Changes in humus forms and soil animal communities in two developmental phases of Norway spruce on an acidic substrate

This study focuses on the relationships between forest dynamics and changes in humus forms and animal communities in three areas of spruce forest at two developmental phases: two areas with 24-year-old trees (mean age) and one area with 136-year-old trees located in the Italian Alps. Mesofauna and macroarthropods were identified at the level of zoological group, order, super-family or family, when possible. They were then classified into morphotypes on the basis of features observable under a magnifying glass. The humus form varied from amphimull to dysmull in the 24-year-old spruce stands (regeneration), and was a dysmoder under the 136-year-old trees (adult). Results of correspondence analysis and ANOVA indicated that soil pH was higher in regeneration stands, where animal communities were mainly characterized by high densities of collembola, centipedes, and macrofauna. The diversity of zoological (and functional) groups was also higher in regeneration stands. The two regeneration stands differed only in the abundance of gamasids and Protura. Soil carbon and nitrogen contents increased in the mature spruce stand, as did the C/N ratio. The abundance of animals increased due to higher densities of mites, particularly oribatids while collembola, macrofauna, zoological diversity and the rate of disappearance of the litter decreased. The comparison of morphotype assemblages by hierarchical agglomerative clustering indicated that the shift between 24-year-old and 136-year-old trees, observed for humus forms, also occurred for animal communities. Differences in soil characteristics and animal communities between the two phases of the forest cycle corresponded to those generally observed between mull and moder, except for collembola which are generally more abundant in the thick organic layers of moder soils. Our findings confirm the change in humus form previously noted by other authors in the French Alps, but are inconsistent with data on humus forms and collembola and oribatid mite populations from Northern Europe. We conclude that changes in invertebrate communities, particularly arthropods, with the growth phases of spruce may be attributed to (1) soil impoverishment, due to the increased rate of nutrients and water uptake by growing trees; and (2) increase in recalcitrant litter input, mostly transformed by oribatid mites. Our data indicated that animal communities and humus forms could be a reliable means of assessing functional characteristics of the ecosystem corresponding to each growth phase of spruce.