Water balance components of temperate tree species as driven by plant traits and environmental factors
Authors:   , Pflug Stefanie 
Published: June 2024

Vegetation plays an important role in the water cycle by transporting water through plants from root to leaf (transpiration) and by precipitation interception and retention on plant surfaces and its evaporation from these surfaces (interception evaporation). Global warming leads to changes in precipitation patterns, with precipitation becoming both more variable and more intense, which in turn affect soil moisture availability.
This thesis aimed at improving our understanding of tree water fluxes. More specifically, the aims were to (1) determine transpiration rates, rainfall interception and the sensitivity of stomatal conductance to soil drought of 10 tree species common to northwest Europe, and to (2) evaluate the explanatory value of tree traits for these fluxes. We set up an outdoor experiment with potted trees on a grass field in the Netherlands to create comparable environmental conditions for each tree (same soil type, nutrient and water supply, as well as very similar meteorological conditions) which allowed studying species-specific effects. Studied were broadleaved deciduous (Acer pseudoplatanus L., Betula pendula Roth, Carpinus betulus L., Fagus sylvatica L., Populus tremula L., Sorbus aucuparia L.) and coniferous evergreen species (Picea abies (L.) Karsten, Pinus sylvestris L., Pseudotsuga menziesii (Mirb.) Franco). The trees were, depending on species, 2.7 to 7.2 m tall.
Our results showed that transpiration in the 10-week period in summer 2018, when all 80 trees had a full canopy, varied up to three-fold between the species with the highest (A. pseudoplatanus, 429 mm) and the lowest transpiration (B. pendula, 136 mm). The differences between species cumulative transpiration could be explained by differences in species response of daily transpiration to reference evapotranspiration and vapour pressure deficit. In the summer period, the cumulative transpiration did not differ between the broadleaved and the coniferous species-groups. Due to the extended growing season of the coniferous species, total (June to December) transpiration was higher for the coniferous than for the broadleaved species.
Rainfall interception was measured directly on 10 trees, i.e., one of each species. We found that the differences between the species within one leaf habit group (broadleaved vs. coniferous) were less pronounced than the differences between these two groups. In all seasons, the coniferous species had higher interception ratios than the broadleaved species. The winter interception ratio of the broadleaved species, despite leaf loss, was still 56% of their mean summer interception ratio.
During the experimental drought, the stomatal conductance of the broadleaved species was measured and related to the (declining) soil water potential. The estimated soil water potential at which stomatal conductance reduction reached its maximum differed within and between species, ranging from -23 kPa for P. tremula to 273 kPa for C. betulus. While the maximum stomatal conductance correlated well with the pre-drought cumulative summer transpiration, the transpiration rates under well-watered conditions could not explain species-drought responses.
Both summer transpiration and interception of all species were positively related to the density of the crown, expressed as the branch dry weight per projected crown area. The group of coniferous species was characterized by a denser crown, which could explain the higher transpiration and interception rates compared to the group of the broadleaved species.
This study has revealed and quantified substantial differences in species transpiration responses to vapour pressure deficit and stomatal responses to drought. These results emphasize the relevance of a species-specific approach for transpiration assessment under current and future climatic conditions. Rainfall interception, though, was more dependent on leaf habit type than on species. The crown density had positive effects on both transpiration and interception and deserves further research to non-invasively estimate plant water use under field conditions.