Species diversity in a beech windthrow

On the basis of long-term surveys of permanent plots and traps we examined the communities of saproxylic beetles, fungi, herbs and trees on an untreated 22 hectare large beech forest windthrow and asked whether the results lend support to the intermediate disturbance hypothesis (IDH). We studied species richness and the similarity of community composition. Additionally, we grouped species by their frequency trend over time to successional model types to examine whether, corresponding to the IDH, the diversity of these groups explained peak richness at intermediate intervals after the disturbance.

In line with the IDH, species richness showed a hump-backed temporal course for alpha and gamma diversity. We found evidence for a linear succession directly after the disturbance. This, however, did not continue, and in all species groups a partial recovery of the initial community was observed. In the case of fungi, herbs and trees, but not for saproxylic beetles, alpha diversity was driven by the diversity of the successional model types.

Our results underline that the mechanisms driving species richness after disturbances are more complex than the IDH suggests, and that these mechanisms vary with species group. We assumed that, besides competition, legacy effects, facilitation, habitat heterogeneity and random saturation of the species pool are important. In case of trees and herbs we found indications for strong legacy and competition effects. For fungi and beetles, substrate heterogeneity and microclimate were assumed to be important. We concluded that disturbances contribute to increasing species richness not only by reducing the effectiveness of competitors but also by increasing the amount and diversity of resources, as well as their rate of change over time.

We have archived the data of species occurences over years per plot. 

Methods
The data were collected in the field by different methods per species group. Tree and herb species were registered on permanent plots. Beetles (Coleptera) were caught in trunk-eclectors. Fungi were registered and, in part, collected on permanent plots. The determination of species was made in the field in case of trees and herbs, afterwards in case of beetles and partly afterwards in case of fungi. Data prcoessing is described in detail in the published paper.