Abstract
Effects of tree species diversity on decomposition can operate via a multitude of mechanism, including alterations of microclimate by the forest canopy. Studying such effects in natural settings is complicated by the fact that topography also affects microclimate and thus decomposition, so that effects of diversity are more difficult to isolate. Here, we quantified decomposition rates of standard litter in young subtropical forest stands, separating effects of canopy tree species richness and topography, and quantifying their direct and micro-climate-mediated components. Our litterbag study was carried out at two experimental sites of a biodiversity-ecosystem functioning field experiment in south-east China (BEF-China). The field sites display strong topographical heterogeneity and were planted with tree communities ranging from monocultures to mixtures of 24 native subtropical tree species. Litter bags filled with senescent leaves of three native tree species were placed from Nov. 2011 to Oct. 2012 on 134 plots along the tree species diversity gradient. Topographic features were measured for all and microclimate in a subset of plots. Stand species richness, topography and microclimate explained important fractions of the variations in litter decomposition rates, with diversity and topographic effects in part mediated by microclimatic changes. Tree stands were 2–3 years old, but nevertheless tree species diversity explained more variation (54.3%) in decomposition than topography (7.7%). Tree species richness slowed litter decomposition, an effect that slightly depended on litter species identity. A large part of the variance in decomposition was explained by tree species composition, with the presence of three tree species playing a significant role. Microclimate explained 31.4% of the variance in decomposition, and was related to lower soil moisture. Within this microclimate effect, species diversity (without composition) explained 8.9% and topography 34.4% of variance. Topography mainly affected diurnal temperature amplitudes by varying incident solar radiation.
Highlights
The degradation of plant litter is a key process in terrestrial global carbon (C) and nutrient cycling
The results of our “diversity-topography” model (Tables 1–3) revealed negative effects of planted plot tree species richness on litter decomposition rate constants (P < 0.05; Fig 3) which vanished in the “diversity-topography-microclimate” model, and when diversity was tested after the presence of the species Rhus chinensis, Castanopsis eyrie and Cyclobalanopsis glauca within the plot level (Table 1)
The presence of the species Rhus chinensis and Castanopsis eyrie decreased and the presence of Cyclobalanopsis glauca increased litter decomposition rates (P < 0.05). These effects vanished in the “diversity-topographymicroclimate” model with R. chinensis still showing a tendency to negative effects (P < 0.1)
Summary
The degradation of plant litter is a key process in terrestrial global carbon (C) and nutrient cycling. Plant litter provides the main supply of energy to soil fauna and micro-organisms and shapes interactions between plant and soil communities [1,2]. Litter decomposition rates depend on a range of biotic (e.g. species identity and diversity of plants, litter and decomposers) and abiotic factors (e.g. temperature, moisture and radiation). Quantifying such effects in natural settings is complicated because these factors are strongly interconnected and often act via a range of direct and indirect pathways (Fig 1). Less is known on whether and how the tree species diversity of forest stands affects leaf litter decomposition rates through mechanisms other than the leaf litter composition
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