Responses of stomatal conductance (gs) to increasing vapour pressure deficit (D) generally follow an exponential decrease described equally well by several empirical functions. However, the magnitude of the decrease ‐ the stomatal sensitivity ‐ varies considerably both within and between species. Here we analysed data from a variety of sources employing both porometric and sap flux estimates of gs to evaluate the hypothesis that stomatal sensitivity is proportional to the magnitude of gs at low D (£ 1k Pa). To test this relationship we used the function gs = gsref ‐ m ·l nD where m is the stomatal sensitivity and gsref = gs at D = 1 kPa. Regardless of species or methodology, m was highly correlated with gsref (average r 2 = 0·75) with a slope of approximately 0·6. We demonstrate that this empirical slope is consistent with the theoretical slope derived from a simple hydraulic model that assumes stomatal regulation of leaf water potential. The theoretical slope is robust to deviations from underlying assumptions and variation in model parameters. The relationships within and among species are close to theoretical predictions, regardless of whether the analysis is based on porometric measurements of gs in relation to leaf-surface D (Ds), or on sap flux-based stomatal conductance of whole trees (GSi), or stand-level stomatal conductance (GS) in relation to D. Thus, individuals, species, and stands with high stomatal conductance at low D show a greater sensitivity to D, as required by the role of stomata in regulating leaf water potential.

/pdf/survey-and-synthesis-of-intra-and-interspecific-variation-in-4kvj0q6kxg.pdf

Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit

Weighing lysimeters, large-tree potometers, ventilated chambers, radioisotopes, stable isotopes and an array of heat balance/heat dissipation methods have been used to provide quantitative estimates of whole-tree water use. A survey of 52 studies conducted since 1970 indicated that rates of water use ranged from 10 kg day(-1) for trees in a 32-year-old plantation of Quercus petraea L. ex Liebl. in eastern France to 1,180 kg day(-1) for an overstory Euperua purpurea Bth. tree growing in the Amazonian rainforest. The studies included in this survey reported whole-tree estimates of water use for 67 species in over 35 genera. Almost 90% of the observations indicated maximum rates of daily water use between 10 and 200 kg day(-1) for trees that averaged 21 m in height. The thermal techniques that made many of these estimates possible have gained widespread acceptance, and energy-balance, heat dissipation and heat-pulse systems are now routinely used with leaf-level measurements to investigate the relative importance of stomatal and boundary layer conductances in controlling canopy transpiration, whole-tree hydraulic conductance, coordinated control of whole-plant water transport, movement of water to and from sapwood storage, and whole-plant vulnerability of water transport to xylem cavitation. Techniques for estimating whole-tree water use complement existing approaches to calculating catchment water balance and provide the forest hydrologist with another tool for managing water resources. Energy-balance, heat dissipation and heat-pulse methods can be used to compare transpiration in different parts of a watershed or between adjacent trees, or to assess the contribution of transpiration from overstory and understory trees. Such studies often require that rates of water use be extrapolated from individual trees to that of stands and plantations. The ultimate success of this extrapolation depends in part on whether data covering short time sequences can be applied to longer periods of time. We conclude that techniques for estimating whole-tree water use have provided valuable tools for conducting basic and applied research. Future studies that emphasize the use of these techniques by both tree physiologists and forest hydrologists should be encouraged.

/pdf/a-review-of-whole-plant-water-use-studies-in-tree-cx4nvivue8.pdf

A review of whole-plant water use studies in tree.

Stem water storage capacity and diurnal patterns of water use were studied in five canopy trees of a seasonal tropical forest in Panama. Sap flow was measured simultaneously at the top and at the base of each tree using constant energy input thermal probes inserted in the sapwood. The daily stem storage capacity was calculated by comparing the diurnal patterns of basal and crown sap flow. The amount of water withdrawn from storage and subsequently replaced daily ranged from 4 kg d–1 in a 0·20-m-diameter individual of Cecropia longipes to 54 kg d–1 in a 1·02-m-diameter individual of Anacardium excelsum, representing 9–15% of the total daily water loss, respectively. Ficus insipida, Luehea seemannii and Spondias mombin had intermediate diurnal water storage capacities. Trees with greater storage capacity maintained maximum rates of transpiration for a substantially longer fraction of the day than trees with smaller water storage capacity. All five trees conformed to a common linear relationship between diurnal storage capacity and basal sapwood area, suggesting that this relationship was species-independent and size-specific for trees at the study site. According to this relationship there was an increment of 10 kg of diurnal water storage capacity for every 0·1 m2 increase in basal sapwood area. The diurnal withdrawal of water from, and refill of, internal stores was a dynamic process, tightly coupled to fluctuations in environmental conditions. The variations in basal and crown sap flow were more synchronized after 1100 h when internal reserves were mostly depleted. Stem water storage may partially compensate for increases in axial hydraulic resistance with tree size and thus play an important role in regulating the water status of leaves exposed to the large diurnal variations in evaporative demand that occur in the upper canopy of seasonal lowland tropical forests.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.1998.00273.x

Stem water storage and diurnal patterns of water use in tropical forest canopy trees

The empirical calibration of Granier-type heat dissipation sap flow probes that relate temperature difference (DeltaT) to sap velocity (v) was reevaluated in stems of three tropical tree species. The original calibration was confirmed when the entire heated probe was in contact with conducting xylem, but mean v was underestimated when part of the probe was in contact with nonconducting xylem or bark. Analysis of the effects of nonuniform sap velocity profiles on heat dissipation estimates showed that errors increased as v and the proportion of the probe in nonconducting wood increased. If half of a 20-mm probe is in sapwood with a v of 0.15 mm s(-1) and the other half is in nonconducting wood, then mean v for the whole probe can be underestimated by as much as 50%. A correction was developed that can be used if the proportion of the probe in nonconducting wood is known. Even with the entire heated probe in contact with conducting xylem, v would be underestimated when radial velocity gradients are present. In this case, the error would be smaller except when velocity gradients are very steep, as can occur in species with ring-porous wood anatomy. Errors occur because the relationship between DeltaT and v is nonlinear. Mean DeltaT along the probe is therefore not a measure of mean v, and users of heat dissipation probes should not assume that v is integrated along the length of the probe. The same type of error can occur when DeltaT is averaged through time while v is changing, but the error is small unless there are sudden, step changes between zero and high sap velocity. It is recommended that relatively short probes (20 mm or less) be used and that probes longer than the depth of conducting sapwood be avoided. Multiple probes inserted to a range of depths should be used in situations where steep gradients in v are expected. If these conditions are met, heat dissipation probes remain useful and widely applicable for measuring sap flow in woody stems.

Potential errors in measurement of nonuniform sap flow using heat dissipation probes

The main impacts of riparian vegetation on hydrological processes are briefly reviewed in order to highlight needs and perspectives for research and management goals. This review is based upon three distinct influences of riparian vegetation on hydrological processes: (i) the control of runoff, i.e. the physical impact of living and dead plants on hydraulics, (ii) the impact of plant physiology on water uptake, storage and return to the atmosphere, and (iii) the impact of riparian vegetation functioning on water quality. Riparian vegetation influences runoff through complex hydraulic interactions during baseflows as well as overbank flows. The contribution of fine vegetational structures to landscape hydrological roughness needs to be considered in relation to the spatial complexity (patchiness, vertical stratification, rhizosphere) and temporal variability (phenology, succession) of plant communities. With the exception of some woody species, the uptake, storage and return of water to the atmosphere is poorly known for riparian communities, and therefore the assessment of the regional hydrological importance of the riparian corridor remains difficult to estimate. Although better understood than the above two influences of riparian vegetation on hydrological processes, there are still a number of unresolved issues concerning the role of riparian vegetation in controlling water quality. In particular, little is known about the coupling of microbial and vegetational functions in nutrient cycling and the dynamics of carbon release from coarse and fine plant debris. The influence of vegetation complexity and plant diversity on both qualitative and quantitative aspects of water cycling remains an important area for future research. Fundamental and management issues are identified in relation to the use of riparian vegetation as a model and as a tool.

Impacts of riparian vegetation on hydrological processes

The transpiration, sap flow, stomatal conductance and water relations ofPinus pinaster were determined during spring and summer in a 64-year-old stand in Ribatejo (Portugal). The transpiration of the pine canopy was determined from sap flow or eddy covariance techniques. Canopy conductance values (g c) were estimated from inversion methods using eddy covariance or sap flow data, respectively, and from scaling-up methods using stomatal conductance values measured in the field and leaf area index (LAI) values. The transpiration was closely controlled by the stomatal conductance of pines (Ω was 0.05-0.15). For wet soil conditions, the various estimates ofg c showed reasonable agreement.g c peaked in the morning at 0.01 m×s-1, exhibited a midday depression and showed a secondary peak in late afternoon. This behaviour could be predicted simply on the basis of the stomatal sensitivity to air vapour pressure deficit. On a seasonal basis, monthly average values ofg c decreased from 4×10-3 m×s-1 in spring to 1.7×10-3 m·s-1 in late summer. Accordingly, the transpiration peaked at 3 mm×d-1 on wet soil in May. It decreased progressively during the summer drought to 0.8 mm×d-1 at the end of August. The minimal value of needle water potential was maintained at -1.9 MPa but predawn values decreased from -0.6 MPa in May to -0.9 MPa in July. It may have reached lower values in August. The amount of water stored in the trunk accounted for a 12% (10 kg×tree-1×day-1) of the daily transpiration in spring. The storage capacity of the canopy was within the same order of magnitude. The trunk storage increased to 25% (13 kg×tree-1×day-1) of the daily transpiration at the end of summer under drought conditions. The sap flow beneath the crown lagged accordingly behind transpiration with a time constant estimated between 26 min in spring and 40 min at the end of summer.

P Roumagnac

Papers

Transpiration of a 64-year-old maritime pine stand in Portugal : 1. Seasonal course of water flux through maritime pine.