Soil macrofaunal response to forest conversion from pure coniferous stands into semi-natural montane forests

We studied the response of the macrofauna to forest conversion from pure coniferous stands into semi-natural montane forests in the southern Black Forest (Germany). The investigation was carried out by comparing existing stands that represent the four major stages of the envisaged conversion process. Major results are: (i) environmental parameters indicate a significant alteration of the soil environment, (ii) neither an overall change nor a clear trend in macrofaunal richness and abundance could be established, and (iii) the data for the different taxa suggest specific responses to the conditions of the individual conversion stages. In general, saprophagous taxa seem to be mainly driven by alterations of the resource base (litter quality, microbial parameters) while predatory taxa tend to respond to changes at the consumer level. Associated alterations in the functional structure occurred for macroengineers (earthworms), primary consumers (diplopods, isopods) as well as a wide range of predators. These changes could partly be explained by changes in environmental conditions that did not follow a gradual adjustment during the conversion process. One essential step is the shift from an organic layer composed of low-quality coniferous litter that is dominated by fungi to a litter layer with higher quality food sources. Within this framework, the specific response of the macrofauna is modulated by factors such as differences in structural features of the ground vegetation and availability of woody litter in a stage-specific way. Since the response of the macrofauna depended far less on site conditions than any of the other environmental factors, this invertebrate group may be used as a valuable indicator of the changes associated with different stages of the conversion process. As long as no dramatic change of soil conditions – in particular pH – occurs, however, no fundamental restructuring of the soil community is to be expected.