown twigs. After the initial stomatal closure, upper-crown twigs maintained high transpiration rates by continuously regulating stomatal aperture, whereas stomatal aperture decreased rapidly in lower-crown twigs and did not increase later in the day. Midday stomatal closure in lower-crown twigs occurred on average 1 h later than in upper-crown twigs. However, the microclimate at the time of initial stomatal closure was similar at both crown locations except that lower-crown twigs received significantly less solar radiation than upper-crown twigs both at the time of initial stomatal closure and afterwards. High rates of sap flow in twigs did not always lead to stomatal closure and therefore could not explain the phenomenon. We conclude that stomatal conductance can be modeled accurately only when both local microclimatic conditions and tree water status are known. Further, we hypothesize that both the quantity and quality of light play an important role in the reopening of closed stomata during the day.  相似文献   

14.
Environmental controls on sap flow in a northern hardwood forest   总被引：1，自引：0，他引：1  
Our objective was to gain a detailed understanding of how photosynthetically active radiation (PAR), vapor pressure deficit (D) and soil water interact to control transpiration in the dominant canopy species of a mixed hardwood forest in northern Lower Michigan. An improved understanding of how these environmental factors affect whole-tree water use in unmanaged ecosystems is necessary in assessing the consequences of climate change on the terrestrial water cycle. We used continuously heated sap flow sensors to measure transpiration in mature trees of four species during two successive drought events. The measurements were scaled to the stand level for comparison with eddy covariance estimates of ecosystem water flux (Fw). Photosynthetically active radiation and D together explained 82% of the daytime hourly variation in plot-level transpiration, and low soil water content generally resulted in increased stomatal sensitivity to increasing D. There were also species-specific responses to drought. Quercus rubra L. showed low water use during both dry and wet conditions, and during periods of high D. Among the study species, Acer rubrum L. showed the greatest degree of stomatal closure in response to low soil water availability. Moderate increases in stomatal sensitivity to D during dry periods were observed in Populus grandidentata Michx. and Betula papyrifera Marsh. Sap flow scaled to the plot level and Fw demonstrated similar temporal patterns of water loss suggesting that the mechanisms controlling sap flow of an individual tree also control ecosystem evapotranspiration. However, the absolute magnitude of scaled sap flow estimates was consistently lower than Fw. We conclude that species-specific responses to PAR, D and soil water content are key elements to understanding current and future water fluxes in this ecosystem.  相似文献   

15.
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   

16.
Transpiration from a hawthorn (Crataegus monogyna L.) dominated hedgerow in southern England was measured continuously over two growing seasons by the sap flow technique. Accompanying measurements of structural parameters, microclimate and leaf stomatal and boundary layer conductances were used to establish the driving factors of hedgerow transpiration. Observed transpiration rates, reaching peak values of around 8 mm day(-1) and a seasonal mean of about 3.5 mm day(-1), were higher than those reported for most other temperate deciduous woodlands, except short-rotation coppice and wet woodlands. The high rates were caused by the structural and physiological characteristics of hawthorn leaves, which exhibited much higher stomatal and boundary-layer conductances than those of the second-most abundant woody species in the hedgerow, field maple (Acer campestre L.). Only in the hot summer of 2003 did stomatal conductance, and thus transpiration, decrease substantially. The hedgerow canopy was always closely coupled to the atmosphere. Hedgerow transpiration equaled potential evaporation (calculated by the Priestley-Taylor formula) in 2003 and exceeded it in 2004, which meant that a substantial fraction of the energy (21% in 2003 and more than 37% in 2004) came from advection. Hedgerow canopy conductance (g(c)), as inferred from the sap flow data by inverting the Penman-Monteith equation, responded to solar radiation (R(G)) and vapor pressure deficit (D). Although the response to R(G) showed no systematic temporal variation, the response to D, described as g(c)(D) = g(cref) - mln(D), changed seasonally. The reference g(c) depended on leaf area index and the ratio of -m/g(cref) on long-term mean daytime D. A model is proposed based on these observations that predicts canopy conductance for the hawthorn hedge from standard weather data.  相似文献   

17.
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   

18.
In a world of diminishing water reservoirs and a rising demand for food, the practice and development of water stress indicators and sensors are in rapid progress. The heat dissipation method, originally established by Granier, is herein applied and modified to enable sap flow measurements in date palm trees in the southern Arava desert of Israel. A long and tough sensor was constructed to withstand insertion into the date palm's hard exterior stem. This stem is wide and fibrous, surrounded by an even tougher external non-conducting layer of dead leaf bases. Furthermore, being a monocot species, water flow does not necessarily occur through the outer part of the palm's stem, as in most trees. Therefore, it is highly important to investigate the variations of the sap flux densities and determine the preferable location for sap flow sensing within the stem. Once installed into fully grown date palm trees stationed on weighing lysimeters, sap flow as measured by the modified sensors was compared with the actual transpiration. Sap flow was found to be well correlated with transpiration, especially when using a recent calibration equation rather than the original Granier equation. Furthermore, inducing the axial variability of the sap flux densities was found to be highly important for accurate assessments of transpiration by sap flow measurements. The sensors indicated no transpiration at night, a high increase of transpiration from 06:00 to 09:00, maximum transpiration at 12:00, followed by a moderate reduction until 08:00; when transpiration ceased. These results were reinforced by the lysimeters' output. Reduced sap flux densities were detected at the stem's mantle when compared with its center. These results were reinforced by mechanistic measurements of the stem's specific hydraulic conductivity. Variance on the vertical axis was also observed, indicating an accelerated flow towards the upper parts of the tree and raising a hypothesis concerning dehydrating mechanisms of the date palm tree. Finally, the sensors indicated reduction in flow almost immediately after irrigation of field-grown trees was withheld, at a time when no climatic or phenological conditions could have led to reduction in transpiration.  相似文献   

19.
Transpiration is generally assumed to be insignificant at night when stomata close in response to the lack of photosynthetically active radiation. However, there is increasing evidence that the stomata of some species remain open at night, which would allow for nighttime transpiration if there were a sufficient environmental driving force. We examined nighttime water use in co-occurring species in a mixed deciduous stand at Harvard Forest, MA, using whole-tree and leaf-level measurements. Diurnal whole-tree water use was monitored continuously with Granier-style sap flux sensors in paper birch (Betula papyrifera Marsh.), red oak (Quercus rubra L.) and red maple (Acer rubrum L.). An analysis was conducted in which nighttime water flux could be partitioned between refilling of internal water stores and transpiration. Substantial nighttime sap flux was observed in all species and much of this flux was attributed to the refilling of depleted water stores. However, in paper birch, nighttime sap flux frequently exceeded recharge estimates. Over 10% of the total daily sap flux during the growing season was due to transpiration at night in paper birch. Nighttime sap flux was over 8% of the total daily flux in red oak and 2% in red maple; however, this flux was mainly associated with recharge. On nights with elevated vapor pressure deficit, sap flux continued through the night in paper birch, whereas it reached zero during the night in red oak and red maple. Measurements of leaf-level gas exchange on a night with elevated vapor pressure deficit showed stomatal conductance dropping by only 25% in paper birch, while approaching zero in red oak and red maple. The study highlighted differences in ecophysiological controls on sap flux exerted by co-occurring species. Paper birch is a fast-growing, shade-intolerant species with an earlier successional status than red oak and red maple. Risking water loss through nighttime transpiration may provide paper birch with an ecological advantage by enabling the species to maximize photosynthesis and support rapid growth. Nighttime transpiration may also be a mechanism for delivering oxygen to respiring cells in the deep sapwood of paper birch.  相似文献   

20.
Sap flow density and meteorological variables were monitored in a very dense Acacia melanoxylon stand (about 9,000 trees/ha) in north-western Iberian Peninsula during the growing season of 2006 (from 8 June to 24 August). Evidences of an increment of stomatal control on transpiration were observed during the study period, probably as a consequence of higher evaporative demand of the atmosphere. However, high sap flow density values observed for the whole study period (from 1.14 to 52.73 dm3 dm−2 day−1) were similar than those found for other fast-growing species. Mean transpiration for the whole study period was 2.21 mm day−1, with a maximum value of 3.17 mm day−1 and a minimum of 1.23 mm day−1. Mean sap flow density values were correlated with crown length and crown ratio, relationships being fairly weak with other dendrometric parameters such as tree diameter or height. Mean transpiration values were correlated with main dendrometric parameters (diameter at breast height, total height, crown length, sapwood area and leaf biomass). It was found that the degree of competition per tree could be used as a good index for sap flow density. Taking into account the high tree density of the stand and the sap flow density values, water consumptions of A. melanoxylon can be very high, playing a relevant role in the hydrological balances of the watersheds where it grows.  相似文献   

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