Temperature-driven vapor pressure deficit structures forest bryophyte communities across the landscape

Abstract. Atmospheric vapor pressure deficit (VPD) controls local plant physiology and global vegetation productivity. However, at ecologically crucial intermediate spatial scales, the role of VPD variability in forest bryophyte community assembly and the processes controlling this variability are little known. To explore VPD effects on bryophyte community composition and richness and to disentangle processes controlling landscape-scale VPD variability, we recorded bryophyte communities and simultaneously measured forest microclimate air temperature and relative humidity across a topographically diverse landscape representing a bryophyte diversity hotspot in temperate Europe. Based on VPD importance for plant physiology, we hypothesize that VPD can be important also for bryophyte community assembly and that VPD variability will be jointly driven by saturated and actual vapor pressure across the topographically diverse landscape with contrasting forest types and steep microclimatic gradients. Contrary to our expectation, VPD variability was dictated by temperature-driven differences in saturated vapor pressure, while actual vapor pressure was surprisingly constant across the landscape. Gradients in species composition, species richness and community structure of bryophyte assemblages followed closely the VPD variability. The average daily mean VPD was a much better predictor of species composition than average daily maximum VPD. The mean VPD also explained significantly more variation in species composition and richness than maximum temperature, indicating that time-averaged evaporative stress is more relevant for bryophyte communities than microclimatic extremes. While mesic forest bryophytes occurred along the whole VPD gradient, species occurring near their distributional limits and locally rare species preferred sites with low VPD. Consequently, low VPD sites represent species-rich microclimatic refugia within the landscape, where regionally abundant mesic forest bryophytes coexist with rare species occurring near their distributional range limits. Our results showed that VPD variability at ecologically crucial landscape scales is controlled by temperature-driven saturated vapor pressure. Future climate warming will thus increase evaporative stress and reshuffle VPD-sensitive forest bryophyte communities even in topographically diverse landscapes, which are traditionally considered as microclimatic refugia buffered against climate change. Bryophyte species occurring near their distributional range limits in microclimatic refugia with low VPD will be especially vulnerable to the future changes in atmospheric VPD.