Explore the vast range of titles available.

Predicting responses of tropical forests to climate change-type drought is challenging because of high species diversity. Detailed characterization of tropical tree hydraulic physiology is necessary to evaluate community drought vulnerability and improve model parameterization.Here, we measured xylem hydraulic conductivity (hydraulic efficiency), xylem vulnerability curves (hydraulic safety), sapwood pressure-volume curves (drought avoidance) and wood density on emergent branches of 14 common species of Eastern Amazonian canopy trees in Paracou, French Guiana across species with the densest and lightest wood in the plot. Our objectives were to evaluate relationships among hydraulic traits to identify strategies and test the ability of easy-to-measure traits as proxies for hard-to-measure hydraulic traits.Xylem efficiency was related to capacitance, sapwood water content and turgor loss point, and other drought avoidance traits, but not to xylem safety (P-50). Wood density was correlated (r = -0.57 to -0.97) with sapwood pressure-volume traits, forming an axis of hydraulic strategy variation.In contrast to drier sites where hydraulic safety plays a greater role, tropical trees in this humid tropical site varied along an axis with low wood density, high xylem efficiency and high capacitance at one end of the spectrum, and high wood density and low turgor loss point at the other.

Woody stems comprise a large biological carbon fraction and determine water transport between roots and leaves; their structure and function can influence both carbon and hydrological cycles. While angiosperm wood anatomy and density determine hydraulic conductivity and mechanical strength, little is known about interrelations across many species. We compiled a global data set comprising two anatomical traits for 3005 woody angiosperms: mean vessel lumen area (Ā) and number per unit area (N). From these, we calculated vessel lumen fraction (F = ĀN) and size to number ratio (S = Ā/N), a new vessel composition index. We examined the extent to which F and S influenced potential sapwood specific stem conductivity (KS) and wood density (D; dry mass/fresh volume). F and S varied essentially independently across angiosperms. Variation in KS was driven primarily by S, and variation in D was virtually unrelated to F and S. Tissue density outside vessel lumens (DN) must predominantly influence D. High S should confer faster KS but incur greater freeze-thaw embolism risk. F should also affect KS, and both F and DN should influence mechanical strength, capacitance, and construction costs. Improved theory and quantification are needed to better understand ecological costs and benefits of these three distinct dimensions.

Drought‐induced mortality and regional dieback of woody vegetation are reported from numerous locations around the world. Yet within any one site, predicting which species are most likely to survive global change‐type drought is a challenge. We studied the diversity of drought survival traits of a community of 15 woody plant species in a desert‐chaparral ecotone. The vegetation was a mix of chaparral and desert shrubs, as well as endemic species that only occur along this margin. This vegetation boundary has large potential for drought‐induced mortality because nearly all species are at the edge of their range. Drought survival traits studied were vulnerability to drought‐induced xylem cavitation, sapwood capacitance, deciduousness, photosynthetic stems, deep roots, photosynthetic responses to leaf water potential and hydraulic architecture. Drought survival strategies were evaluated as combinations of traits that could be effective in dealing with drought. The large variation in seasonal predawn water potential of leaves and stem xylem ranged from −6·82 to −0·29 MPa and −6·92 to −0·27 MPa, respectively. The water potential at which photosynthesis ceases ranged from −9·42 to −3·44 MPa. Architecture was a determinant of hydraulic traits, with species supporting large leaf area per sapwood area exhibiting high rates of water transport, but also xylem that is vulnerable to drought‐induced cavitation. Species with more negative midday leaf water potential during the growing season also showed access to deeper water sources based on hydrogen isotope analysis. Drought survival mechanisms comprised of drought deciduousness, photosynthetic stems, tolerance of low minimum seasonal tissue water potential and vulnerability to drought‐induced xylem cavitation thus varied orthogonally among species, and promote a diverse array of drought survival strategies in an arid ecosystem of considerable floristic complexity.

Background and Aims Plants rely on water storage capacity to increase accessibility of water for transpiration, reduce competition for water with neighboring plants, and buffer water supply during dry periods. The resulting benefits, typically a decrease in plant water stress and increase in productivity, are highly climate dependent and vary with soil moisture, vapor pressure deficit, and solar radiation. This paper analyzes the effects of plant water storage capacity on the relationship between soil moisture and carbon assimilation in woody plants. Methods A resistance-capacitance model is used to examine the role of plant water storage at various soil moisture levels. Hydraulic traits are co-varied according to empirical relationships, and effects of sapwood volume and wood density on carbon assimilation are explored. The time scales of plant water storage and withdrawal are analyzed as a function of plant hydraulic capacitance, water storage capacity, and resistance to transport between water storage tissue and xylem. Results The effects of plant water storage on carbon assimilation are found to depend strongly on soil moisture levels. The theoretically optimal sapwood volume lies near naturally occurring ranges and increases with increasing soil moisture. The theoretically optimal wood density also lies within expected ranges and decreases with increasing soil moisture. Conclusions A large portion of sapwood volume appears to be justified by its role in buffering diurnal variability in evaporative demand. The outlined coordination between soil moisture and optimal hydraulic traits is consistent with observed increases in sapwood capacitance and decreases in wood density across increasing rainfall gradients. This coordination provides support for the drought-tolerance vs. drought-avoidance hypothesis.

Taking advantage of novel IoT technologies, a new multifunctional device, the “TreeTalker”, was developed to monitor real-time ecophysical and biological parameters of individual trees, as well as climatic variables related to their surrounding environment, principally, air temperature and air relative humidity. Here, IoT applied to plant ecophysiology and hydrology aims to unravel the vulnerability of trees to climatic stress via a single tree assessment at costs that enable massive deployment. We present the performance of the TreeTalker to elucidate the functional relation between the stem water content in trees and respective internal/external (stem hydraulic activity/abiotic) drivers. Continuous stem water content records are provided by an in-house-designed capacitance sensor, hosted in the reference probe of the TreeTalker sap flow measuring system, based on the transient thermal dissipation (TTD) method. In order to demonstrate the capability of the TreeTalker, a three-phase experimental process was performed including (1) sensor sensitivity analysis, (2) sensor calibration, and (3) long-term field data monitoring. A negative linear correlation was demonstrated under temperature sensitivity analysis, and for calibration, multiple linear regression was applied on harvested field samples, explaining the relationship between the sample volumetric water content and the sensor output signal. Furthermore, in a field scenario, TreeTalkers were mounted on adult Fagus sylvatica L. and Quercus petraea L. trees, from June 2020 to October 2021, in a beech-dominated forest near Marburg, Germany, where they continuously monitored sap flux density and stem volumetric water content (stem VWC). The results show that the range of stem VWC registered is highly influenced by the seasonal variability of climatic conditions. Depending on tree characteristics, edaphic and microclimatic conditions, variations in stem VWC and reactions to atmospheric events occurred. Low sapwood water storage occurs in response to drought, which illustrates the high dependency of trees on stem VWC under water stress. Consistent daily variations in stem VWC were also clearly detectable. Stem VWC constitutes a significant portion of daily transpiration (using TreeTalkers, up to 4% for the beech forest in our experimental site). The diurnal–nocturnal pattern of stem VWC and sap flow revealed an inverse relationship. Such a finding, still under investigation, may be explained by the importance of water recharge during the night, likely due to sapwood volume changes and lateral water distribution rather than by a vertical flow rate. Overall, TreeTalker demonstrated the potential of autonomous devices for monitoring sap density and relative stem VWC in the field of plant ecophysiology and hydrology.

Research Highlights: novel fast and easily assessable proxies for vulnerability to cavitation of conifer sapwood are proposed that allow reliable estimation at the species level. Background and Objectives: global warming calls for fast and easily applicable methods to measure hydraulic vulnerability in conifers since they are one of the most sensitive plant groups regarding drought stress. Classical methods to determine P12, P50 and P88, i.e., the water potentials resulting in 12, 50 and 88% conductivity loss, respectively, are labour intensive, prone to errors and/or restricted to special facilities. Vulnerability proxies were established based on empirical relationships between hydraulic traits, basic density and sapwood anatomy. Materials and Methods: reference values for hydraulic traits were obtained by means of the air injection method on six conifer species. Datasets for potential P50 proxies comprised relative water loss (RWL), basic density, saturated water content as well as anatomical traits such as double wall thickness, tracheid lumen diameter and wall/lumen ratio. Results: our novel proxy P25W, defined as 25% RWL induced by air injection, was the most reliable estimate for P50 (r = 0.95) and P88 (r = 0.96). Basic wood density (r = −0.92), tangential lumen diameters in earlywood (r = 0.88), wall/lumen ratios measured in the tangential direction (r = −0.86) and the number of radial cell files/mm circumference (CF/mm, r = −0.85) were also strongly related to P50. Moreover, CF/mm was a very good predictor for P12 (r = −0.93). Conclusions: the proxy P25W is regarded a strong phenotyping tool for screening conifer species for vulnerability to cavitation assuming that the relationship between RWL and conductivity loss is robust in conifer sapwood. We also see a high potential for the fast and easily applicable proxy CF/mm as a screening tool for drought sensitivity and for application in dendroecological studies that investigate forest dieback.

The capacitative release of water from sapwood allows photosynthesis to continue for longer into dry periods, both diurnally and seasonally. However, costs of high capacitance include increased vulnerability to xylem cavitation. The degree of reliance on stored water is predicted to differ among environments as a result of this trade‐off. Xylem water potential and sapwood capacitance were measured on 32 evergreen sclerophyll shrub and tree species in eastern Australia, sampled from four sites contrasting in soil nutrients and rainfall. Capacitance calculated over species' typical shoot water potential operating range was threefold higher for species from high compared to low‐rainfall sites, and 1·5‐fold higher for species from high compared to low‐nutrient sites. To determine whether these site differences were related to extrinsic (e.g. water availability) or intrinsic (e.g. species anatomical construction) factors, we calculated capacitance at two common operating ranges; that is, the mean range in water potential observed for low‐rainfall species (ΔΨₗₒw ᵣₐᵢₙ) and the mean range for high‐rainfall species (ΔΨₕᵢgₕ ᵣₐᵢₙ). While no difference was seen between low‐ and high‐rainfall species in release of stored water across ΔΨₕᵢgₕ ᵣₐᵢₙ, across ΔΨₗₒw ᵣₐᵢₙ, the high‐rainfall species released 38% more stored water than low‐rainfall species. Presumably these differences reflect underlying differences in anatomy, such as wood density, which was lower in high‐rainfall species. These results accord with predictions that (i) species from wetter sites exhibit less negative stem water potentials and high sapwood capacitance, enabling them to maintain function under variable conditions characterized by many short, dry periods, while (ii) species from low‐rainfall sites have wood anatomies conferring tolerance to very low water potentials, with low sapwood capacitance, enabling them to survive longer through unpredictable and extended periods of low rainfall. The finding that the degree to which species rely on stem‐stored water varies with site rainfall suggests that changes in drought regimes (e.g. incidence, duration and severity) under future climates could differentially affect species according to the capacitance properties of their woody tissues.

Stomatal regulation of transpiration constrains leaf water potential (ΨL) within species-specific ranges that presumably avoid excessive tension and embolism in the stem xylem upstream. However, the hydraulic resistance of leaves can be highly variable over short time scales, uncoupling tension in the xylem of leaves from that in the stems to which they are attached. We evaluated a suite of leaf and stem functional traits governing water relations in individuals of 11 lowland tropical forest tree species to determine the manner in which the traits were coordinated with stem xylem vulnerability to embolism. Stomatal regulation of ΨL was associated with minimum values of water potential in branches (Ψbr) whose functional significance was similar across species. Minimum values of Ψbr coincided with the bulk sapwood tissue osmotic potential at zero turgor derived from pressure-volume curves and with the transition from a linear to exponential increase in xylem embolism with increasing sapwood water deficits. Branch xylem pressure corresponding to 50% loss of hydraulic conductivity (P ₅₀) declined linearly with daily minimum Ψbr in a manner that caused the difference between Ψbr and P ₅₀ to increase from 0.4 MPa in the species with the least negative Ψbr to 1.2 MPa in the species with the most negative Ψbr. Both branch P ₅₀ and minimum Ψbr increased linearly with sapwood capacitance (C) such that the difference between Ψbr and P ₅₀, an estimate of the safety margin for avoiding runaway embolism, decreased with increasing sapwood C. The results implied a trade-off between maximizing water transport and minimizing the risk of xylem embolism, suggesting a prominent role for the buffering effect of C in preserving the integrity of xylem water transport. At the whole-tree level, discharge and recharge of internal C appeared to generate variations in apparent leaf-specific conductance to which stomata respond dynamically.

Leaf and stem functional traits related to plant water relations were studied for six congeneric species pairs, each composed of one tree species typical of savanna habitats and another typical of adjacent forest habitats, to determine whether there were intrinsic differences in plant hydraulics between these two functional types. Only individuals growing in savanna habitats were studied. Most stem traits, including wood density, the xylem water potential at 50% loss of hydraulic conductivity, sapwood area specific conductivity, and leaf area specific conductivity did not differ significantly between savanna and forest species. However, maximum leaf hydraulic conductance$(K_{leaf} )$and leaf capacitance tended to be higher in savanna species. Predawn leaf water potential and leaf mass per area were also higher in savanna species in all congeneric pairs. Hydraulic vulnerability curves of stems and leaves indicated that leaves were more vulnerable to drought-induced cavitation than terminal branches regardless of genus. The midday$K_{leaf} $values estimated from leaf vulnerability curves were very low implying that daily embolism repair may occur in leaves. An electric circuit analog model predicted that, compared to forest species, savanna species took longer for their leaf water potentials to drop from predawn values to values corresponding to 50% loss of$K_{leaf} $or to the turgor loss points, suggesting that savanna species were more buffered from changes in leaf water potential. The results of this study suggest that the relative success of savanna over forest species in savanna is related in part to their ability to cope with drought, which is determined more by leaf than by stem hydraulic traits. Variation among genera accounted for a large proportion of the total variance in most traits, which indicates that, despite different selective pressures in savanna and forest habitats, phylogeny has a stronger effect than habitat in determining most hydraulic traits.

Heat and stable isotope tracers were used to study axial and radial water transport in relation to sapwood anatomical characteristics and internal water storage in four canopy tree species of a seasonally dry tropical forest in Panama. Anatomical characteristics of the wood and radial profiles of sap flow were measured at the base, upper trunk, and crown of a single individual of Anacardium excelsum, Ficus insipida, Schefflera morototoni, and Cordia alliodora during two consecutive dry seasons. Vessel lumen diameter and vessel density did not exhibit a consistent trend axially from the base of the stem to the base of the crown. However, lumen diameter decreased sharply from the base of the crown to the terminal branches. The ratio of vessel lumen area to sapwood cross-sectional area was consistently higher at the base of the crown than at the base of the trunk in A. excelsum, F. insipida and C. alliodora, but no axial trend was apparent in S. morototoni. Radial profiles of the preceding wood anatomical characteristics varied according to species and the height at which the wood samples were obtained. Radial profiles of sap flux density measured with thermal dissipation sensors of variable length near the base of the crown were highly correlated with radial profiles of specific hydraulic conductivity ($k_{\\text{s}}$) calculated from xylem anatomical characteristics. The relationship between sap flux density and$k_{\\text{s}}$was species-independent. Deuterium oxide (D₂O) injected into the base of the trunk of the four study trees was detected in the water transpired from the upper crown after only 1 day in the 26-m-tall C. alliodora tree, 2 days in the 28-m-tall F. insipida tree, 3 days in the 38-m-tall A. excelsum tree, and 5 days in the 22-m-tall S. morototoni tree. Radial transport of injected D₂O was detected in A. excelsum, F. insipida and S. morototoni, but not C. alliodora. The rate of axial D₂O transport, a surrogate for maximum sap velocity, was positively correlated with the predicted sapwood$k_{\\text{s}}$and with tree height normalized by the relative diurnal water storage capacity. Residence times for the disappearance of the D₂O tracer in transpired water ranged from 2 days in C. alliodora to 22 days in A. excelsum and were positively correlated with a normalized index of diurnal water storage capacity. Capacitive exchange of water between stem storage compartments and the transpiration stream thus had a profound influence on apparent rates of axial water transport, the magnitude of radial water movement, and the retention time in the tree of water taken up by the roots. The inverse relationship between internal water exchange capacity and$k_{\\text{s}}$was consistent with a trade-off contributing to stability of leaf water status through highly efficient water transport at one extreme and release of stored water at the other extreme.