Vegetation Litter: An Underestimated Variable in Hydrology and Geomorphology

Abstract

The influence of vegetation litter on a variety of geomorphological and hydrological processes is stressed, notably as regards retention of rainfall, evaporation and drought in soils, overland flow, rainsplash erosion, weathering, soil characteristics, frost processes and ground vegetation. Results of a laboratory study of retention of rainfall by the litter of two contrasting species, pine (Pinus sylvestris) and beech (Fagus sylvaticus), are presented. The retention properties of immersed single leaves and needles, and of aggregate litters under simulated rainfall conditions are described. Individual beech leaves retain (wt. for wt.) one and a half times the amount of water that pine needles do. This is related to the contrasting physical charac- teristics of dead leaves and needles, and notably to the much higher surface area/ weight ratio of beech leaves. The chief factors controlling absolute amounts of retention of rainfall by aggregate litters are the type of litter (beech retains substantially more water than does pine), and the amount of litter, since the greater the amount of litter, the greater the retention. The retention capacity of litter was soon reached in most of the experiments, and so the percentage of rainfall retained by litter shows a strong inverse relationship with rainfall amount. Differences in percolation in pine litter as compared to beech litter are stressed. Percolation is more direct in pine litter, whereas defined streams of water tend to develop in beech litter. Finally, a tentative model of the retention of rainfall by litter is presented, laying emphasis on the following factors: the amount of litter; type of litter; rainfall characteristics; and evaporation rates. The great variability in amounts (and hence effects) of litter as between different climatic regions of the world is discussed and stressed.