Physiognomic and physiologic changes in mountain grasslands in response to environmental and anthropogenic factors

Abstract

Mountain grasslands provide valuable ecosystem services for sustainable development and human wellbeing. These habitats have suffered important changes related with their physiognomic (biomass) and physiologic (greenness) properties. Some of these changes received significant attention i.e. woody encroachment, while others, like the changes in biomass and greenness of those grasslands that have not experienced woody encroachment are almost unknown. We calculated physiognomic and physiologic properties for dense grasslands not affected by woody encroachment through the Normalized Difference Vegetation Index (NDVI) and Normalized Difference Infrared Index (NDII) from Landsat-5 Thematic Mapper. Imagery taken in the late-1980s and late-2000s in the Spanish Pyrenees were analyzed with multi-temporal vectors to detect increases or decreases of biomass and greenness. To understand the source of these changes, we modeled them with anthropogenic (land use, i.e. grazing, ski resorts, and related infrastructures) and environmental factors (topographic, lithologic and climatic). Anthropogenic factors were most strongly correlated with decrease in the biomass and greenness, showing degradation patterns of the grasslands at localized patches. Nonetheless, environmental factors were most strongly correlated with positive changes in both indices, detecting a continuous pattern in the increase in biomass and greenness. In areas that had high livestock stocking rates, grasslands biomass and greenness decrease, while in areas that had low stocking rates, biomass and greenness increases. Grasslands at low elevation showing decrease in biomass and greenness were either on gentle slopes and largely affected by human activities, or on steep slopes locally affected by ski resorts. In areas that have been disturbed by anthropogenic factors, the increase of rain in early summer trigger erosion processes, enhancing the negative effect of anthropic pressure on grassland greenness and biomass. In contrast, grasslands at high elevations, on steep slopes, and those that had north or west aspects and that had an acidic lithology, with less continentality, and that received more rain, had the most increase in biomass and greenness. Those results suggest that changes in mountain grasslands, apart from woody encroachment, are deeply altering their physiology and physiognomy, pointing out direct relationships with current management practices and climate trends.