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 temperate wet heathland vegetation, pin-point cover data from 42 Danish sites sampled during a 15-year period was regressed onto selected environmental variables. The effects of nitrogen deposition, soil pH, soil C–N ratio, soil type, precipitation, and grazing on the heathland vegetation was modeled in a spatiotemporal structural equation model using a Bayesian hierarchical model structure. The results suggest that the modeled 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 temporal 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 is 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.