Dokument

Summary:
Litter decomposition is the breakdown of dead organic matter along with the transformation and liberation of its components as inorganic forms. This process is of high importance in ecosystem ecology, as it determines the available resources to below and aboveground communities, as well as nutrient and carbon dynamics and soil formation. Climate, vegetation (via litter traits) and decomposers are the main drivers of litter decomposition. However, these factors interact with each other, which makes the evaluation of their relative importance for decomposition a difficult task. For example, climate controls have both direct (e.g. via moisture and temperature) and indirect (via changes in species abundance, composition and litter traits) influences. Studies along natural gradients and litter transplant experiments can help to disentangle these effects. In this doctoral research, I particularly studied the role of climate and litter traits in litter decomposition across a large climatic gradient in the Chilean coastal range, by using different litterbag experiments and litter from species with a high variation of functional traits (i.e. litter quality). In the first study, I tested whether soil decomposers are “adapted” to local litter types and thus, these decompose faster compared to the decomposition of non-local litter with similar quality. Under the assumptions of this so-called “home-field advantage” (HFA) hypothesis, I tested whether this adaptation occurs and differs across a wide range of ecosystems, where litter input and microbial specialization may vary. I used a reciprocal litter translocation experiment with 20 species of different litter quality among four different study sites distributed along the Chilean costal range. In addition to mass loss, I used the loss ratios of decomposable and leachable fractions of litter (relative N/K and P/K loss) to understand the specific contribution of decomposers to decomposition and to avoid confounding climatic effects. The results showed no support for the HFA hypothesis in any ecosystem, since the mass and nutrient loss ranking of litter species was consistent along the climatic gradient, i.e. in every site, litter from the arid sites always decomposed the fastest, and litter from the mediterranean and temperate sites decomposed the slowest. These results supports the hypothesis that, in the studied ecosystems, litter quality drives decomposer activity independently of litter origin, and that the decomposer community can probably quickly adjust when foreign litter enters their ecosystem. In the second study, I unraveled the relative importance of litter quality and microclimate (soil moisture and temperature) for litter decomposition, and identified how their effects varied along the decomposition process. By using a reciprocal litter translocation experiment along the climatic gradient in Chile, I followed the decomposition of 30 species with a wide spectrum of functional traits for two years. Litter traits had a strong impact on litter decomposition across the gradient, while an increase in decomposition with soil moisture was observed only in the wettest climates. Overall, litter traits drove decomposition in the first year of decomposition after which soil moisture increased considerably in importance. Moreover, statistical analyses of subsets of the 30 species showed that litter trait effects on litter decomposition gain in importance when the variation in trait values was larger. Thus, the relative effects of litter traits and climate on decomposition depend on the ranges in climate and litter traits considered in the study, and also change with time. In the last study, I evaluated the role of diversity (species number and functional dispersion, FDis) on litter mixture decomposition across ecosystems. I used FDis values based on litter traits related to nutrient transfer among litters or litter recalcitrance, two mechanisms that could explain litter mixture effects. I found only a small number of significant mixture effects on decomposition (both positive and negative) along the climatic gradient, which occurred more often in the most arid sites. These mixture effects were independent of the number of species in the litter mixtures at all sites, but were stronger with increasing FDis at the two most arid sites. At these sites, FDis based on litter traits related to nutrient content correlated with positive mixture effects on decomposition, whereas traits related to inhibitory secondary compounds correlated with negative mixture effects. Overall, this study indicates that mixture effects on decomposition are rather rare across the climatic gradient. However, it suggests that a mechanistic approach to functional diversity metrics could help to further understand under which conditions and in which direction diversity influences decomposition. Altogether, this thesis highlights the importance of litter traits in litter decomposition: this factor not only drives the affinity of decomposers and determines species rankings in decomposability, but can also exert additional controls via functional diversity. I demonstrated that the study of a broad range of litter traits and litter species is decisive to correctly predict the relative importance of litter quality on decomposition, and likely controls the occurrence of litter mixture effects. Similarly, the use of a large climatic range allows to detect critical differences among ecosystems. These results are of particular importance to correctly predict litter decomposition feedbacks on climate and highlight the importance of studies including representative ranges in climate and vegetation. Of particular interest are the underrepresented ecosystems, such as arid and semi-arid areas. In these ecosystems, I showed that litter quality can strongly drive decomposition and litter mixture effects, in contrast to the results from mediterranean and temperate forests. The importance of litter quality, highlighted in all three studies, opens a frame for new research focusing in the understanding of human-driven changes in the functional composition of vegetation for decomposition and thus, for carbon and nutrient cycling.

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