Litter Decomposition in Mediterranean Pine Forests Subjected to Climate Change

Decomposition of dead organic matter is a fundamental process in the carbon cycle and is essential for the sustainment of biomass production in terrestrial ecosystems. Climatic factors are key drivers of decay of plant residues (litter), and, thus, changing those factors might affect litter decomposition. In this chapter, we assess the potential impact of climate change on litter decomposition in Mediterranean pine forests by direct climatic influences and by indirect impacts through changes in forest structure, composition and microclimate. We also present important insights from litter decomposition in Mediterranean pine forests that could be relevant to more mesic forests. Models project reductions in precipitation amounts for Mediterranean-type climate zones, and water deficits are expected to grow in all Mediterranean regions because of rising temperatures. These conditions can lead to decreased canopy densities, either by tree mortality and defoliation, or by adaptive management decisions. Decreased water availability reduces the rates of rain-based microbial degradation of plant litter, while increasing the openness of canopies, and thus enhancing abiotic decay by photochemical and thermal degradation, and microbial degradation driven by non-rainfall water sources (air humidity, dew, fog). We hypothesize that abiotic and humidity-enhanced biotic degradation compensates for the decrease in classic rain-driven microbial degradation in the hotter and drier, but not in the cooler and moister Mediterranean pine forests. The Mediterranean-type climate is projected to expand into more mesic regions, bringing abiotic and humidity-enhanced biotic mechanisms of decay into play in some of today’s temperate forests.