Disturbance induced changes in species and functional diversity in southern Patagonian forest-steppe ecotone

Abstract

Ecotone areas are the most dynamic areas of the world where small changes in some condition produce rapid and abrupt responses such as shifts in the distribution of dominant species and associated community's patches. Studying southwestern Santa Cruz forest-steppe ecotone is an ideal landscape to explore potential feedbacks of grazing and fire on vegetation diversity because of the juxtaposition of fire-resistant forests dominated by obligate seedlers with fire-sprouting shrublands and the prevalence of wild cattle since the early XX century. In this study, we analyzed how climate (precipitation, temperature and water balance), stand characteristics (basal area, quantity of cohorts and exotic species cover) and disturbances (fire and grazing) affect native species diversity, Plant Functional Types (PFTs) diversity and PFTs response in the forest-steppe ecotone of southern Patagonia. The study was conducted on 124 plots located on the eastern slope of the Andes (between 48°50' and 50°50'S) including forest-steppe ecotone sites between 1000 and 400. mm of annual precipitation. Native species and PFTs diversity indices modeling were carried out by generalized least squares and generalized lineal models. Stand characteristics, disturbance type and climate variables were used as factors over native species and PFTs diversity variables. An ordination and a Spearman rank correlation analysis were achieved between scores of the two first axes with total basal area, exotic species cover, mean annual temperature, annual precipitation, and water balance in order to explore PFTs responses to biotic or abiotic ecological conditions. The relationship between native species and PFTs richness (number of PFTs per plot) was modeled in order to evaluate the redundancy degree of PFTs under different disturbance types by fitting nonlinear power models to both richness measurements for each disturbance type. Fire impact over forest-steppe ecotone communities is one of the most important top down factor driving major increases on PFTs redundancy and heliophilous plants species abundance. At stand level, multicohort fire-disturbed stands support the highest native species diversity. Thus, mimicking this natural pattern on silvicultural practices could safeguard higher understory native species diversity than managing policies creating homogeneous conditions. Even if closed forest communities present lower native species diversity values than open canopy communities, they sustain different PFTs that present high conservation values for forest fauna. Grazing pressure represents a threatening agent diminishing native forest-steppe biodiversity. The coexistence of different stands at different development stages in the same landscape ensures the seed bank pools of shade tolerant and heliophilous species.