Effect of forest canopy density on water status and photosynthesis of 14 herbaceous species in temperate forest

The density of forest canopy is affecting understory light conditions as well as soil water availability. Trees either suppress the herbaceous understory species by competition for resources or help them to cope with increased climatic warming by canopy shading. Nevertheless, little is known about the light × water interaction on herbaceous species performance. In shaded environments, such as in the forest understory, the energy needed for stomatal opening may not be entirely offset by the imminent carbon gain, which may give a partial advantage to anisohydric species. The combination of shade and additional drought might damage the light-harvesting part of the photosynthetic systems because of the strong competition between species for light, which may be specifically critical for the anisohydric species. In this study, we compare seasonal water status, photosynthetic and fluorescence performance of 14 forest understorey herb species in open and closed forest stands, including perennials, annuals and grasses under favourable and mild drought-stress conditions. 
We found midday and predawn water potentials are different between several different water-managing species which were divided into rather isohydric, rather anisohydric and neutral species. Most of the species were less drought-stressed during the midday in the closed forest compared to open forest. In contrast, all species under the sparse canopy kept the same or higher light-saturated photosynthetic yield than those growing in the closed forest. Few species growing under the open forest had lower values of intercellular CO2 concentration compared to those growing in the closed forest both under favourable and mild drought-stress conditions. We found that almost every individual species had a different seasonal variation in stomatal conductance values and led to differences in the photorespiration rates, which were mostly driven by light conditions.
We conclude that herbaceous species growing in the closed forest were less drought stress and the response of studied physiological traits to the combined effect of drought and light was found to be species-specific.