Evidence from Amazonian forests is consistent with isohydric control of leaf water potential

Abstract

Climate modelling studies predict that the rain forests of the Eastern Amazon basin are likely to experience reductions in rainfall of up to 50% over the next 50-100 years. Efforts to predict the effects of changing climate, especially drought stress, on forest gas exchange are currently limited by uncertainty about the mechanism that controls stomatal closure in response to low soil moisture. At a through-fall exclusion experiment in Eastern Amazonia where water was experimentally excluded from the soil, we tested the hypothesis that plants are isohydric, that is, when water is scarce, the stomata act to prevent leaf water potential from dropping below a critical threshold level. We made diurnal measurements of leaf water potential ( Ψ Ψ Ψ Ψ l ), stomatal conductance ( g s ), sap flow and stem water potential ( Ψ Ψ Ψ Ψ stem ) in the wet and dry seasons. We compared the data with the predictions of the soil-plant-atmosphere (SPA) model, which embeds the isohydric hypothesis within its stomatal conductance algorithm. The model inputs for meteorology, leaf area index (LAI), soil water potential and soil-to-leaf hydraulic resistance ( R ) were altered between seasons in accordance with measured values. No optimization parameters were used to adjust the model. This 'mechanistic' model of stomatal function was able to explain the individual tree-level seasonal changes in water relations ( r 2 = = = = 0.85, 0.90 and 0.58 for Ψ Ψ Ψ Ψ l , sap flow and g s , respectively). The model indicated that the measured increase in R was the dominant cause of restricted water use during the dry season, resulting in a modelled restriction of sap flow four times greater than that caused by reduced soil water potential. Higher resistance during the dry season resulted from an increase in below-ground resistance (including root and soil-to-root resistance) to water flow.

Growth, leaf and stomatal traits of crabwood (Carapa guianensis Aubl.) in central Amazonia

“…Similar diurnal patterns of Ψ L between rainfall seasons suggest that g s had an important role in controlling water loss via transpiration. This is in agreement with results reported by Fisher et al (2006) for other Amazonian tree species In this experiment, the drop in Ψ L had an effect on both A max and g s , which was consistently lower in the dry season. The effect of a decline of Ψ L on A max was more pronounced in the afternoon, particularly in the dry season, as restrictions in stomatal aperture reduce carbon uptake.…”

“…Diurnal variations in Ψ L found in this experiment are similar to those observed in other studies (BARKER; PEREZ-SALICRUP, 2000;FISHER et al, 2006). On average, Ψ L did not decrease below -0.8 MPa, which appears to be the minimum critical Ψ L for this species, at least under our experimental conditions.…”

“…This makes difficult to indubitably ascribe the presence of crypts to variations in water regime during plant growth. Indeed, it has been suggested that crypts have a role in facilitating CO 2 diffusion from abaxial to adaxial palisade cells (HASSIOTOU et al, 2009). Although the correlation between S D and S S was significant (r = 0.65, p < 0.05), it seems that encrypted stomata tend to be less responsive to variation in size, due to reduced peristomatal transpiration, perhaps because in comparison with non-sunked stomata, guard cells of encrypted stomata are less prone to suffer dehydration.…”

Predicting the response of the Amazon rainforest to persistent drought conditions under current and future climates: a major challenge for global land surface models

“…The use of different WSF formulations in different land surface models (Egea et al, 2011;Zhou et al, 2013) reflects our inability to define the general behavior(s) for multi-species biomes in which the physiological processes are not yet fully understood. The use of hydrodynamic models that do not include empirical soil moisture response functions, but instead predict drought-induced stomatal closure from the simulation of hydraulic potential, in the continuum soil-plant-atmosphere, has demonstrated some promising results (Williams et al, 2001;Fisher et al, 2006Fisher et al, , 2007Zeppel et al, 2008;McDowell et al, 2013).…”

Coordination of leaf and stem water transport properties in tropical forest trees

“…In so-called isohydric species, minimum values of *P L remain essentially constant despite variation in soil water availability and atmospheric demand, whereas anisohydric species are generally characterized by increasingly negative values of minimum *P L as soil water availability declines (Turner et al 1984;Tardieu and Simonneau 1998;Bucci etal. 2005;Fisher et al 2006). A third type of plant *P regulation, isohydrodynamic, in which a constant root-to-shoot A*P is maintained seasonally, has recently been described (Franks et al 2007).…”