Variation In Transpiration Research Articles

Scaling Individual Tree Transpiration With Thermal Cameras Reveals Interspecies Differences to Drought Vulnerability

AbstractUnderstanding tree transpiration variability is vital for assessing ecosystem water‐use efficiency and forest health amid climate change, yet most landscape‐level measurements do not differentiate individual trees. Using canopy temperature data from thermal cameras, we estimated the transpiration rates of individual trees at Harvard Forest and Niwot Ridge. PT‐JPL model was used to derive latent heat flux from thermal images at the canopy‐level, showing strong agreement with tower measurements (R2 = 0.70–0.96 at Niwot, 0.59–0.78 at Harvard at half‐hourly to monthly scales) and daily RMSE of 33.5 W/m2 (Niwot) and 52.8 W/m2 (Harvard). Tree‐level analysis revealed species‐specific responses to drought, with lodgepole pine exhibiting greater tolerance than Engelmann spruce at Niwot and red oak showing heightened resistance than red maple at Harvard. These findings show how ecophysiological differences between species result in varying responses to drought and demonstrate that these responses can be characterized by deriving transpiration from crown temperature measurements.

A gravimetric method to monitor transpiration under water stress conditions in wheat

AbstractThe water‐limited potential yield of wheat (Triticum aestivum L.) may be largely dependent on plant transpiration behavior. Research into plant transpiration dynamics is limited due to the difficulty of monitoring water use over an extended period of time. The advent of fully automated gravimetric platforms makes this research possible. The methods for using this equipment to study transpiration of wheat in water‐deficit environments are not well established. The objective of this greenhouse study was to develop a methodology to evaluate plant transpiration under terminal water stress using a gravimetric platform. Using a plant–pot system designed to limit nonplant water loss, the methodology proved capable of monitoring plant transpiration over a 42‐day period and detected significant differences between well‐watered and water‐stressed treatments. However, the high degree of variation in transpiration limited the system's ability to detect significant differences between genotypes of similar watering regimes. Both environmental and human error likely contributed to the observed variation. Recommendations are provided to reduce this variation and improve upon the methodology for future studies. This work lays the foundation for further research into plant water use and the identification of genotypes capable of high transpiration despite exposure to water‐deficit conditions.

Fractional coefficient for estimating physiologically meaningful and diurnal transpiration cycle weighted VPD from daily ambient environmental data

Appropriate representation of environmental conditions via meaningful metrics is critical for their effectiveness in modeling crop physiological and resource use processes. Vapor pressure deficit (VPD) is a primary determinant of crop water use efficiency, commonly reported as daily average. However, VPD can vary substantially during the day in relation to diurnal cycle of transpiration, in which case daily average VPD may not suffice. In such scenarios, integrating VPD over the course of diurnal transpiration cycle into a weighted VPD (VPDw) can be physiologically meaningful as a metric. The goal of this research is to characterize the fractional coefficient (Frac) that can be used to effectively calculate VPDw from daily temperature and relative humidity data without the need for sub-daily environmental data. Site-specific and seasonal estimates of Frac, which represent the relative positioning of VPDw to diurnal extremes of saturation vapor pressure are currently lacking for semi-arid regions. To address this, we quantified Frac at 101 sites in the U.S. Central Plains using long-term (1982–2019) hourly and daily radiation and VPD datasets. Frac varied substantially across the region, with growing season mean values ranging from 0.59 to 0.92. Summer months had the highest Frac values with relatively lower uncertainty as compared to lower Frac during winter. Local observations of Frac deviated significantly from previously assumed stationary estimates implying that site-specific and month-specific Frac may be necessary for robust water use estimations. VPDw measured at an experimental site in Central U.S. Plains explained greater variance in daytime crop transpiration than daily mean VPD in maize, soybean, and sorghum canopies. Site-specific Frac was also assessed to perform better than commonly used Frac estimate of 0.75 to explain day-to-day variation in transpiration, demonstrating effective use of long-term mean Frac values presented here to estimate VPDw using daily datasets.

Species-specific control of DBH and landscape characteristics on tree-to-tree variability of sap velocity

The substantial tree-to-tree variability of transpiration poses a major challenge to a reliable stand-scale quantification of transpiration. The diameter at breast height (DBH) and landscape characteristics have been identified as drivers of tree-to-tree variability, but it remains unclear if their control on sap velocity varies between species-specific water-use and environmental conditions. We hypothesized that their controls are specie-specific, such as the temporal dynamic of their relative importance. To test our hypotheses, we used a multi-species stand that include 37 trees equipped with sap-flow sensors from four species representing the dominant species in central Europe. We analysed the daily relative importance of DBH, landscape slope, aspect, flow accumulation, and topographical position. We found that tree-to-tree variability of sap velocity was mainly dependent on DBH for oak (twice higher relative importance than other species) and on landscape characteristics for beech (36% higher relative importance than other species) and conifers. The temporal dynamics of the relative importance of most tested drivers was found to be species-specific and linked to root-related aspects in response to hydro-meteorological conditions. During dry summer months, the daily relative importance of oak's DBH increased to almost 60% to be three times higher than the value for beech. In contrast, the relative importance of flow accumulation was always two to three times higher for beech trees than oak and conifer trees. This indicated that larger oak trees accessed deeper water sources than smaller oaks. However, the shallower root architecture of beech trees involved a higher dependence on shallow soil water because a larger DBH is seemingly not enhancing the tree's capacity to explore deeper soils. These new insights emphasize the critical importance of accounting for DBH and landscape characteristics through a species-specific and temporally dynamic correction in further approaches for upscaling of sap-flow data from individual tree to stand-scale.

Transpiration of Anatolian black pine and sessile oak forest stands in a sub-humid region of Turkey

Transpiration is a key component of the watershed water budget. Therefore, determining the transpiration of forest stands with different characteristics have been of interest for watershed hydrology and forest management practices in a wide range of environments. The objectives of this study were to compare transpiration of Anatolian black pine (Pinus nigra Arn. subsp. pallasiana (Lamb.) Holmboe) and sessile oak (Quercus petraea (Matt.) Liebl.) tree stands and to model transpiration based on the measured climatic factors. Stand transpirations were calculated from sap flow measurements made by the trunk heat balance method. We conducted an exploratory factor analysis (PCR) to detect affecting meteorological factors of stand transpiration, and we developed linear regression equations to predict transpiration of pine and oak stands. Mean daily and yearly canopy transpiration (Ec) were 1.05 mm day-1 and 378.3 mm year-1 for the pine stand and 3.52 mm day-1 and 801.7 mm year-1 for the oak stand. There was a highly positive correlation between daily stand transpiration and wind speed, global radiation, air vapour pressure deficit and air temperature, but a negative correlation with relative humidity for both stands. Soil water potential had little effect on stand transpiration. The model equations accounted for 81% of the variations in transpiration for the pine stand and 85% for the oak stand. Therefore, the transpiration of forest stands should be considered for effective vegetation management practices, as model equations to estimate the transpiration of pine and oak stands in the region.

New Representation of Plant Hydraulics Improves the Estimates of Transpiration in Land Surface Model

Transpiration represents more than 30% of the global land–atmosphere water exchange but is highly uncertain. Plant hydraulics was ignored in traditional land surface modeling, but recently plant hydraulics has been found to play an essential role in transpiration simulation. A new physical-based representation of plant hydraulic schemes (PHS) was recently developed and implemented in the Common Land Model (CoLM). However, it is unclear to what extent PHS can reduce these uncertainties. Here, we evaluated the PHS against measurements obtained at 81 FLUXNET sites. The transpiration of each site was estimated using an empirical evapotranspiration partitioning approach. The metric scores defined by the International Land Model Benchmarking Project (ILAMB) were used to evaluate the model performance and compare it with that of the CoLM default scheme (soil moisture stress (SMS)). The bias score of transpiration in PHS was higher than SMS for most sites, and more significant improvements were found in semi-arid and arid sites where transpiration was limited by soil moisture. The hydraulic redistribution in PHS optimized the soil water supply and thus improved the transpiration estimates. In humid sites, no significant improvement in seasonal or interannual variability of transpiration was simulated by PHS, which can be explained by the insensitivity of transpiration demand coupled to the photosynthesis response to precipitation. In arid and semi-arid sites, seasonal or interannual variability of transpiration was better captured by PHS than SMS, which was interpreted by the improved drought sensitivity for transpiration. Arid land is widespread and is expected to expand due to climate change, thus there is an urgent need to couple PHS in land surface models.

Stomata coordinate with plant hydraulics to regulate transpiration response to vapour pressure deficit in wheat.

Genotypic variation in transpiration (Tr) response to vapour pressure deficit (VPD) has been studied in many crop species. There is debate over whether shoots or roots drive these responses. We investigated how stomata coordinate with plant hydraulics to mediate Tr response to VPD and influence leaf water status in wheat (Triticum aestivum L.). We measured Tr and stomatal conductance (gs) responses to VPD in well-watered, water-stressed and de-rooted shoots of eight wheat genotypes. Tr response to VPD was related to stomatal sensitivity to VPD and proportional to gs at low VPD, except in the water-stressed treatment, which induced strong stomatal closure at all VPD levels. Moreover, gs response to VPD was driven by adaxial stomata. A simple linear Tr response to VPD was associated with unresponsive gs to VPD. In contrast, segmented linear Tr to VPD response was mostly a function of gs with the breakpoint depending on the capacity to meet transpirational demand and set by the shoots. However, the magnitude of Tr response to VPD was influenced by roots, soil water content and stomatal sensitivity to VPD. These findings, along with a theoretical model suggest that stomata coordinate with plant hydraulics to regulate Tr response to VPD in wheat.

Leaf elongation response to blue light is mediated by stomatal-induced variations in transpiration in Festuca arundinacea.

Reduced blue light irradiance is known to enhance leaf elongation rate (LER) in grasses, but the mechanisms involved have not yet been elucidated. We investigated whether leaf elongation response to reduced blue light could be mediated by stomata-induced variations of plant transpiration. Two experiments were carried out on tall fescue in order to monitor LER and transpiration under reduced blue light irradiance. Additionally, LER dynamics were compared with those observed in the response to vapour pressure deficit (VPD)-induced variations of transpiration. Finally, we developed a model of water flow within a tiller to simulate the observed short-term response of LER to various transpiration regimes. LER dramatically increased in response to blue light reduction and then reached new steady states, which remained higher than the control. Reduced blue light triggered a simultaneous stomatal closure which induced an immediate decrease of leaf transpiration. The hydraulic model of leaf elongation accurately predicted the LER response to blue light and VPD, resulting from an increase in the growth-induced water potential gradient in the leaf growth zone. Our results suggest that the blue light signal is sensed by stomata of expanded leaves and transduced to the leaf growth zone through the hydraulic architecture of the tiller.

Impact of Water Deficit on Seasonal and Diurnal Dynamics of European Beech Transpiration and Time-Lag Effect between Stand Transpiration and Environmental Drivers

In-situ measurements of tree sap flow enable the analysis of derived forest transpiration and also the water state of the entire ecosystem. The process of water transport (by sap flow) and transpiration through vegetation organisms are strongly influenced by the synergistic effect of numerous external factors, some of which are predicted to alter due to climate change. The study was carried out by in-situ monitoring sap flow and related environmental factors in the years 2014 and 2015 on a research plot in Bienska dolina (Slovakia). We evaluated the relationship between derived transpiration of the adult beech (Fagus sylvatica L.) forest stand, environmental conditions, and soil water deficit. Seasonal beech transpiration (from May to September) achieved 59% of potential evapotranspiration (PET) in 2014 and 46% in 2015. Our study confirmed that soil water deficit leads to a radical limitation of transpiration and fundamentally affects the relationship between transpiration and environmental drivers. The ratio of transpiration (E) against PET was significantly affected by a deficit of soil water and in dry September 2015 decreased to the value of 0.2. The maximum monthly value (0.8) of E/PET was recorded in August and September 2014. It was demonstrated that a time lag exists between the course of transpiration and environmental factors on a diurnal basis. An application of the time lags within the analysis increased the strength of the association between transpiration and the variables. However, the length of these time lags changed in conditions of soil drought (on average by 25 min). Transpiration is driven by energy income and connected evaporative demand, provided a sufficient amount of extractable soil water. A multiple regression model constructed from measured global radiation (RS), air temperature (AT), and air humidity (RH) explained 69% of the variability in beech stand transpiration (entire season), whereas (RS) was the primary driving force. The same factors that were shifted in time explained 73% of the transpiration variability. Cross-correlation analysis of data measured in time without water deficit demonstrated a tighter dependency of transpiration (E) on environmental drivers shifted in time (−60 min RS, +40 min RH and +20 min vapour pressure deficit against E). Due to an occurrence and duration of soil water stress, the dependence of transpiration on the environmental variables became weaker, and at the same time, the time lags were prolonged. Hence, the course of transpiration lagged behind the course of global radiation by 60 (R2 = 0.76) and 80 (R2 = 0.69) minutes in conditions without and with water deficit, respectively.

Performance assessment of the AquaCrop model for film-mulched maize with full drip irrigation in Northwest China

Research on field water consumption is critical for optimizing crop growth and policy-making, in which the computer models play an increasingly important role. As a water-driven crop model, the AquaCrop model has been used in a large number of studies since its launch in 2009. However, how the model performs in predicting the ecohydrological process of farmland under film-mulched drip irrigation is still unclear, especially its application on partitioning crop evapotranspiration is very rarely reported. To make up for the above insufficiency, maize experiments were conducted under full mulch drip irrigation with observation instruments of eddy covariance systems, heat balance stem-flow gauges, micro-lysimeters and other tools, during seasons of 2014–2018. The AquaCrop model was first calibrated using measured data in 2014, and subsequently validated with data in 2015–2018. Results indicate that the parameterized model could precisely simulate the canopy cover ( $${R}^{2}$$ = 0.97), biomass ( $${R}^{2}$$ = 0.99) and grain yield (standard deviation was 4.13%), as well as reflect the patterns of daily variation in transpiration and evapotranspiration with satisfactory $${R}^{2}$$ of 0.91 and 0.87, respectively. Nevertheless, the $${R}^{2}$$ values of soil water content and evaporation were not good, ranging between 0.23 and 0.45, and 0.26 and 0.75, respectively. The AquaCrop model adopts canopy cover instead of leaf area index to describe the growing process of crops; this is an important innovation for model extension and application but also may lead to some inaccuracies in water balance simulation. Summarizing, this study shows that the AquaCrop model is appropriate for supporting crop production but not for predicting the soil moisture content and evaporation variation for maize under film-mulching drip irrigation.

A new phenomenological model to describe root-soil interactions based on percolation theory

In his paper on net primary productivity of terrestrial communities predicted from climatological data, Rosenzweig (1968) argued that variability in productivity is well accounted for by (evapo)-transpiration, and that water from transpiration is, on global scales, the most variable component in the photosynthesis reaction. The goal of this paper is to investigate whether variability in plant growth on local scales and within species is primarily related to transpiration under several scenarios including different terrain curvature, slope aspect, soil characteristics, and climate ranges. We test the hypothesis that this relationship exists because root growth into the surface soil layers (0–2 m) tends to follow paths with minima in resistance, which in turn maximizes water flow and nutrient delivery rates that regulate growth. The set of all connected paths with individual pore-to-pore flow resistances less than a critical, percolating, value forms a cluster with mass fractal dimensionality, df. We propose that roots follow paths through the 2D percolation cluster, defining the set of all optimal flow paths, making the 2D value of df from percolation relevant to root fractal dimensionality. The tortuosity of such optimal paths as defined in percolation theory should then relate root length to root radial extent, linking the parameters of root tortuosity and plant productivity. Our analysis of large data sets across species implies that root radial extent and tree height are both proportional to cumulative transpiration until trees approached maximum height, and their growth rates are proportional to the transpiration rate, not to the moisture content. Local variations in tree height as functions of the variables investigated appear generally consistent with deduced variations in transpiration. Here this correlation is investigated more closely in the context of studies addressing individual tree species.

Water Use and Soil Moisture Relationships on Western Juniper Trees at Different Growth Stages

An enhanced understanding of plant water uptake is critical for making better-informed management decisions involving vegetative manipulation practices aimed to improve site productivity. This is particularly true in arid and semiarid locations where water is a scarce, yet precious commodity. In this project, we evaluated the interannual and seasonal variability of soil moisture and transpiration in sapling, juvenile, and mature western juniper (Juniperus occidentalis) trees in a semiarid rangeland ecosystem of central Oregon, USA. Transpiration levels were greatest in mature juniper trees in an untreated juniper watershed (Jensen WS), while the lowest transpiration levels were observed in juniper saplings in a treated watershed (Mays WS) where most mature juniper trees were removed in 2005. Significant differences (p ≤ 0.05) in leaf water potential levels observed between predawn and midday readings for all juniper growth stages indicated water is lost over the course of the day. Results showed seasonal precipitation was highly variable over the course of the study (2017 through 2019) and this was reflected in soil water available for tree uptake. This resulted in considerable intra- and inter-annual variation in transpiration. In years with greater winter precipitation amounts (2017 and 2019), juniper transpiration rates were highest during the summer, followed by spring, autumn, and winter. On average, transpiration rates during the summer in the wettest (329 mm) year 2017 were 115 and 2.76 L day−1 for mature and sapling trees, respectively. No data were collected for juvenile trees in 2017. In the drier (245 mm) year 2018, higher transpiration rates were observed in the spring. On average, spring transpiration rates were 72.7, 1.61, and 1.00 L day−1 for mature, juvenile, and sapling trees, respectively. Study results highlight the sensitivity of western juniper woodlands to variations in seasonal precipitation and soil moisture availability.

Maize transpiration and water productivity of two irrigated fields with varying groundwater depths in an arid area

A better understanding of crop transpiration and its influencing factors in arid areas where irrigation is required contributes to improving agricultural water productivity and water resources management. However, previous studies from different regions have not come to a consensus on which factors and how they control crop transpiration, and little is known for areas with a shallow groundwater table. In this study, maize (Zea mays L.) transpiration was investigated under two groundwater table depths (WTDs) during two growing seasons. Sap velocity (SV), micrometeorological conditions and water regimes were monitored to evaluate transpiration rates under different WTDs and their relationships to environmental and hydrologic conditions. Results show that the combination of photosynthetic active radiation (PAR) and vapor pressure deficit (VPD) mainly controlled the diurnal dynamics of SV. PAR better explained the temporal variability of transpiration than solar radiation at hourly scale. At daily scale, the two-segment transpiration response to VPD was observed, suggesting that the studied species likely has a capacity of drought-tolerance. On average, SV was found to be larger during midday for the shallower WTD in 2016 than the deeper one. For different seasons, the shallower WTD in 2017 also led to larger transpiration than the deeper one in 2016. Given similar yields for all cases, the maize field with the deeper WTD in 2016 was estimated to have higher water productivity. An optimum water use efficiency WTD was determined by assessing the normalized transpiration difference with the varying WTD and it was about 2.1–2.2 m for maize in this area. These results shed light on the most effective utilization of the valuable water confronting the changing climate in arid areas with shallow groundwater.

Hydrology of Drained Peatland Forest: Numerical Experiment on the Role of Tree Stand Heterogeneity and Management

A prerequisite for sustainable peatland forestry is sufficiently low water table (WT) level for profitable tree production. This requires better understanding on controls and feedbacks between tree stand and its evapotranspiration, drainage network condition, climate, and WT levels. This study explores the role of spatial tree stand distribution in the spatiotemporal distribution of WT levels and site water balance. A numerical experiment was conducted by a three-dimensional (3-D) hydrological model (FLUSH) applied to a 0.5 ha peatland forest assuming (1) spatially uniform interception and transpiration, (2) interception and transpiration scaled with spatial distributions of tree crown and root biomass, and (3) the combination of spatially scaled interception and uniform transpiration. Site water balance and WT levels were simulated for two meteorologically contrasting years. Spatial variations in transpiration were found to control WT levels even in a forest with relatively low stand stem volume (<100 m3/ha). Forest management scenarios demonstrated how stand thinning and reduced drainage efficiency raised WT levels and increased the area and duration of excessively wet conditions having potentially negative economic (reduced tree growth) and environmental (e.g., methane emissions, phosphorus mobilization) consequences. In practice, silvicultural treatment manipulating spatial stand structure should be optimized to avoid emergence of wet spots.

Hydraulic anatomy affects genotypic variation in plant water use and shows differential organ specific plasticity to drought in Sorghum bicolor

Genotypic variability and plasticity in hydraulic anatomy are not well-understood in herbaceous monocots. In this study, we used Sorghum bicolor, a monocotyledonous, tropical grass model, to understand whether differential plant water use is associated with xylem anatomy and if whole-plant xylem anatomy responds to water stress, justifying differential genotypic sensitivity to drought. In a greenhouse environment, we studied four sorghum genotypes that are known to genetically differ in growth and exhibit differential sensitivity to drought. Under well-watered scenario, transpiration variability and plant growth traits correlated with xylem anatomical traits at both the leaf and stem level, including xylem area and predicted xylem-specific hydraulic conductivity. High water use genotypes had inherently higher hydraulic capacity, but under drought, their transpiration declined at higher fractions of transpirable soil water (FTSW) and they showed greater plasticity in hydraulic anatomy. However, lower FTSW thresholds and modest anatomical changes were identified in the low water use genotypes with inherently lower hydraulic conductivity. Drought, induced modular phenotypic plasticity in hydraulic anatomy, whereby plasticity in leaf xylem traits was remarkably higher than stem xylem, while root xylem showed a reverse nature of vascular modification. Xylem traits were in agreement with phloem anatomy, irrespective of water regime. Our study indicates that hydraulic anatomy can be critical for herbaceous monocots in determining limits to plant water use and genotypic response to drought with implications on whole-plant functions and habitat ecology.

Genotypic variation in transpiration of coppiced poplar during the third rotation of a short-rotation bio-energy culture.

The productivity of short‐rotation coppice (SRC) plantations with poplar (Populus spp.) strongly depends on soil water availability, which limits the future development of its cultivation, and makes the study of the transpirational water loss particularly timely under the ongoing climate change (more frequent drought and floods). This study assesses the transpiration at different scales (leaf, tree and stand) of four poplar genotypes belonging to different species and from a different genetic background grown under an SRC regime. Measurements were performed for an entire growing season during the third year of the third rotation in a commercial scale multigenotype SRC plantation in Flanders (Belgium). Measurements at leaf level were performed on specific days with a contrasted evaporative demand, temperature and incoming shortwave radiation and included stomatal conductance, stem and leaf water potential. Leaf transpiration and leaf hydraulic conductance were obtained from these measurements. To determine the transpiration at the tree level, single‐stem sap flow using the stem heat balance (SHB) method and daily stem diameter variations were measured during the entire growing season. Sap flow‐based canopy transpiration (E c), seasonal dry biomass yield, and water use efficiency (WUE; g aboveground dry matter/kg water transpired) of the four poplar genotypes were also calculated. The genotypes had contrasting physiological responses to environmental drivers and to soil conditions. Sap flow was tightly linked to the phenological stage of the trees and to the environmental variables (photosynthetically active radiation and vapor pressure deficit). The total E c for the 2016 growing season was of 334, 350, 483 and 618 mm for the four poplar genotypes, Bakan, Koster, Oudenberg and Grimminge, respectively. The differences in physiological traits and in transpiration of the four genotypes resulted in different responses of WUE.

Drought and irrigation affect transpiration rate and morning tree water status of a mature European beech (Fagus sylvatica L.) forest in Central Europe

AbstractSeverity and frequency of recurrent droughts is likely to be amplified by recent climate changes and may adversely affect large areas of temperate forests. This study addresses the ecophysiological responses of a mature beech forest in Central Europe (Slovakia, 450 m a.s.l.) exposed to drought during the growing seasons of 2012–2014. Sap flow and stem circumference changes of European beech (Fagus sylvatica L.) were measured in two contrasting treatments (drought vs. irrigation). Limited water availability reflected in the morning tree water status (morning stem contraction, ΔW) significantly reduced transpiration. Trees stressed by drought in the years 2012 and 2013 showed remarkably lower values of morning ΔW than those in the irrigated group. This suggests (a) lower water potential of the stem‐conducting tissues and (b) increased use of internally stored water to maintain daily transpiration. Additionally, morning ΔW showed a close relationship with soil water potential, particularly in the nonirrigated trees. Potential evapotranspiration (PET) explained less variability in the transpiration of the control stand than that of the irrigated stand. The use of a simple analysis of covariance model comprising morning ΔW and PET interaction significantly increased the explained variability in transpiration in the control stand. Morning ΔW of mature beech trees seems to be a useful and easily obtainable non‐invasive bioindicator of their ability to reduce and regulate transpiration in relation to atmospheric evaporative demands in response to drought.

Modeling transpiration and leaf temperature of urban trees – A case study evaluating the microclimate model ENVI-met against measurement data

Increasing vegetation cover in cities is a key approach to mitigating urban heat excess. However, both the effect of vegetation on microclimate and the plants’ vitality need to be assessed to support and quantify the effects of such strategies. One way to assess the interactions between vegetation and the urban environment is through microclimate models that can simulate the effects of vegetation onto the urban microclimate as well as effects of urban environments onto vegetation. To provide reliable estimates microclimate models need to be parameterized based on empirically obtained data. In this paper we compare modeled transpiration rates and leaf temperatures of a leading microclimate model, ENVI-met V4, with in-situ measured stem sap flow and leaf temperatures of two different trees in an urban courtyard. The vegetation model of ENVI-met is evaluated considering four synoptic situations including varying cloud covers ranging from fully cloudy to clear sky. The comparison of simulation results with empirical data reveals a high agreement. The model is capable of capturing the magnitude as well as short-term variations in transpiration caused by microclimatic changes. However, substantial deviations were found in situations with low photosynthetic active radiation. Modeled and observed diurnal tree transpiration and leaf temperature showed good agreement. These findings indicate that ENVI-met is capable of simulating transpiration rates and leaf temperatures of trees in complex urban environments.

Temporal variations in transpiration of Vitellaria paradoxa in West African agroforestry parklands

Lack of data on water use of key species of drylands constitutes an obstacle to understanding their role in hydrological processes in this environment. To elucidate seasonal variation in water consumption by Vitellaria paradoxa, the dominant species of parklands of the semi-arid areas of West Africa, we’ve measured its transpiration using heat ratio method (HRM) and seven potential explanatory variables. Sap flux was found to be significantly different among years with 0.64, 0.59 and 0.67 L h−1 dm−2 in 2008, 2009 and 2010, respectively. Sap flux was significantly higher in the dry (0.73 L h−1 dm−2) than in the wet season (0.53 L h−1 dm−2). Nighttime sap flux during dry season (0.48 L h−1 dm−2) was significantly higher than that of the wet season (0.20 L h−1 dm−2) and it contributes on average to 26% of daily sap flow with a maximum reaching 49%. The mean transpiration rate per tree was 151 L day−1 and all measured variables except rainfall and soil water content were significantly correlated with sap flux. These correlations were stronger (higher R value) during the rainy than in the dry season. Vapor Pressure Deficit (VPD) explained the highest proportion of sap flux variation and their curve was of parabolic type (R2 = 0.54) indicating that V. paradoxa can probably down-regulate its canopy conductance beyond a certain threshold of VPD, which is about 3 kPa in the present study. Future studies should investigate such hypothesis as well as the impacts of the variation of V. paradoxa transpiration due to climatic variables on hydrological cycles.

Rubber tree transpiration in the lowlands of Sumatra

AbstractThe expansion of rubber cultivation in Southeast Asia raises concerns about the integrity of the hydrological cycle. From mainland Asia, high evapotranspiration from rubber plantations was reported. Our study was conducted in the Sumatran lowlands (Indonesia), where rubber is grown by small holders under maritime climate. We assessed patterns of water use with sap flux methods, focusing on influences of tree age and size. We first tested a field measurement scheme in methodological experiments and subsequently applied it to 10 plots in monocultural rubber plantations. Among fully leaved, mature stands, maximum sap flux densities decreased with increasing tree diameter in 14‐ and 16‐year‐old plantations, but not in 7‐ and 8‐year‐old ones. Consequently, tree water use increased more steeply with increasing diameter in the younger than in the older plantations. In contrast to this, among the same five mature plantations, stand‐scale transpiration decreased with increasing mean tree diameter and height. This was due to a negative linear relationship between diameter and stand density. Among seven fully leaved plantations, stand age explained 95% of the site‐to‐site variability in transpiration. Temporally, rubber transpiration showed pronounced seasonality due to leaf shedding. Transpiration in our study was substantially lower than in rubber plantations in mainland Asia; reasons include differences in methods, management, and climate. In Sumatra, rubber may be ecohydrologically less concerning than, for example, oil palm plantations, due to low transpiration and periodical leaf shedding. Our study endorses the importance of considering age, management, climate, and species in ecohydrological assessments of tropical plantation landscapes.