Ecosystem dynamics in wet heathlands: spatial and temporal effects of environmental drivers on the vegetation

To understand and estimate the effects of environmental drivers on wet heathland vegetation, pin-point cover data from 42 sites sampled during a 15-year period was regressed onto environmental variables (nitrogen deposition, soil pH, soil C-N ratio, soil type, precipitation and grazing) in a spatio-temporal structural equation model using a Bayesian hierarchical model structure with latent variables to model the effect of measurement and sampling uncertainties. The results suggest that the modelled environmental variables have various regulating effects on the large-scale spatial variation as well as plant community dynamics in wet heathlands. Most noticeably, nitrogen deposition and yearly precipitation had relatively large and opposite effects on the characteristic species Erica tetralix and Molinia caerulea, where nitrogen deposition had negative effects on E. tetralix and positive effects on M. caerulea. The results of this study differed in important qualitative aspects from the findings of an earlier study where comparable data from a shorter time-series (7 years instead of 15 years) were analyzed with a similar model, which suggests that relatively long time-series are needed for studying ecosystem dynamics. Furthermore, it was concluded that the effect of nitrogen deposition on plant community dynamics mainly was through direct effects, whereas the effect of soil type on plant community dynamics was both direct and indirect mediated by the effect of soil type on soil pH. It was concluded that the modeled environmental variables are sufficient for predicting the average plant community dynamics of wet heathlands. However, caution and humbleness are required if the fitted model is used for generating local ecological predictions as input to a process of generating adaptive management plans for specific wet heathland sites. Moreover, the results suggest that the ratio between the two species E. tetralix and M. caerulea may be used as an indicator for the conservation status of wet heathlands.