Species replacement and transitional zones in natural grasslands with subtle environmental gradations

Abstract

Heterogeneity creates borders, limits and contrasts between patches. In turn, this pattern generates new processes. Linked to heterogeneity, ecological systems have limits where layout is essential to understanding system organisation. This study analysed internal heterogeneity in natural grassland using: (1) estimation of species replacement in space, and (2) identification of transitional zones and definition of their ecological features in an extremely flat area of the Flooding Pampa, Argentina. Transition analysis was conducted at three sites 77 plots in total. In each plot, all species, their cover percentage and the proportion of bare soil were recorded and soil samples (0-20 cm depth) measured for pH and electrical conductivity. With floristic data, we explored the relationship between pairwise similarities in community structure and pairwise differences in space, adjusting similarity as a linear or non-linear function of geographical distance. We then conducted a cluster analysis to identify transitional zones, which were associated with large vegetation units described for the Flooding Pampa. Floristic composition in transitional zones was analysed according to the proportions of species and combinations characteristic of the different plant communities. Finally, we synthetised topo-edaphic measured information with a Principal Component Analysis to analyse transitional zone environmental properties. Joining hierarchical classification methods and Indicator Species Analysis for single species and combinations, we highlighted internal heterogeneity, often not noticed in these environments. Rotational livestock grazing contribute to heterogeneity that is typical of these grasslands. Knowledge about transitional zones strongly supports the development of management strategies to prevent grassland degradation.