How environment and grazing influence floristic composition of dry Puna in the southern Peruvian Andes

High mountain environments located in the tropics account for as much as 10 % of the total surface area of high mountain ecosystems worldwide, yet their ecology has been studied much less thoroughly than outside the tropics. The dry Puna is the largest ecosystem of the high tropical Andes and one of the leading biodiversity hotspots worldwide. In this high mountain environment, the main stress factors are the long period of water shortage, high degree of potential evapotranspiration, wind erosion, low soil nutrient content, and extreme thermal variation. The aim of this study was to deepen our understanding of the relation between the dry Puna species composition and diversity, and environmental/anthropogenic variables. Species cover was determined and soil samples were analysed from 121 plots, along altitudinal transects located between 3,900 and 4,900 m a.s.l. in South Peru. The data were statistically analysed by means of Multivariate regression tree analysis, Indicator Species analysis and Redundancy analysis. The hypothesis that in the dry Puna, altitudinal gradient, aspect and rockiness are key factors that mainly determine the floristic composition of the vegetation was confirmed. The local soil features emerged as a secondary driver in the plant community composition. Moreover, disturbance intensity appeared to be a main factor in determining changes in the plant community diversity, also modifying the site ecology (nutrient content, pH) and the structure of ecosystems (from grass dominated systems to dwarf shrub-dominated communities). High grazing intensities caused the spread of species with avoidance strategies such as thorny dwarf shrubs (Tetraglochin cristaturn), annual and prostrate plants. Harsh conditions (thin and coarse soil, severe solar radiation, low temperatures and thermal fluctuations) promoted the spread of cushion plants (Pycnophyllurn sp. pl.), while aspect and soil features (pH, silt % and potassium content) shaped the species composition of Festuca orthophylla tall grassland, the dominant plant community in the dry Puna.