Apparent Controls on Leaf Conductance by Soil Water Availability and via Light‐Acclimation of Foliage Structural and Physiological Properties in a Mixed Deciduous, Temperate Forest

Abstract

Controls on leaf stomatal conductances imposed by soil water availability and foliage acclimation to long‐term integrated irradiance were studied in a natural mixed deciduous stand composed of shade‐intolerant Populus tremula L. and shade‐tolerant Tilia cordata Mill. Positive relationships between maximum stomatal conductance and seasonal integrated average daily quantum flux density ($$Q_{\mathrm{int}\,}$$, mol m$$^{-2}$$ d$$^{-1}$$) were observed in both species, whereas the slope of this relationship declined with increasing soil water limitations. There were negative correlations between $$Q_{\mathrm{int}\,}$$ and leaf water and osmotic potentials, and stomatal conductances reached in conditions of severe water stress were relatively lower in the upper than in the lower canopy in both species. Thus, even at a constant soil water availability, foliar water stress increased with increasing seasonal average integrated light in the canopy. A number of plastic structural and chemical adjustments improving the apparent tolerance of water deficits were observed in the foliage along the light gradient. Leaf dry mass per unit area and the content of leaf osmotica per unit area increased, and an estimate of symplasmic leaf fraction—leaf water contents per unit leaf dry mass—decreased with increasing $$Q_{\mathrm{int}\,}$$. In addition to stomatal closure, the leaves had a substantial and rapid capacity for osmotic adjustment of leaf water potential in response to water stress, and because of light‐related foliar modifications, this capacity was greatest in the upper canopy leaves. Across the whole set of data, there was a negative correlation between minimum daily leaf water potential and stomatal conductance, because both variables covaried with irradiance. When the covariation with light was accounted for by a multiple linear regression analysis, minimum leaf water potential had no significant effect on stomatal aperture. Instead, stomatal conductance correlated positively with soil water potential in both species. Concentration of xylem sap abscisic acid (CABA) was analyzed to assess its role as a potential messenger of soil water status. Instantaneous CABA was negatively correlated with soil water potential, and with stomatal conductance only in the drought‐stressed leaves. Thus, there was evidence that stomatal sensitivity to abscisic acid (ABA) increased with advancing water stress. Tilia cordata, stomata of which were more sensitive to water limitations, also appeared to be more responsive to abscisic acid. We found a negative correlation between CABA and $$Q_{\mathrm{int}\,}$$, which possibly resulted from greater transpiration rates at greater $$Q_{\mathrm{int}\,}$$. Since stomatal conductances were more responsive to drought in the upper canopy, where ABA concentrations were the lowest, we suggest that stomatal sensitivity to ABA is not constant along the canopy light gradient.