Plant diversity across five vegetation belts in the Pyrenees (Catalonia, Spain)

In this paper we analyze the size and habitat partitioning of the vascular floras of five areas of the NE Iberian Peninsula, representing five distinct vegetation belts and three floristic regions: Mediterranean (basal belt), medio-European (submontane and montane belts) and Boreo-Alpine (subalpine and alpine belts). Each area covered over 1000 ha, and was fairly uniform in terms of potential vegetation, bedrock and bioclimate. They excluded large villages and field areas, the landscape being mainly natural or moderately anthropized. The size of the local floras showed a unimodal function with respect to altitude, with the highest number of species (568) in the submontane belt. The decrease in species richness was moderate toward the neighbouring belts (basal and montane) but more marked from the montane to the subalpine belt and then to the alpine belt (the poorest area, with 217 species). However, considering the size of the areas studied, the montane belt showed higher species richness than the submontane belt. The species impoverishment toward higher altitudes followed general tendencies found in other altitude and latitude gradients, and reflects the increasing environmental constraints and the decreasing habitat diversity of colder landscapes. On the contrary, the richer floras of the submontane and montane areas reflect less stressful general conditions, but also the occurrence within them of a number of taxa from the neighbouring regions (Mediterranean and Boreo-Alpine). The forest flora in the basal to subalpine landscapes accounted for a small percentage of the local floras (6.5-13.2%). Among the other species, roughly one half (42-46%) made up diverse serial communities (forest clearings, scrubs, pastures) while the other half (44-51%) occurred in particular habitats (water sides, rocky areas) or in anthropogenic habitats. In the alpine area almost half of the taxa were found in the pasture mosaic, and a similar amount thrived in rocky and water-related habitats. Across the studied areas, the size of the flora particular to each main habitat showed no relationship with the surface area occupied by these habitats. Ongoing changes in land use –mainly rural abandonment– would have little effect on the ruderal floras, which can persist even in very small disturbed patches, and may have no effect on the plants of rocky and water-related habitats. But these changes clearly reduce the extent of secondary habitats (scrubs, pastures), and thus lead to population reduction and fragmentation, and eventually local extinction of the most rare species in these habitats.