The Effects of Soil Texture on Transpiration Losses During Droughts: A Field Study of Three Oak Forests in an Inner Alpine Valley of Switzerland

As safety mechanism during droughts, plants close their stomata to reduce transpiration losses and prevent excessively negative water potentials. Although the coordination between stomatal closure, xylem vulnerability and leaf traits has been extensively investigated, the role of soil hydraulic limitation remains elusive. Here, we test the hypothesis that stomatal closure is triggered by the loss of soil hydraulic conductivity in the root zone. As the soil hydraulic conductivity is a function of soil texture, we further hypothesize that stomatal closure, and more precisely the relation between stomatal conductance and leaf water potential, are soil texture dependent. An alternative hypothesis is that the shoot to root ratio adapts to the specific soil conditions, and it is lower in soils with low hydraulic conductivity. We compared three field sites in an inner alpine valley in Switzerland (yearly rainfall of 600 mm) with the same oak tree species (Quercus pubescens) and varying soil textures. We used the model of Carminati and Javaux (2020), which predicts that the decline in transpiration rate is controlled by the soil hydraulic conductivity, and, consequently, it is more abrupt in 1) coarse textured soils compared to fine textured soils and 2) in plants with high shoot to root ratio.

To test these working hypotheses, stem water status and soil matric potential were measured at various depths of three oak sites. The soil matric potentials were then linked to the hydraulic conductivity and soil water content using a new pedotransfer function (PTF) for forest soils, which overcomes the limits of existing PTFs that were trained for arable soils. A simple water balance model based on changes in water content (deduced from PTF and measured potentials) was used to calculate transpiration rates and was compared with sap flow measurements conducted on two trees per site. Leaf water potentials were estimated from dendrometers after calibration with pressure chamber measurements of leaves. The sap flow measurements correlate well with estimated transpiration rates (R²=0.7).

Comparisons between sites show a similar decrease in stomatal conductance at all three sites during drought, regardless of soil texture. Rather, our data suggest the trees hydraulically adapt to the local soil texture by adjusting their leaf area index and root length density to the local water demand and supply. In coarse-textured soils, oak had a low leaf area index, reducing their water demand, and high fine root length density, increasing their supply. In conclusion, our study suggests a hydraulic adaptation of trees to their local soil texture by adapting their “shoot to root ratio” as quantified here by leaf area index and root density.