Effects of environmental factors and plantation forests on endangered cactus diversity and composition in subtropical South American grasslands

摘要

We investigated the relative importance of various natural and human-related environmental factors on the structure of 36 cacti communities occurring on rocky outcrops in southern Brazil grasslands. Outcrops were embedded in either open grasslands grazed by livestock or five year-old eucalypt plantations. Total richness equaled 10 species, nine of which were globose and only one cylindrical, all of small size. Outcrop-scale true diversity was positively related to outcrop area, slope, and rock cover, and negatively related to the distance to the nearest outcrop. The within- (α) and between-plot (β _1) components corresponded to 16.1% and 30.1% of total true diversity, while the between-outcrop (β _2) and between-management (β _3) levels corresponded to 35.2% and 18.6% of total true diversity. Observed diversities were significantly different from expected values, and the largest differences occurred in the β _2 and β _3 components. Altogether, environmental variables and spatial structure explained 41% of the variance in species abundances in the rocky outcrops. Landscape management and the spatial structure had a significant effect on cacti communities structure, explaining 11% and 9% of total variation, respectively. The first axis in an NMDS ordination reflected compositional differences in cacti communities between management regimes, with the grassland rocky outcrops concentrated in the left, and the plantation outcrops scattered to the right. Management-mediated changes in species abundances can be driven by released succession in rocky outcrops embedded in plantations.