Overhead tree canopy species has limited effect on leaf litter decomposition and decomposer communities in a floristically diverse, southern temperate rainforest

Abstract

To decipher the <30% of unexplained variation in leaf litter decomposition, a process important for nutrient cycling yet vulnerable to rising CO2 levels, numerous hypotheses have been proposed. The home-field advantage (HFA) hypothesis states that leaf litter decomposes more rapidly beneath canopies of tree species from which the leaves originate (home environment), than beneath other tree species. Most HFA studies typically involve reciprocal litter transplants between habitat types (i.e. forest vs. grassland) or successional stages (i.e. early vs. late). The mixed and often conflicting results pertaining to this hypothesis, mostly assessed at the landscape-level, merit a finer scale analysis of decomposition variability, i.e. at the tree species level. Despite homogenous litter build up close to any given tree individual in a mixed forest, along with evidence of varying decomposition rates under different tree species, we still lack basic understanding of forest floor decomposition dynamics at the tree species level, strongly driven by host selection by fungi as well as arthropod detritivores. To address these gaps, we first determined differences in decomposition rates between leaves of three dominant forest tree species. We then compared decomposition rates between leaf litter placed beneath the canopies of source species vs. non-source species in a mixed-tree natural, temperate rainforest, accounting for the activity of both fungi (microbiota) and arthropod detritivores (both meso- and macrofauna). We found that leaf-litter decomposition rates can differ significantly depending on the source-tree species. Despite this, we found no effect of overhead tree species on decomposition rates of leaf litter, irrespective whether under source species or non-source species. Only one species' leaf litter showed differences in arthropod diversity between leaf litter placed beneath source tree species vs. non-source species, with higher arthropod diversity in leaf litter placed beneath source trees. Also, arthropods here contributed significantly more than fungi to overall decomposition. However, bacteria (not measured here) may also have decomposed litter greatly, with litter loss not solely due to our focal groups. We conclude that, in a single, diverse forest, perhaps due to differences in litter fall rate and nutrient content of leaf litter, only some plant species might reveal small-scale patterns for detritivore diversity, with yet unknown effects overall decomposition. As such, landscape-level studies on decomposition should incorporate and account for the effects of tree species-level decomposition variability.