Sap Flux Measurements Research Articles

Biotic and Abiotic Drivers of Sap Flux in Mature Green Ash Trees (Fraxinus pennsylvanica) Experiencing Varying Levels of Emerald Ash Borer (Agrilus planipennis) Infestation

While the relationship between abiotic drivers of sap flux are well established, the role of biotic disturbances on sap flux remain understudied. The invasion of the emerald ash borer (Agrilus planipennis Fairmaire, EAB) into North America in the 1990s represents a significant threat to ash trees (Fraxinus spp.), which are a substantial component of temperate forests. Serpentine feeding galleries excavated by EAB larvae in the cambial and phloem tissue are linked to rapid tree mortality. To assess how varying levels of EAB infestation impact the plant water status and stress levels of mature green ash (Fraxinus pennsylvanica Marshall) trees, we combined tree-level sap flux measurements with leaf-level gas exchange, isotopes, morphology and labile carbohydrate measurements. Results show sap flux and whole tree water use are reduced by as much as 80% as EAB damage increases. Heavily EAB impacted trees exhibited reduced leaf area and leaf mass, but maintained constant levels of specific leaf area relative to lightly EAB-impacted trees. Altered foliar gas exchange (reduced light saturated assimilation, internal CO2 concentrations) paired with depleted foliar δ13C values of heavily EAB impacted trees point to chronic water stress at the canopy level, indicative of xylem damage. Reduced photosynthetic rates in trees more impacted by EAB likely contributed to the lack of nonstructural carbohydrate (soluble sugars and starch) accumulation in leaf tissue, further supporting the notion that EAB damages not only phloem, but xylem tissue as well, resulting in reduced water availability. These findings can be incorporated into modeling efforts to untangle post disturbance shifts in ecosystem hydrology.

Canopy transpiration of Larix principis-rupprechtii plantation and its impact factors in diffe-rent slope locations at the south side of Liupan Mountains, China.

Based on a continuous field observation in the Larix principis-rupprechtii plantation plots, located at the upper (P1), middle-upper (P2), middle (P3), middle-lower (P4), and lower (P5) positions, in a southeast-facing slope of the Xiangshuihe watershed of Liupan Mountains, China, the stem sap flow was observed with the thermal diffusivity probe method. The soil water potential and meteorological factors were monitored from May to October, 2014. We found significant differences among slope positions in the daily forest transpiration (Tr, mm·d-1), with an order of P2 (0.975)＞P4 (0.876)＞P3 (0.726)＞P1 (0.653)＞P5 (0.628). Tr was significantly positively correlated with the daily maximum temperature (Tmax), daily mean solar radiation (SR), daily mean saturated vapor pressure deficit (VPD), potential evapotranspiration (PET), and daily mean soil water potential (Ψ), but negatively correlated with the daily mean air relative humidity (RH), daily precipitation (P), and daily minimum temperature (Tmin). According to the upper boundary line ana-lysis, significant differences were found in the degree of Tr responding to each single environmental factor among slope positions. The degree of its responses gradually decreased for average daily air temperature (T), RH, VPD, PET and Ψ, whereas increased for the SR and daily average volumetric soil water content (VSM) from the upper position to the lower. Results from regression and partial correlation analysis showed that variation of Tr was mainly controlled by VPD, PET and RH in different slope positions. Tr was also strongly affected by Ψ and T in the upper-slope positions and by SR, Tmin and VSM at the lower-slope positions. Generally, the Tr difference among slope positions was a consequence of joint contributions of soil water and meteorological factors. It is necessary to consider the changes of soil water and meteorological factors in different positions along the slope when examining slope-scale or watershed-scale forest transpiration with sap flow estimated from xylem sap flux density measurements of a particular plot.

The accuracy of the thermal dissipation technique for estimating sap flow is affected by the radial distribution of conduit diameter and density

There are conflicting reports on the accuracy of the thermal dissipation probe (TDP, the Granier method) measurement using the original formula, which is widely used to estimate the transpiration of individual trees and forest stands. In this article, six woody species of three wood types were used to study a possible association between TDP measurement accuracy and wood anatomical characteristics, including the vessel diameter and density, as well as sapwood depth. We found that TDP technique with Granier’s original equation underestimated the sap flux density in six species to various degrees, dependent on conduit size and sap flux. Our calibration using two conifers with small diameters and a high density of tracheids was relatively consistent with Granier’s calibration; however, because there were larger diameters and lower densities of vessels in the two diffuse-porous species, the original calibration significantly underestimated sap flow. Two ring-porous species had the largest diameters and lowest densities of vessels. In particular, Robinia pseudoacacia possessed the shallowest sap wood depth, less than a probe length. Our calibration for the ring-porous species, especially R. pseudoacacia, deviated far from the original calibration, which mostly underestimated the sap flow. The degree of underestimation was well associated with sap wood depth and the radial diameter and density distribution of conduits. Our results demonstrated that a new calibration must be operated for each species together with the sapwood depth determination and more probes may be applied for one stem in the field to obtain the more accurate sap flux. In addition, we investigated the effects of different environmental temperature and perfusing fluid composition on the TDP-based sap flux measurement. We found that an environmental temperature reduction from 25 to 0 °C did not alter the values of the maximum temperature difference (ΔTm) between a heated probe and a reference probe when there was no sap flow, verifying that ΔTm measured at night can be used as a reference in daytime.

New design of external heat-ratio method for measuring low and reverse rates of sap flow in thin stems

Sap-flow techniques had limited application to thin-stemmed woody and herbaceous species and to diverse functional plant organs until the recent development of the external heat-ratio (EHR) method. Existing EHR techniques using miniature gauge configurations, however, are limited to thin stems with diameters of 2–5 mm. This study introduces a new design of an EHR gauge adapted to thin stems with diameters up to 15 mm and sap-flux densities <50 cm h−1. The gauge was calibrated on cut stems of the shrubs Caragana korshinskii and Salix psammophila, with the measured heat-pulse velocity (Vh) compared to gravimetric measurements of sap-flux density (Vs) under controlled conditions. A validation test was also carried out by comparing EHR method with the stem heat-balance (SHB) method through in situ and long-term monitoring of Vs on standing stems of C. korshinskii. Vh in the tested cut stems of both species was linearly correlated with Vs up to approximately 50 cm h−1 (R2 up to 0.96, P < 0.001) in a range of stem diameters of 4.1–15.6 mm. An empirical multiplier for converting the measured Vh to Vs, however, varied between the two species, 2.02 and 1.15 for C. korshinskii and S. psammophila, respectively. The EHR technique sensitively captured the diurnal dynamics of Vs in field tests, within a range from zero to nearly 30 cm h−1 on C. korshinskii stems, and hourly Vs was linearly correlated with the reference evapotranspiration (R2 = 0.70, P < 0.001) over 26 successive days without drought stress. The tested SHB method, however, poorly detected the sap-flux density, especially at low densities. The gauges for the EHR method were easy to build and capable of accurate estimating bidirectional sap flow, especially at low densities. This technique, with variable EHR gauge configurations, has broader applications than SHB methods for understanding plant-water relations in understories, shrubs and ecosystems dominated by herbage.

Comparison of water-use characteristics of landscape tree (Tabebuia argentea) and palm (Ptychosperma macarthurii) species in a tropical roof garden with implications for urban water management

In cities, landscape palms and trees are planted for scenic views and normally maintained with frequent irrigation. With similar water input, their morphological difference may lead to different water-use that varies seasonally. Thus, evaluating their water-use patterns is necessary to achieve proper urban water management. Here, we examined water-use characteristics of Tabebuia argentea and Ptychosperma macarthurii, a tree and a palm species, in a roof garden in Bangkok, Thailand. Self-constructed Granier’s probes were used for sap flux measurement and monitored for five months in 2016. Results showed that palm transpired 3–4 times higher water than tree daily. With frequent watering, tree was more sensitive to increasing vapor pressure deficit (D) than palm on rain-free days. When rainfall occurred, however, both species became less sensitive to D and continued consuming more water at high D. Extending the analysis to include a hypothetical warming climate with +4.8 °C temperature, we found that tree saved more water than palm on rain-free days. However, daily transpiration of both species increased similarly by 14–18% on rainy days. These results imply that Tabebuia argentea may provide ecosystem benefits by conservatively using water under drought and decelerating runoff from the garden when storms come. In contrast, Ptychosperma macarthurii may provide ecosystem disservice with high water-use in dry season but may slow down runoff in wet season and therefore benefiting downstream ecosystems. These findings are useful for selective planting associated with future changes in precipitation patterns, leading to sustainable urban water management.

Effects of soil water decline on diurnal and seasonal variations in sap flux density for differently aged Japanese cypress (Chamaecyparis obtusa) trees

The effects of soil drought on transpiration are often neglected when predicting transpiration for forests in humid regions under the influence of the Asian monsoon. These effects have indeed been neglected for Japanese cypress, Chamaecyparis obtusa, a major plantation species in Japan and the surrounding area, probably because previous studies have reported no clear effects of soil drought on transpiration for Japanese cypress forests. However, a few studies have reported an apparent reduction in transpiration with soil drought for young Japanese cypress forests. It remains unclear whether such a reduction in transpiration is limited to young Japanese cypress forests or if it is not uncommon for mature Japanese cypress forests, which occupy a large area in Japan. To clarify this point, we conducted sap flux measurements in a year with soil drought on three differently aged Japanese cypress stands including mature (43 years old) and relatively young (23 and 26 years old) trees. In a diurnal time scale, a cross correlation analysis of sap flux density (Fd) and vapor pressure deficit (VPD) showed that the time lags between Fd and VPD were 1-3 h in dry soil conditions. These were larger than those of wet soil conditions (<1 h) for all sample trees. Fd at a given VPD in dry soil conditions was smaller than that in wet soil conditions for all sample trees; a 28%–63% reduction in the rate of change in Fd was observed under dry soil conditions. Because our results were obtained when the non-exceedance probability of recorded monthly precipitation was 9%–18%, the results suggest the need to consider the effects of soil drought more extensively. Those effects should be considered for not only relatively young but also mature Japanese cypress when predicting diurnal and seasonal patterns of transpiration in years with soil drought, and when predicting inter-annual patterns of transpiration for Japanese cypress despite humid temperate climate.

The influence of bamboo culm water content on sap flux measurements with thermal dissipation probes: observations and modeling

Water content fluctuations in bamboo culms significantly influence sap flux measurements with thermal dissipation probes, as indicated and quantified by experimental, monitoring and model analyses. Bamboos and other plants may substantially rely on stem water storage for transpiration. Fluctuations in wood water content (θ wood) may lead to errors when estimating transpiration based on sap flux (J s) measurements with the widely used thermal dissipation probe (TDP) method. To test the effects of θ wood on J s, we conducted a culm dehydration experiment, monitored bamboos with TDPs, and implemented a steady-state thermal model. Based on the model simulation, a mathematical correction method was built. Central to the calculation of J s, and thus a major potential source of error, is the maximum temperature difference between probes (ΔT max) which is often referred to as ‘zero flow’ conditions. In the culm dehydration experiment, we observed that ΔT max decreased when θ wood increased. In long-term field monitoring, ΔT max decreased when soil moisture content increased, potentially indicating changes in θ wood and a seasonal decrease in stem water storage. The steady-state model reproduced the θ wood to ΔT max relationship of the dehydration experiment and underlined a considerable sensitivity of J s estimates to θ wood. Fluctuations in θ wood may lead to a substantial underestimation of J s, and subsequently of transpiration, in commonly applied estimation schemes. However, our model results suggest that such underestimation can be quantified and subsequently corrected for with our correction equations when key wood properties are known. Our study gives insights into the relationship between θ wood and TDP-derived J s and examines potential estimation biases.

Calibration and comparison of thermal dissipation, heat ratio and heat field deformation sap flow probes for diffuse-porous trees

Sap flow probes are routinely used in forest and horticulture hydrology for estimating tree water use. This requires unbiased measurements when upscaling from tree to stand level, but accuracy and comparability of different thermometric methods have been questioned. Three sap flow measuring techniques were compared against gravimetric flow measurement in cut stem segments: ‘Granier-type’ thermal dissipation probes (TDP; three different sensor types), the heat field deformation method (HFD), and the heat ratio method (HRM). For the empirical methods (TDP and HFD), new calibration parameters were estimated using a nonlinear hierarchical modelling approach. 66 stem segments from five temperate, diffuse-porous tree species (9–16cm stem diameter, 100cm stem length) were exposed to a wide range of flux densities by applying subatmospheric pressure (−50 to −650hPa) analogous to natural flow conditions in the field.All TDP probes underestimated flux density by 23–45% when calculated with Granier's original calibration parameters, with the deviation increasing with flux rate. The accuracy was significantly improved by estimating new calibration parameters, especially for probes differing from Granier's original sensor design. Species-specific parameters further improved accuracy, although the species differences might partially be explained by variation in the observed ranges of sap flux. The HFD sensor overestimated gravimetric flow by ∼11%; empirical calibration did not improve its accuracy compared to the manufacturer's equation. At low to medium flow rates, the HRM system achieved higher accuracy than the other probes (0.8% underestimation), while performing poorly at high flux rates under our measurement settings (energy input of 25J). Both for TDP and HFD sensors, we observed a surprisingly large variability in calibration parameters between different stems of the same species.We conclude that (i) TDP and HFD sensors require species-specific calibration to measure sap flux with high accuracy, (ii) the original Granier equation cannot be used for TDP probes with deviating design, and (iii), at low to medium flow rates, the highest accuracy can be achieved with HRM sensors. Our results help to increase the accuracy of tree sap flow measurements with thermal dissipation probes, and to assess various levels of errors related to the different thermometric methods. This is important when synthesizing forest transpiration data on regional and global scales.

Transpiration of trees in a cool temperate forest on Mt. Aso, Japan: comparison of model simulation and measurements

AbstractIn the large caldera of Mt. Aso, Japan, artificial grasslands were converted into forests of different species due to the decline of the livestock industry in this region. These changes in species composition are thought to have changed not only the transpiration rates (E), but also the responses in E to variations in environmental conditions. For three introduced forest types, we monitored E using sap flux sensors and computed E with a multilayer model parameterized by independently obtained leaf‐scale ecophysiological traits. Modeled E replicated the time series of measured E reasonably well, but did not reproduce a decrease in E lasting 20 days after a short rainless period in conifer plantation. Mean leaf‐scale stomatal conductance of two deciduous broadleaved tree species was as low as that of trees under dry conditions in other studies, possibly an adaptation for the avoidance of excessive E during rainless periods. These results suggest that land use change influenced E and its response to the rainfall patterns of the region and that drought plays an important role in influencing species‐specific E characteristics in this rainy region. The multilayer model in combination with sap flux measurements was found to be a useful tool not only for the extrapolation of E, but also for the detection of the unexpected events, as long as the measured parameter values capture the species‐specific seasonality in leaf ecophysiological traits.

Tree soil water uptake and transpiration in mono-cultural and jungle rubber stands of Sumatra

Rubber tree mono-cultural plantations are expanding. Also, there is an increasing search for ‘green’ rubber production. Rubber tree cultivation in stands with admixed, spontaneously established native trees, referred to as jungle rubber, has a long tradition on Sumatra. For rubber tree monocultures on mainland Asia, concerns have been raised because of potentially very high tree transpiration rates. The objectives of our study were to analyze tree water use rates and tree soil water uptake depths in mono-cultural and jungle rubber stands with a focus on the role of tree diameter. Sap flux measurements suggest similar water use rates for rubber trees in the two cultivation systems. Stand-level transpiration in jungle rubber was 27% higher than in rubber monocultures, which was related to higher stand densities in jungle rubber stands. A water stable isotope (δ18O and δD) approach suggests different soil water uptake depths for the rubber trees in the two cultivation systems. In a relatively dry period, the main tree water uptake in the monoculture was relatively close to the soil surface, whereas rubber trees in jungle rubber stands mainly took up water from deeper soil strata; here the native trees had their main uptake depth relatively close to the soil surface. This pattern indicates plasticity in rubber tree water uptake and points to competitive displacement. Across rubber trees in both cultivation systems and also among the native trees, there was a clear relationship between tree diameter and soil water uptake depth: bigger trees tended to take up soil water closer to the soil surface. Diameter and density regulation by thinning of big native trees thus appears as a potential management option for influencing water uptake in jungle rubber stands in favor of rubber trees.

The effects of probe misalignment on sap flux density measurements and in situ probe spacing correction methods

The accurate measurement of sap flow is important for estimating transpiration. This paper is focused on the errors in sap flux density (J) caused by probe misalignment and on how to reduce these errors. We found that probe misalignment caused errors in the heat pulse velocity (Vh). Furthermore, probe misalignment had a large influence on thermal diffusivity (κ) and the determination of zero flow points. Assuming uniform axial thermal diffusivity and zero sap flow conditions, we developed a new in situ probe spacing correction method based on three temperature sensor measurements inside the same temperature probe, which can be used for correcting both linear and nonlinear misalignments. Laboratory experimental results verified that our method could significantly reduce the errors in determinations of J by using the Compensation Heat Pulse velocity (CHP) method, the Heat Ratio (HR) method and the T-max method.

Combining sap flow measurements and modelling to assess water needs in an oasis farmland shelterbelt of Populus simonii Carr in Northwest China

Farmland shelterbelts provide an ecological protection screen for an oasis but exhibit high mortality in the face of water shortage. It is necessary to understand farmland shelterbelt tree transpiration under different levels of water stress and stand ages for proper management. Sap flux measurement techniques and models are among the most useful method to detect water stress and to evaluate plant water consumption. The usefulness of both methods decreases, however, when applied to species, such as Populus simonii Carr, that have an outstanding tolerance to drought and a remarkable capacity to take up water from drying soils. Our hypothesis is that analysis using simultaneous measurements of sap flow and models in the same trees is useful for assessing the irrigation needs in farmland shelterbelts. To test our hypothesis, we analysed the relationships between canopy transpiration, canopy conductance, relative extractable water and atmospheric factors in a farmland shelterbelt and evaluated the effectiveness of the model. Measurements were made during one growing season. The time courses of sap flow measured and modelled on days of contrasting weather and soil water conditions were analysed to evaluate the usefulness of the method to assess the crop water needs. We calculated the daily tree water consumption from sap flow measurements and the parameterized modified Jarvis-Stewart model, and we evaluated the model’s usefulness to assess the final water needs under water stress and stand ages for farmland shelterbelt irrigation. The transpiration decreased as the soil drought increased, and it increased as the atmospheric drought increased. The time course of the water needs showed that the occurrence of water stress in the farmland shelterbelt trees had a large impact on their water consumption, which increased as the water stress decreased, following the equation y=1/[1+e−60.67×(REWx−0.402)]. The simultaneous use of modelling and tree structural data increased the reliability of assessing water needs from youth to maturity. A similar analysis with the water consumption values, from which stand age values were derived, showed that water needs increased with the tree age following the equation y=847−844/[1+(x/87.9)1.9]. We conclude that compared to the use of sap flow records alone, the simultaneous use of sap flow records and model values provides more detailed information to assess water needs in a farmland shelterbelt, which has an important significance for farmland shelterbelt protection.

Sap flux – a real time assessment of health status in Norway spruce

ABSTRACTTop dieback of Norway spruce (Picea abies), triggered by drought in 2004–2006, has been observed in Southeast Norway and trees died within four years after appearance of the first symptoms. The aim of our study was to use sap flux measurements as a diagnostic method for assessment of tree vitality. We used the heat field deformation method to monitor the sap flux density (SFD) in four pairs of healthy and declining trees in situ. To provide retrospective information on hydraulic performance of the trees we took samples for wood anatomical analysis. After felling the trees we used the modified differential translucence method (MDT) as a proxy for the SFD measurements. Healthy trees had three times higher SFD values as declining trees. In some healthy trees we detected decreasing SFD with time. The MDT method agreed with the SFD measurements. In conclusion, we detected sap flux dysfunction in declining trees and showed that the SFD reduction may occur during a short period, prior to occurrence of any visual symptoms. We suggest incorporating the SFD measurements into the repertoire of diagnostic tools in forest pathology.

Water Use Patterns of Four Tropical Bamboo Species Assessed with Sap Flux Measurements.

Bamboos are grasses (Poaceae) that are widespread in tropical and subtropical regions. We aimed at exploring water use patterns of four tropical bamboo species (Bambusa vulgaris, Dendrocalamus asper, Gigantochloa atroviolacea, and G. apus) with sap flux measurement techniques. Our approach included three experimental steps: (1) a pot experiment with a comparison of thermal dissipation probes (TDPs), the stem heat balance (SHB) method and gravimetric readings using potted B. vulgaris culms, (2) an in situ calibration of TDPs with the SHB method for the four bamboo species, and (3) field monitoring of sap flux of the four bamboo species along with three tropical tree species (Gmelina arborea, Shorea leprosula, and Hevea brasiliensis) during a dry and a wet period. In the pot experiment, it was confirmed that the SHB method is well suited for bamboos but that TDPs need to be calibrated. In situ, species-specific parameters for such calibration formulas were derived. During field monitoring we found that some bamboo species reached high maximum sap flux densities. Across bamboo species, maximal sap flux density increased with decreasing culm diameter. In the diurnal course, sap flux densities in bamboos peaked much earlier than radiation and vapor pressure deficit (VPD), and also much earlier than sap flux densities in trees. There was a pronounced hysteresis between sap flux density and VPD in bamboos, which was less pronounced in trees. Three of the four bamboo species showed reduced sap flux densities at high VPD values during the dry period, which was associated with a decrease in soil moisture content. Possible roles of internal water storage, root pressure and stomatal sensitivity are discussed.

Water scarcity and oil palm expansion: social views and environmental processes

Merten, J., A. Röll, T. Guillaume, A. Meijide, S. Tarigan, H. Agusta, C. Dislich, C. Dittrich, H. Faust, D. Gunawan, J. Hein, . Hendrayanto, A. Knohl, Y. Kuzyakov, K. Wiegand, and D. Hölscher. 2016. Water scarcity and oil palm expansion: social views and environmental processes. Ecology and Society 21(2):5.http://dx.doi.org/10.5751/ES-08214-210205

Drought-induced stomatal closure probably cannot explain divergent white spruce growth in the Brooks Range, Alaska, USA.

Increment cores from the boreal forest have long been used to reconstruct past climates. However, in recent years, numerous studies have revealed a deterioration of the correlation between temperature and tree growth that is commonly referred to as divergence. In the Brooks Range of northern Alaska, USA, studies of white spruce (Picea glauca) revealed that trees in the west generally showed positive growth trends, while trees in the central and eastern Brooks Range showed mixed and negative trends during late 20th century warming. The growing season climate of the eastern Brooks Range is thought to be drier than the west. On this basis, divergent tree growth in the eastern Brooks Range has been attributed to drought stress. To investigate the hypothesis that drought-induced stomatal closure can explain divergence in the Brooks Range, we synthesized all of the Brooks Range white spruce data available in the International Tree Ring Data Bank (ITRDB) and collected increment cores from our primary sites in each of four watersheds along a west-to-east gradient near the Arctic treeline. For cores from our sites, we measured ring widths and calculated carbon isotope discrimination (δ13C), intrinsic water-use efficiency (iWUE), and needle intercellular CO2 concentration (C(i)) from δ13C in tree-ring alpha-cellulose. We hypothesized that trees exhibiting divergence would show a corresponding decline in δ13C, a decline in C(i), and a strong increase in iWUE. Consistent with the ITRDB data, trees at our western and central sites generally showed an increase in the strength of the temperature-growth correlation during late 20th century warming, while trees at our eastern site showed strong divergence. Divergent tree growth was not, however, associated with declining δ13C. Meanwhile, estimates of C(i) showed a strong increase at all of our study sites, indicating that more substrate was available for photosynthesis in the early 21st than in the early 20th century. Our results, which are corroborated by measurements of xylem sap flux density, needle gas exchange, and measurements of growth and δ13C along moisture gradients within each watershed, suggest that drought-induced stomatal closure is probably not the cause of 20th century divergence in the Brooks Range.

Effect of irrigation on sap flux density variability and water use estimate in cherry (Prunus avium) for timber production: Azimuthal profile, radial profile and sapwood estimation

Information on tree water use in plantations for high quality wood is scarce, thus studies are needed to properly estimate the irrigation demand of these plantations. Plant water use estimation with sap flow sensors has been used extensively. However, biases in tree sap flow estimate can arise from variations on radial and azimuthal profiles of sap flux density and also from the sapwood area considered for the up-scaling from sap flux density to sap flux. This work aimed to (1) study the spatial variations of sap flux density in cherry trees in a timber orientated plantation, (2) compare several methods to estimate sapwood depth in cherry trees and (3) to evaluate the effect of drip irrigation on these factors. The results showed that most of the studied trees had decreasing radial sap flux density profiles with depth as expected. However, the three irrigated trees of bigger sizes still showed high sap flux densities in their inner tissues, at contrast with the rest of the trees and especially with the non irrigated ones of similar size with values close to 20% of the sap flux density measured at 1cm depth from cambium. On the other hand, the different methods tested to estimate sapwood depth gave significantly different results and only the two methods of visual identification in wood cores based on color change and measurements of sap flux densities along the xylem radius may be considered suitable for scaling purposes. Moreover, azimuthal variation pattern was found to be random in all the studied trees, and the ranking between the aspects (north, south, east and west) was not affected by either drip irrigation or sun exposition, and thus measuring sap flux density in any particular aspect has been shown to be suitable to estimate the overall tree sap flux. We conclude that more studies are necessary to properly assess the radial profile of sap flux density, especially when considering the high sap flux density in the inner tissues of the three bigger irrigated trees as compared to the other trees, and also how this pattern seemed to indicate sapwood depths values very contrasted to the ones estimated from color change in wood cores.

Modeling Potential Impacts of Planting Palms or Tree in Small Holder Fruit Plantations on Ecohydrological Processes in the Central Amazon

Native fruiting plants are widely cultivated in the Amazon, but little information on their water use characteristics can be found in the literature. To explore the potential impacts of plantations on local to regional water balance, we studied plant water use characteristics of two native fruit plants commonly occurring in the Amazon region. The study was conducted in a mixed fruit plantation containing a dicot tree species (Cupuaçu, Theobroma grandiflorum) and a monocot palm species (Açai, Euterpe oleracea) close to the city of Manaus, in the Central Amazon. Scaling from sap flux measurements, palms had a 3.5-fold higher water consumption compared to trees with a similar diameter. Despite the high transpiration rates of the palms, our plantation had only one third of the potential water recycling capacity of natural forests in the area. Converting natural forest into such plantations will thus result in significantly higher runoff rates.

Interrelationships among dry season leaf fall, leaf flush and transpiration: insights from sap flux measurements in a tropical dry deciduous forest

AbstractWe measured the sap flux densities of 12 deciduous trees in a tropical dry deciduous forest with high seasonality of available water located in Cambodia and evaluated the seasonal trends in transpiration and leaf phenology. For all trees, the minimum transpiration was recorded in the middle of the dry season, and almost all trees restarted transpiration before the first monsoon rain. The occurrence of the ‘paradox’ of leaf phenology was confirmed. During the dry season, transpiration was controlled by leaf phenology and decreased with an increase in the duration of the leafless period. In contrast, during the wet season, daily changes in transpiration were determined by changes in evaporative demand. Transpiration during the dry season accounted for more than 30% of the annual total among trees, and at the stand scale, the dry season contribution was 38%. The dry season transpiration was not negligible for the water balance in this ecosystem. The soil water condition in the shallow layer, where the main root system is located, was not the source of transpiration during the dry season. This implied that the root probably extended to a deep layer and absorbed water. The relationships between the mean canopy stomatal conductance and vapour pressure deficit revealed that most trees were isohydric. Isohydric behaviour controlling stomatal openness to avoid xylem hydraulic failure was also confirmed at the stand scale and was advantageous for these trees, allowing them to continue transpiring under the high evaporative demand during the dry season. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Oil palm water use: calibration of a sap flux method and a field measurement scheme.

Oil palm (Elaeis guineensis Jacq.) water use was assessed by sap flux density measurements with the aim to establish the method and derive water-use characteristics. Thermal dissipation probes were inserted into leaf petioles of mature oil palms. In the laboratory, we tested our set-up against gravimetric measurements and derived new parameters for the original calibration equation that are specific to oil palm petioles. In the lowlands of Jambi, Indonesia, in a 12-year-old monoculture plantation, 56 leaves on 10 palms were equipped with one sensor per leaf. A 10-fold variation in individual leaf water use among leaves was observed, but we did not find significant correlations to the variables trunk height and diameter, leaf azimuthal orientation, leaf inclination or estimated horizontal leaf shading. We thus took an un-stratified approach to determine an appropriate sampling design to estimate stand transpiration (Es, mm day(-1)) rates of oil palm. We used the relative standard error of the mean (SEn, %) as a measure for the potential estimation error of Es associated with sample size. It was 14% for a sample size of 13 leaves to determine the average leaf water use and four palms to determine the average number of leaves per palm. Increasing these sample sizes only led to minor further decreases of the SEn of Es. The observed 90-day average of Es was 1.1 mm day(-1) (error margin ± 0.2 mm day(-1)), which seems relatively low, but does not contradict Penman-Monteith-derived estimates of evapotranspiration. Examining the environmental drivers of Es on an intra-daily scale indicates an early, pre-noon maximum of Es rates (11 am) due to a very sensitive reaction of Es to increasing vapor pressure deficit in the morning. This early peak is followed by a steady decline of Es rates for the rest of the day, despite further rising levels of vapor pressure deficit and radiation; this results in pronounced hysteresis, particularly between Es and vapor pressure deficit.