Protecting Spain's mountain ecosystems is of great importance to preserve ecosystem services. Global warming is increasing the vulnerability of these habitats, especially in peninsular Spain. Biosphere Reserves are internationally protected areas that seek to protect biodiversity and, at the same time, promote sustainable development. Evaluating these protected areas is essential to verify environmental changes and establish priorities in their management. In this work, we have studied the time trends of NDVI in the high mountain Biosphere Reserves of Spain, in the period from 2001 to 2016, to check if the trend patterns are associated with: the Biosphere Reserves studied, surface temperature, water stress, altitude, slope, habitat type, biogeographic region, zonation, and distance to population centres. Significant differences were found between NDVI trends and high mountain Biosphere Reserves. First, significant positive trends in NDVI were observed when analyzing both reserves together. However, significant differences were found between the two reserves. The Ordesa-Viñamala Reserve shows higher positive NDVI trends and lower negative trends, while this pattern is reversed in the Sierra Nevada. Temperature and water stress affect the Sierra Nevada Reserve to a greater extent, increasing the number of negative NDVI trends. Habitat types show different patterns depending on the Reserve, with less resistance and, therefore, more vulnerable habitats. It has been observed that the zoning of the reserves significantly affects NDVI trends, following a pattern of increasing negative trends as we move away from the core. The height variable appears to substantially affect NDVI trends, with an increase in positive trends as we move in altitude. Finally, the distance to population centers has significantly influenced NDVI trends; generally, greater negative trends have been observed in areas closer to municipalities
