Contrasting effects of deadwood and gaps on the trophic structure of forest soil microarthropods

Understanding the mechanisms driving forest biodiversity is challenging,
especially in soil. Trophic niche is the most important perspective to
understand how biodiversity changes with disturbances. Deadwood input and
gap formation are major disturbances resulting from tree mortality. Both
alter the ecological niches of forest organisms, yet their individual and
interactive effects on soil animals remain unclear. Trophic diversity,
i.e., the area of trophic niches occupied by organisms, links to the
resource processing within food webs and, consequently, to ecosystem
functions. Changes in the trophic diversity of Collembola communities may
result from both changes in species composition and from trophic shifts of
individual species. To investigate how the trophic structure of soil fauna
responds to forest disturbances, we examined trophic niches of Collembola
across three regions in Germany, as influenced by deadwood addition and
gap formation using stable isotope analysis (13C, 15N). Changes in the
trophic structure of Collembola associated with deadwood addition and gap
formation were distinct and independent. The deadwood addition resulted in
higher trophic level (Δ15N) and greater trophic diversity of Collembola
communities. These shifts were likely driven by increased habitat
heterogeneity, resulting in higher trophic differentiation (rather than
higher species richness) of Collembola communities. The results highlight
the importance of habitat structure in driving soil animal functional
diversity. By contrast, gap formation promoted the growth of understory
plants, leading to increased use of plant-derived resources (lower Δ13C)
by Collembola. Further, gap formation reduced the trophic diversity of
euedaphic (soil-dwelling) Collembola, pointing to trophic homogenisation,
likely driven by increased use of root-derived resources from understory
plants, and this was also accompanied by higher biomass of euedaphic
Collembola in forest gaps. Overall, our findings highlight that deadwood
addition and gap formation affect the trophic structure of detritivore
communities in an independent way, with deadwood enhancing habitat
heterogeneity and trophic differentiation, while gap formation results in
trophic homogenisation and increased resource supply to euedaphic
Collembola. Thereby, deadwood addition and gap formation complement each
other in affecting the trophic structure of detritivore soil animals and
thereby their functioning.