Article III. Testing for the effects of leaf qualities and forest type on leaf-litter decomposition in mountains of the Central Highlands of Chiapas, México
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Leaf-litter decomposition rates may be determined by the structural and biochemical components of litter and by the forest understory environments and soil substrate conditions under which decomposition takes place; it is highly likely that these two sets of factors interact. The relationships between functional traits and growth, recruitment and mortality in plant species are now quite well known, but trait effects on ecosystem processes like litter decomposition are much less well-known, and it is not known whether the functional characteristics of forest stands are related to decomposition rates in situ. Research in this area can make a valuable contribution to the development of functional ecology. For tropical montane forests of the Central Highlands, state of Chiapas, Mexico, this study evaluated the effect of functional leaf traits of individual tree species and forest environments on leaf-litter decomposition. For this, two complementary experiments were conducted. In a greenhouse experiment, the direct and indirect associations of litter decomposition rates with 10 functional traits were assessed in 20 dominant tree species. Six of these traits were measured on fresh leaves (leaf area, LA; specific leaf area, SLA; leaf dry matter content, LDMC; leaf tensile strength, LTS; leaf phosphorous concentration, LPC and leaf nitrogen concentration, LNC), and four on dry leaf-litter (lignin, cellulose and hemicellulose concentrations -carbon fractions). We hypothesised that leaf litter decomposition rates would be lower in conservative species such as some Quercus spp. which have low SLA, LNC and LPC and high LDMC and carbon fractions, and higher in tree species of the understorey and sub-canopy of the forest, which have higher SLA, LNC and LPC and lower LDMC and carbon fractions. In a field experiment we assessed the effect of four different forest association (mixed broad-leaved, pine-oak, oak and pine dominated) on the decomposition of six litter types (four combinations of field-collected leaf litter, one for each forest association, and two reference species) tested in all four forest associations. Because of the exploratory nature of the field experiment, we tested a simple hypothesis that decomposition rates would differ between forest associations. Hypotheses were met in the greenhouse experiment. Conservative Quercus spp. with high LTS, LDMC and lignin had slow decomposition, though so did Pinus spp. which have low SLA and high LTS. Sub-canopy and understorey species with more acquisitive leaves had high decomposition rates. In contrast, in the field experiment, decomposition rates were similar between the four different forest types, supporting the hypothesis that leaf characteristics of species that persist after senescence, rather than forest stand characteristics, are the most important drivers of decomposition processes. Nevertheless, there was evidence of “home-field advantage” with oak litter decomposing fastest in the oak forest and pine litter in the pine forest, indicating that the interaction between forest association and litter type is also an important driver of decomposition. This interaction may depend on the characteristics of leaf-litter substrates determined by forest functional composition. Overall, the results of this study support the emerging principle of functional ecology that traits of living plants and their leaves, and possibly also the soil surface conditions of the stands they dominate, can drive leaf litter decomposition rates, with important implications for understanding of ecosystem functioning and ecosystem service provision.