Soil seed bank and seedling dynamics in badlands of the Upper Llobregat basin (Pyrenees)

In the badlands of the Upper Llobregat basin, vegetation is sparse and heterogeneous. The harsh conditions generated with the formation of the badlands involve a regressive succession. On the upper part of the slope, a sharp change can be seen between dense communities and denuded badland slopes. Nevertheless, recolonization of the badlands rarely occurs. Scarce seed availability or low capacity of seeds to germinate in such unfavorable conditions may be the main causes. The soil seed bank was sampled in two micro-catchments; a total of 105 soil cores were extracted along three parallel transects. Soil samples were washed through a set of sieves, and seeds were separated and identified. Seedling emergence and mortality were also recorded monthly from April 1990 to December 1991, on two permanent plots in a nearby micro-catchment. An unexpected soil seed bank was found. A relatively high species number has been found with rather small and slightly heterometric seeds. Strong relation between seed bank composition and established vegetation has been detected. Seedling emergence occurred mainly from May to August. The species composition was also dominated by the most frequent species in the badlands and in the soil seed bank. Seedling mortality does not appear to be directly associated with the erosive processes, as it occurred throughout the year. It seems that seedling survival rates could be sufficient to ensure persistence of the current vegetation in badland areas. We conclude that there is no single reason that can explain the low vegetation cover in badlands but rather there is a combination of several restrictive factors. Lack of seed availability within the reduced favorable period for germination and low seedling establishment contribute in part to this low colonization.