Abstract:
Climate change is likely to alter plant composition, productivity and litter decomposition. As ecosystems are shaped by specific small-scale climatic environments, the responses of plants to climate changes are likely system specific. Inter-annual precipitation is expected to become more variable with longer dry spells and sporadic but intense precipitation events. It is therefore important to understand the responses of plant communities to climate change in both directions (drought and sporadic but intense precipitation). Litter decomposition, a key component of the global carbon cycle, is greatly affected by the interplay of climate, decomposers and litter quality. Unfortunately, our current understanding of climate-change effects on plant composition and litter decomposition stems mainly from space-for-time studies along climate gradients, where biotic and climatic effects on litter decomposition are confounded. Experimental studies separating indirect from direct climate effects are needed that test the validity of the space-for-time approach.
In order to assess the influence of the magnitude of climate change on biomass production and community composition (richness, diversity and evenness) I translocated soil from two climates at a micro-climatic (opposite slopes) and a macro-climatic scale (among climates) in chapter 2. I found that plant communities do not respond to micro-climatic changes, except for biomass production, which was unexpectedly consistently higher on the drier slopes than on the wetter slopes. Macro-climatic changes triggered several responses: species richness had a consistent response to climate change and was higher in the semi-arid climate in both translocations, but diversity and evenness had contradicting, non-opposite responses. The non-generality of responses might be an indication that changes that occur during drier years are not easily recuperated during wetter years. In the third chapter, I test the hypothesis that the decomposer community may be locally adapted to litter quality, providing a home-field advantage (HFA) resulting in accelerated decomposition of local compared to non-local litter, after accounting for decomposition differences due to litter quality. Although widely tested in temperate forests, this hypothesis remains controversial and lacks a deep understanding of its generality across climates. I therefore tested the HFA hypothesis for litter decomposition in four ecosystems along an extensive climatic gradient in Chile, using a translocation experiment involving litter from 20 species. In addition to comparing mass loss, I adopted a novel way to disentangle decomposer effects from climate effects, based on loss rates of decomposable vs. leachable nutrient fractions. I used the ratios of N and K losses and P and K losses to unravel the relative role of biotic mineralization (N and P loss) vs. physical leaching (K loss, driven by precipitation) along the climate gradient. My findings unequivocally contradicted the HFA hypothesis across a wide range of environments and 20 different litter types. A HFA effect was not found, and litter quality influenced litter decomposition much more strongly than origin or location of the litter. Our study questions the applicability of the HFA for litter decomposition and calls for more studies that include a large range of climatic conditions to understand the context-dependency of the HFA.
In the fourth chapter I combined large- and small scale reciprocal litter translocations, in situ precipitation manipulation, and a prominent climate gradient to study climate effects on litter decomposition. Interestingly, all experiments indicated clear positive effects of precipitation on
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decomposition, but the decomposition of local litter at their home site indicated the opposite, due to indirect climate effects on litter quality. This indicates that space cannot substitute for time and highlights the need for experimental evidence in litter decomposition studies. Such evidence would improve predictions of models of the global carbon cycle that include interactions between climate and vegetation.
Even though plant communities and soil properties seem relatively robust to cope with inter-annual precipitation variability, the predicted precipitation variability for the next decades will most likely alter decomposition rates, which will affect carbon and nutrient cycling. To better predict the influence of climate change on the nutrient cycle, future studies should quantify litter production and plant community changes. A combination of a detailed quantification of plant community litter production with observations, translocations and manipulations of litter decomposition, will make it possible to correctly estimate the effect of future climate change on the nutrient cycle.
Litter decomposition
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