LONG-TERM DECOMPOSITION PROCESS OF LEAF-LITTER FROM QUERCUS-PYRENAICAFORESTS ACROSS A RAINFALL GRADIENT (SPANISH CENTRAL SYSTEM)

A long-term (3 years) study has been made of the leaf-litter decomposi tion process in four forest ecosystems across a rainfall gradient. The soil organic contents comply with the factor of rainfall, but this do es not appear to affect the complete decomposition process decisively, since the limiting factor is soil humidity. The rainfall distribution is similar for all the plots studied and the main differences are see n essentially in the amount of water received during the wet season (i n each case with sufficient humidity for leaf decomposition to progres s). Thus, leaf-litter decay is linked to the maintenance of soil humid ity, mineralization slowing down when the soil dries out, and the effe cts of climate therefore predominating over the chemical characteristi cs of the material in the regulation of the decomposition process (at least at short term). In this sense, the summer drought is the most un favourable environmental factor in this type of climate. Ln advanced s tages of decay the rate of weight loss becomes relatively independent of climate. The decomposition rates obtained by the litterbag procedur e are lower than those calculated by litter production and permanent l itter; the reason for these differences is probably the limitation of the mesofaune activity in the litterbags. Leaf decomposition constants ranged from 0.33 on the plot with the highest rainfall to 0.47 (on th e driest plot), corre sponding to mean residence times of 2.0 to 1.1 y ears, respectively. However, each bioelement has a different residence time.