PLANT LITTER - ITS DYNAMICS AND EFFECTS ON PLANT COMMUNITY STRUCTURE

We discuss the dynamics of plant litter, the effects of litter on the chemical and physical environment, the direct and indirect effects of plant litter on plant populations and communities, and different adaptative traits that may be related to litter accumulation. The production of litter depends primarily on the site productivity, but other properties of the environment, as well as chance, may introduce important variation. The existence of time lags between the production of plant organs and their transformation into litter appears as a relevant character of litter dynamics seldom included in models. Herbivory, and other processes that destroy biomass or reduce productivity, may reduce the amount of litter produced. The destruction of litter encompasses a complex of interactions. The main processes, including physical and chemical degradation, consumption by invertebrates and decomposition, are differentially affected by the environment and by the physical and chemical characteristics of the litter itself. The relative importance of those processes varies among systems. Litter alters the physical and chemical environment directly and indirectly. The decomposition of litter may release both nutrients and phytotoxic substances into the soil. The physical changes produced by litter also alter the activity of decomposers, resulting in an indirect effect on the chemical environment. The accumulated litter intercepts light, shading seeds and seedlings, and reduces the thermal amplitude in the soil. By reducing maximum soil temperatures, and creating a barrier to water vapor diffusion, litter reduces evaporation from the soil. However, litter may also diminish water availability when it retains a large proportion of rainfall. Litter creates a physical barrier for seedling and sprout emergence and to seeds reaching the soil. The heterogeneity introduced into the abiotic environment by the patchy accumulation of litter may affect community structure. This effect may be both direct (when the litter of one species affects the performance of a second species) or indirect (when litter produced by one species alters the outcome of the interaction between a second and a third species). Litter tolerance, timing of litterfall to optimize external nutrient recycling, and accumulation of litter to deter competitors (either through physical or chemical effects) have been postulated as strategies associated with litter accumulation. The existing evidence shows that only tolerance to litter accumulation admits adaptive value as the most likely explanation.