Sap Flux Measurements Research Articles

Exposure to moderate concentrations of tropospheric ozone impairs tree stomatal response to carbon dioxide

With rising concentrations of both atmospheric carbon dioxide (CO2) and tropospheric ozone (O3), it is important to better understand the interacting effects of these two trace gases on plant physiology affecting land-atmosphere gas exchange. We investigated the effect of growth under elevated CO2 and O3, singly and in combination, on the primary short-term stomatal response to CO2 concentration in paper birch at the Aspen FACE experiment. Leaves from trees grown in elevated CO2 and/or O3 exhibited weaker short-term responses of stomatal conductance to both an increase and a decrease in CO2 concentration from current ambient level. The impairement of the stomatal CO2 response by O3 most likely developed progressively over the growing season as assessed by sap flux measurements. Our results suggest that expectations of plant water-savings and reduced stomatal air pollution uptake under rising atmospheric CO2 may not hold for northern hardwood forests under concurrently rising tropospheric O3.

A model of heat transfer in sapwood and implications for sap flux density measurements using thermal dissipation probes

A variety of thermal approaches are used to estimate sap flux density in stems of woody plants. Models have proved valuable tools for interpreting the behavior of heat pulse, heat balance and heat field deformation techniques, but have seldom been used to describe heat transfer dynamics for the heat dissipation method. Therefore, to better understand the behavior of heat dissipation probes, a model was developed that takes into account the thermal properties of wood, the physical dimensions and thermal characteristics of the probes, and the conductive and convective heat transfer that occurs due to water flow in the sapwood. Probes were simulated as aluminum tubes 20 mm in length and 2 mm in diameter, whereas sapwood, heartwood and bark each had a density and water fraction that determined their thermal properties. Base simulations assumed a constant sap flux density with sapwood depth and no wounding or physical disruption of xylem beyond the 2 mm diameter hole drilled for probe installation. Simulations across a range of sap flux densities showed that the dimensionless quantity k [defined as (ΔT(m) -ΔT)/ΔT, where ΔT(m) is the temperature differential (ΔT) between the heated and unheated probe under zero-flow conditions] was dependent on the thermal conductivity of the sapwood. The relationship between sap flux density and k was also sensitive to radial gradients in sap flux density and to xylem disruption near the probe. Monte Carlo analysis in which 1000 simulations were conducted while simultaneously varying thermal conductivity and wound diameter revealed that sap flux density and k showed considerable departure from the original calibration equation used with this technique. The departure was greatest for variation in sap flux density typical of ring-porous species. Depending on the specific combination of thermal conductivity and wound diameter, use of the original calibration equation resulted in an 81% under- to 48% overestimation of sap flux density at modest flux rates. Future studies should verify these simulations and assess their utility in estimating sap flux density for this widely used technique.

Transpiration of urban forests in the Los Angeles metropolitan area

Despite its importance for urban planning, landscape management, and water management, there are very few in situ estimates of urban-forest transpiration. Because urban forests contain an unusual and diverse mix of species from many regions worldwide, we hypothesized that species composition would be a more important driver of spatial variability in urban-forest transpiration than meteorological variables in the Los Angeles (California, USA) region. We used constant-heat sap-flow sensors to monitor urban tree water use for 15 species at six locations throughout the Los Angeles metropolitan area. For many of these species no previous data on sap flux, water use, or water relations were available in the literature. To scale sap-flux measurements to whole trees we conducted a literature survey of radial trends in sap flux across multiple species and found consistent relationships for angiosperms vs. gymnosperms. We applied this relationship to our measurements and estimated whole-tree and plot-level transpiration at our sites. The results supported very large species differences in transpiration, with estimates ranging from 3.2 +/- 2.3 kg x tree(-1) x d(-1) in unirrigated Pinus canariensis (Canary Island pine) to 176.9 +/- 75.2 kg x tree(-1) x d(-1) in Platanus hybrida (London planetree) in the month of August. Other species with high daily transpiration rates included Ficus microcarpa (laurel fig), Gleditsia triacanthos (honeylocust), and Platanus racemosa (California sycamore). Despite irrigation and relatively large tree size, Brachychiton populneas (kurrajong), B. discolor (lacebark), Sequoia sempervirens (redwood), and Eucalyptus grandis (grand Eucalyptus) showed relatively low rates of transpiration, with values < 45 kg x tree(-1) x d(-1). When scaled to the plot level, transpiration rates were as high as 2 mm/d for sites that contained both species with high transpiration rates and high densities of planted trees. Because plot-level transpiration is highly dependent on tree density, we modeled transpiration as a function of both species and density to evaluate a likely range of values in irrigated urban forests. The results show that urban forests in irrigated, semi-arid regions can constitute a significant use of water, but water use can be mitigated by appropriate selection of site, management method, and species.

Root functioning in tropical high-elevation forests: Environmental vs. biological control of root water absorption

Lowered temperatures may reduce the root water uptake of tropical trees at high elevations through several mechanisms; however, field studies to test their relevance are lacking. We measured sap flux density ( J) in small-diameter tree roots across a 2000-m elevation transect in a tropical mountain forest for quantifying the effects of temperature ( T a ), VPD and soil moisture ( θ) on root water flow and uptake at different elevations. Recently developed miniature heat balance-sap flow gauges were applied to roots of about 10 mm in diameter in mountain forest stands at 1050, 1890 and 3060 m a.s.l. in the Ecuadorian Andes and the measured flow was related to anatomical properties of the root xylem. Between 1050 and 3060 m, mean J decreased to about a third. VPD was the most influential environmental factor controlling J at 1050 and 1890 m, while T a was the key determinant at 3060 m. Large vessels were absent in the root xylem of high-elevation trees which resulted in a 10-fold decrease of theoretical hydraulic conductivity ( k h theor) between 1050 and 3060 m. We conclude that both physical limitations (reduced VPD, increased viscosity of water) and biological constraints (large decrease of k h theor) result in a significantly reduced J and root water uptake of the trees in high-elevation tropical forests.

Responses of transpiration and canopy conductance to partial defoliation of Eucalyptus globulus trees

Partial defoliation has been shown to affect the water relations and transpiration (gas exchange) of plants. Over one growing season, the water relations in response to partial (∼45%) defoliation were examined in four-year-old Eucalyptus globulus trees in southern Australia. Daily maximum transpiration rates ( E max), maximum canopy conductance ( G C max ), and diurnal patterns of tree water-use were measured over a period of 215 days using the heat-pulse technique in adjacent control (non-defoliated) and defoliated trees. Sap-flux measurements were used to estimate canopy conductance and soil-to-leaf hydraulic conductance ( K P); leaf water potential ( Ψ) and climate data were also collected. Following the removal of the upper canopy layer, defoliated trees exhibited compensatory responses in transpiration rate and canopy conductance of the remaining foliage. Defoliated E. globulus had similar predawn but higher midday Ψ l, transpiration rates ( E), canopy conductance ( G C) and K P compared to the non-defoliated controls, possibly in response to increased water supply per unit leaf area demonstrated by higher midday Ψ l. Higher E in defoliated E. globulus trees was the result of higher G C in the morning and early afternoon. This paper also incorporates the cumulative effect of defoliation, in a phenomenological model of maximum canopy conductance of E. globulus. These results contribute to a mechanistic understanding of plant responses to defoliation, in particular the often observed up-regulation of photosynthesis that also occurs in response to defoliation.

Vertical structure of evapotranspiration at a forest site (a case study)

The components of ecosystem evapotranspiration of a Norway spruce forest ( Picea abies L.) as well as the vertical structure of canopy evapotranspiration were analyzed with a combination of measurements and models for a case study of 5 days in September 2007. Eddy-covariance and sap flux measurements were performed at several heights within the canopy at the FLUXNET site Waldstein–Weidenbrunnen (DE-Bay) in the Fichtelgebirge mountains in Germany. Within and above canopy fluxes were simulated with two stand-scale models, the 1D multilayer model ACASA that includes a third-order turbulence closure and the 3D model STANDFLUX. The soil and understory evapotranspiration captured with the eddy-covariance system in the trunk space constituted 10% of ecosystem evapotranspiration measured with the eddy-covariance system above the canopy. A comparison of transpiration measured with the sap flux technique and inferred from below and above canopy eddy-covariance systems revealed higher estimates from eddy-covariance measurements than for sap flux measurements. The relative influences of possible sources of this mismatch, such as the assumption of negligible contribution of evaporation from intercepted water, and differences between the eddy-covariance flux footprint and the area used for scaling sap flux measurements, were discussed. Ecosystem evapotranspiration as well as canopy transpiration simulated with the two models captured the dynamics of the measurements well, but slightly underestimated eddy-covariance values. Profile measurements and models also gave us the chance to assess in-canopy profiles of canopy evapotranspiration and the contributions of in-canopy layers. For daytime and a coupled or partly coupled canopy, mean simulated profiles of both models agreed well with eddy-covariance measurements, with a similar performance of the ACASA and the STANDFLUX model. Both models underestimated profiles for nighttime and decoupled conditions. During daytime, the upper half of the canopy contributed approximately 80% to canopy evapotranspiration, whereas during nighttime the contribution shifted to lower parts of the canopy.

Calibration of thermal dissipation sap flow probes for ring- and diffuse-porous trees

Thermal dissipation probes (the Granier method) are routinely used in forest ecology and water balance studies to estimate whole-tree transpiration. This method utilizes an empirically derived equation to measure sap flux density, which has been reported as independent of wood characteristics. However, errors in calculated sap flux density may occur when large gradients in sap velocity occur along the sensor length or when sensors are inserted into non-conducting wood. These may be conditions routinely associated with ring-porous species, yet there are few cases in which the original calibration has been validated for ring-porous species. We report results from laboratory calibration measurements conducted on excised stems of four ring-porous species and two diffuse-porous species. Our calibration results for ring-porous species were considerably different compared with the original calibration equation. Calibration equation coefficients obtained in this study differed by as much as two to almost three orders of magnitude when compared with the original equation of Granier. Coefficients also differed between ring-porous species across all pressure gradient conditions considered; however, no differences between calibration slopes were observed for data collected within the range of expected in situ pressure gradients. In addition, dye perfusions showed that in three of the four ring-porous species considered, active sapwood was limited to the outermost growth ring. In contrast, our calibration results for diffuse-porous species showed generally good agreement with the empirically derived Granier calibration, and dye perfusions showed that active sapwood was associated with many annual growth rings. Our results suggest that the original calibration of Granier is not universally applicable to all species and xylem types and that previous estimates of absolute rates of water use for ring-porous species obtained using the original calibration coefficients may be associated with substantial error.

Biological and environmental controls on tree transpiration in a suburban landscape

Tree transpiration provides a variety of ecosystem services in urban areas, including amelioration of urban heat island effects and storm water management. Tree species vary in the magnitude and seasonality of transpiration owing to differences in physiology, response to climate, and biophysical characteristics, thereby complicating efforts to manage evapotranspiration at city scales. We report sap flux measurements during the 2007 and 2008 growing seasons for dominant tree species in a suburban neighborhood of Minneapolis–Saint Paul, Minnesota, USA. Evergreen needleleaf trees had significantly higher growing season means and annual transpiration per unit canopy area (1.90 kg H2O m−2 d−1 and 307 kg H2O m−2 yr−1, respectively) than deciduous broadleaf trees (1.11 kg H2O m−2 d−1 and 153 kg H2O m−2 yr−1, respectively) because of a smaller projected canopy area (31.1 and 73.6 m2, respectively), a higher leaf area index (8.8 and 5.5 m2 m−2, respectively), and a longer growth season (8 and 4 months, respectively). Measurements also showed patterns consistent with the species' differences in xylem anatomy (conifer, ring porous, and diffuse porous). As the growing season progressed, conifer and diffuse porous genera had increased stomatal regulation to high vapor pressure deficit, while ring porous genera maintained greater and more constant stomatal regulation. These results suggest that evaporative responses to climate change in urban ecosystems will depend in part on species composition. Overall, plant functional type differences in canopy structure and growing season length were most important in explaining species' differences in midsummer and annual transpiration, offering an approach to predicting the evapotranspiration component of urban water budgets.

Stand-scale transpiration estimates in a Moso bamboo forest: (I) Applicability of sap flux measurements

The applicability of sap flux ( F d ) measurements to bamboo forests has not been studied. This study was undertaken to establish an optimal and effective design for stand-scale transpiration ( E) estimates in a Moso bamboo forest. To this aim, we validated F d measurements in Moso bamboos in a cut bamboo experiment. In addition, we analyzed how sample sizes affect the reliability of E estimates calculated from F d and conducting culm area ( A S_ b ). In the cut bamboo experiments, we found that F d measurement using a 10 mm probe was a valid means of determining the water-use behavior of a Moso bamboo, although a specific correction was needed. Furthermore, we calculated E from stand A S_ b ( A S_stand ) and mean stand F d ( J S ). Employing Monte Carlo analysis, we examined potential errors associated with sample size in E, A S_stand , and J S using an original dataset with A S_ b and F d measured for 40 and 16 individuals, respectively. Consequently, we determined the optimal sample size for both A S_stand and J S estimates as 11. The optimal sample sizes for J S were almost the same under different vapor pressure deficit and soil moisture conditions. The optimal sample size for J S at the study site was less than that of a coniferous plantation in the same region probably owing to small individual-to-individual variations in sap flux in the Moso bamboo forest. Our study concludes that sap flux measurements are an applicable technique for assessing water use in Moso bamboo forests.

Stand-scale transpiration estimates in a Moso bamboo forest: II. Comparison with coniferous forests

In western Japan, Moso bamboo ( Phyllostachys pubescens) forests have been expanding by replacing surrounding vegetation such as coniferous plantation forests and natural broadleaved forests. It has been speculated that the replacement of surrounding vegetation by bamboo forests could alter the vegetation water cycle and available water resources. We quantified stand-scale transpiration ( E) in a bamboo forest on the basis of sap-flux measurements and compared the E value with values for coniferous forests. The annual E was estimated to be 567 mm. Seasonal trends in E primarily corresponded to seasonal trends in the vapor pressure deficit. Annual E for the bamboo forest was higher than that for the coniferous forests by 12% of annual precipitation ( P). This difference in annual E is comparable with the difference in annual interception evaporation ( I) relative to P between bamboo and coniferous forests; previous studies reported lower I for bamboo forests by ∼10% of P. Thus, the sum of E and I was comparable for bamboo and coniferous forests. As this study is the first measuring E of bamboo forests, further studies are required to examine the generality of our results.

Estimating water use of a black locust plantation by the thermal dissipation probe method in the semiarid region of Loess Plateau, China

Black locust (Robinia pseudoacacia) is a major reforestation species in the semiarid region in the Loess Plateau of China. There has been increasing concern about the sustainability of the plantations because of their possible high water-use. This study was, accordingly, undertaken to quantify the stand-scale water use of a middle-aged black locust plantation in the region. The thermal dissipation probe method was applied to 27 trees to measure sap flux densities in an experimental plot during the growing season of 2008. The monoculture stand has a basal area of 23.3 m2 ha−1 and a maximum plant area index (PAI) of 2.89. Sapwood areas were estimated by use of a regressive relationship with the diameter at breast height (DBH) for scaling up of stand transpiration. The results showed that DBH could be a good predictor of sapwood area of individual trees. The diurnal cycles of average sap flux densities differed among DBH classes. Daily transpiration can be predicted from mean daily daytime vapor pressure deficit (VPDm) using a fitted exponential saturation model. Model variables were different among seasons, probably owing to different soil water conditions and leaf phenology. By using the derived model for each month, stand canopy transpiration over the growing season was estimated to be 73.8 mm, with an average daily value of 0.41 mm day−1 and a maximum of 0.89 mm day−1. The relatively small estimates of stand transpiration might be attributed to low PAI and sap wood area of the middle-aged stand.

A comparison of sap flux density using thermal dissipation, heat pulse velocity and heat field deformation methods

A laboratory test and field evaluation were conducted to determine the accuracy of the three commonly used techniques for measuring sap flux density in trees: heat pulse velocity, thermal dissipation and heat field deformation. In the laboratory test a constant flow rate of water was maintained through freshly cut stem segments of diffuse-porous Fagus grandifolia trees with mean sapwood depths of 4.02 ± 0.14 and 7.44 ± 0.51 cm for sample trees with stem diameter at breast height of 15 and 21 cm, respectively. The three sensor types were measured simultaneously and compared against gravimetric measurements. All three techniques substantially underestimated sap flux density. On average the actual sap flux density was underestimated by 35% using heat pulse velocity (with wound correction), 46% using heat field deformation and 60% using thermal dissipation. These results were consistent across sap flux densities ranging from 5 to 80 cm 3 cm −2 h −1. Heat pulse velocity measurements were more variable than those of the other two techniques, and the least accurate at low sap flux densities. An error analysis was conducted on all parameters of the equations used with each technique. That analysis indicated that each technique has unique sensitivities to errors in parameter estimates which need to be taken into consideration. Except for the use of heat, the three techniques are quite different and there appeared to be no single reason why the methods underestimated actual sap flux density, but rather there were likely multiple errors that compounded to reduce the overall accuracy of each technique. Field measurements supported the relative sensor performance observed in the laboratory. Applying a sensor-specific correction factor based on the laboratory test to the field data produced similar estimates of sap flux density from all three techniques. We conclude that a species-specific calibration is necessary when using any of these techniques to insure that accurate estimates of sap flux density are obtained, at least until a physical basis for an error correction can be proposed.

Contribution of competition for light to within‐species variability in stomatal conductance

Sap flux (JS) measurements were collected across two stands dominated by either trembling aspen or sugar maple in northern Wisconsin. Observed canopy transpiration (EC‐obs) values derived from JS were used to parameterize the Terrestrial Regional Ecosystem Exchange Simulator ecosystem model. Modeled values of stomatal conductance (GS) were used to determine reference stomatal conductance (GSref), a proxy for GS that removes the effects of temporal responses to vapor pressure deficit (D) on spatial patterns of GS. Values of GSref were compared to observations of soil moisture, several physiological variables, and a competition index (CI) derived from a stand inventory, to determine the underlying cause of observed variability. Considerable variability in GSref between individual trees was found, with values ranging from 20 to 200 mmol m−2 s−1 and 20 to 100 mmol m−2 s−1 at the aspen and maple stands, respectively. Model‐derived values of GSref and a sensitivity to D parameter (m) showed good agreement with a known empirical relationship for both stands. At both sites, GSref did not vary with topographic position, as indicated by surface soil moisture. No relationships were observed between GSref and tree height (HT), and a weak correlation with sapwood area (AS) was only significant for aspen. Significant nonlinear inverse relationships between GSref and CI were observed at both stands. Simulations with uniform reductions in incident photosynthetically active radiation (Q0) resulted in better agreement between observed and simulated EC. Our results suggest a link between photosynthesis and plant hydraulics whereby individual trees subject to photosynthetic limitation as a result of competitive shading exhibit a dynamic stomatal response resulting in a more conservative strategy for managing hydrologic resources.

Inter- and intra-specific variation in nocturnal water transport in Eucalyptus

To identify environmental and biological drivers of nocturnal vapour exchange, we quantified intra-annual, intra- and inter-specific variation in nocturnal water transport among ecologically diverse Eucalyptus species. We measured sap flux (J(s)) and leaf physiology (leaf surface conductance (g(s)), transpiration (E) and water potential (Psi(l))) in three to five trees of eight species. Over 1 year, nocturnal J(s) (J(s,n)) contributed 5-7% of total J(s) in the eight species. The principal environmental driver of J(s,n) was the product of atmospheric vapour pressure deficit (D) and wind speed (U). Selected observations suggest that trees with higher proportions of young foliage may exhibit greater J(s,n) and nocturnal g(s) (g(s,n)). Compared with other tree taxa, nocturnal water use in Eucalyptus was relatively low and more variable within than between species, suggesting that (i) Eucalyptus as a group exerts strong nocturnal stomatal control over water loss and (ii) prediction of nocturnal flux in Eucalyptus may depend on simultaneous knowledge of intra-specific tree traits and nocturnal atmospheric conditions.

Effet de la longueur de la portée des sondes de dissipation thermique de type Granier sur des mesures de densité de flux de sève

• Granier-type thermal dissipation sensors measure sap flux density (u ) by using the temperature difference between the heater and the reference probe. To detect u correctly, heat must not be transferred to the reference probe by thermal conduction. The distance across which heat can be transferred by conduction is important for the span length of a sensor and spacing of a number of sensors. • To validate span lengths and spacing of sensors, we used numerical simulations to calculate the potential distance across which heat can be transferred by conduction. We compared measurements with an original and a modified sensor for a Japanese red pine (Pinus densiflora ) from December 2004 to May 2005. The span length of the original and the modified sensor is 15 and 4 cm, respectively. • Numerical simulations showed that span length and spacing of Granier sensors should be more than 10 cm for trees in which u ceases for a few hours before the predawn period. The modified sensor underestimated u by 18–46% in winter (December–March) because its reference temperature was increased by heat transferred by conduction. The modified sensor measured u correctly in warm seasons, and only underestimated the annual amount of transpiration by 6%.

RADIAL SAP FLUX PROFILES AND BEYOND: AN EASY SOFTWARE ANALYSIS TOOL

Sap flow measurements are nowadays used worldwide to validate forest-ecosystem estimates of water use and storage. Tall trees with large radial variability in sap flux across their sapwood still present a challenge in scaling single point sap flux measurements to whole-tree sap flow. Assessment of the radial sap flux profile is, hence, necessary to make a precise scaling possible. A sensor useful for assessing the radial sap flux profile in trees is the heat field deformation (HFD) sensor. This sensor records changes in the heat field around a linear heater at different radial positions in the xylem and links the heat field deformation to sap flow. The technical construction of the HFD-sensor, however, requires the configuration of multiple channels on the data logger (usually 8 to 12 channels per sensor or 48 to 72 channels per one “cross section” of the tree trunk when using 6 sensors in one cross section). Logging each channel with a small scan rate (e.g. 5 min) quickly results in a massive amount of data. The task of analysing these large data sets can become difficult and cumbersome with spreadsheet programs and the large file sizes often prevent efficient data interpretation. The objective of this paper was therefore to develop a software tool for easy analysis of HFD-data sets, including very large ones. Its primary use is to visualise in a fast way the measured radial sap flux profile in both 2D and 3D. The computed results can be saved in a compact form, drastically reducing the resulting file size in comparison to the output generated using classical spreadsheet programs. But the features of the software tool go beyond the analysis of radial sap flux profiles. Due to easy determination, K-values inherent to the sap flux density calculations can now be used to derive stem specific properties (e.g. relative water content) or raw signals can be used to quantify the sap flow index.

Stem‐mediated hydraulic redistribution in large roots on opposing sides of a Douglas‐fir tree following localized irrigation

*Increasing evidence about hydraulic redistribution and its ecological consequences is emerging. Hydraulic redistribution results from an interplay between competing plant and soil water potential gradients. In this work, stem-mediated hydraulic redistribution was studied in a 53-year-old Douglas-fir tree during a period of drought. *Sap flux density measurements using the heat field deformation method were performed at four locations: in two large opposing roots and on two sides of the tree stem. Hydraulic redistribution was induced by localized irrigation on one of the measured roots, creating heterogeneous soil water conditions. *Stem-mediated hydraulic redistribution was detected during night-time conditions when water was redistributed from the wet side of the tree to the nonirrigated dry side. In addition to stem-mediated hydraulic redistribution, bidirectional flow in the dry root was observed, indicating radial sectoring in the xylem. *It was observed that, through stem-mediated hydraulic redistribution, Douglas-fir was unable to increase its transpiration despite the fact that sufficient water was available to one part of the root system. This resulted from the strong water potential gradient created by the dry soil in contact with the nonirrigated part of the root system. A mechanism of stem-mediated hydraulic redistribution is proposed and its possible implications are discussed.

Species-specific tree water use characteristics in reforestation stands in the Philippines

Reforestation in the tropics offers opportunities for restoration of ecosystem services, biodiversity conservation, wood production and carbon sequestration. The potentially high water use rates associated with reforestations will however require that a balance is found between wood production or carbon fixation and the use of water resources. Species selection has been put forward as a measure to control tree water use of reforested stands but species-specific information is at present very limited. We studied ten broad-leaved tree species in 12-year-old reforestations on Leyte island, the Philippines. Objectives were (1) to identify environmental and tree structural controls on water use and (2) to test for differences in water use characteristics across species. The studied species included eight native species, four of which were dipterocarps, and two species not native to the region but widely planted in the tropics. Per species five well-exposed individuals were equipped with thermal dissipation probes to measure sap flux density. A Jarvis-type model, with global radiation and vapour pressure deficit as explanatory variables, was applied at the tree level and successfully predicted measured sap flux densities. The model parameters suggested significant species differences in maximum sap flux density, differing twofold across species, as well as in sap flux response to environmental variables. Maximum transpiration rates per unit crown area varied up to a factor of four across species. Maximum water use rates were strongly related to tree diameter ( R adj 2 = 0.65 ) and weaker, though significant, to estimated aboveground biomass ( R adj 2 = 0.34 ). Despite its size dependence, tree water use rates differed about twofold for a given diameter or biomass depending on species, which leads us to conclude that species selection indeed offers opportunities to control tree water use in reforested stands.

Wide‐area estimates of saltcedar (Tamarix spp.) evapotranspiration on the lower Colorado River measured by heat balance and remote sensing methods

AbstractIn many places along the lower Colorado River, saltcedar (Tamarix spp) has replaced the native shrubs and trees, including arrowweed, mesquite, cottonwood and willows. Some have advocated that by removing saltcedar, we could save water and create environments more favourable to these native species. To test these assumptions we compared sap flux measurements of water used by native species in contrast to saltcedar, and compared soil salinity, ground water depth and soil moisture across a gradient of 200–1500 m from the river's edge on a floodplain terrace at Cibola National Wildlife Refuge (CNWR). We found that the fraction of land covered (fc) with vegetation in 2005–2007 was similar to that occupied by native vegetation in 1938 using satellite‐derived estimates and reprocessed aerial photographs scaled to comparable spatial resolutions (3–4 m). We converted fc to estimates of leaf area index (LAI) through point sampling and destructive analyses (r2 = 0·82). Saltcedar LAI averaged 2·54 with an fc of 0·80, and reached a maximum of 3·7 with an fc of 0·95. The ranges in fc and LAI are similar to those reported for native vegetation elsewhere and from the 1938 photographs over the study site. On‐site measurements of water use and soil and aquifer properties confirmed that although saltcedar grows in areas where salinity has increased much better than native shrubs and trees, rates of transpiration are similar. Annual water use over CNWR was about 1·15 m year−1. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Variability in radial sap flux density patterns and sapwood area among seven co-occurring temperate broad-leaved tree species

Forest transpiration estimates are frequently based on xylem sap flux measurements in the outer sections of the hydro-active stem sapwood. We used Granier's constant-heating technique with heating probes at various xylem depths to analyze radial patterns of sap flux density in the sapwood of seven broad-leaved tree species differing in wood density and xylem structure. Study aims were to (1) compare radial sap flux density profiles between diffuse- and ring-porous trees and (2) analyze the relationship between hydro-active sapwood area and stem diameter. In all investigated species except the diffuse-porous beech (Fagus sylvatica L.) and ring-porous ash (Fraxinus excelsior L.), sap flux density peaked at a depth of 1 to 4 cm beneath the cambium, revealing a hump-shaped curve with species-specific slopes. Beech and ash reached maximum sap flux densities immediately beneath the cambium in the youngest annual growth rings. Experiments with dyes showed that the hydro-active sapwood occupied 70 to 90% of the stem cross-sectional area in mature trees of diffuse-porous species, whereas it occupied only about 21% in ring-porous ash. Dendrochronological analyses indicated that vessels in the older sapwood may remain functional for 100 years or more in diffuse-porous species and for up to 27 years in ring-porous ash. We conclude that radial sap flux density patterns are largely dependent on tree species, which may introduce serious bias in sap-flux-derived forest transpiration estimates, if non-specific sap flux profiles are assumed.