Rare frost events reinforce tropical savanna-forest boundaries

1. The ability of vegetation to ameliorate or exacerbate environmental extremes can generate feedbacks that mediate the distribution of biomes. It has been suggested that feedbacks between vegetation and frost damage may be important for maintaining savanna, particularly at the edge of the tropics. 2. We quantified frost damage and air temperature across a network of 30 permanent plots distributed across tropical savanna-forest boundaries in Brazil during an uncommonly hard frost. 3. Tree cover strongly buffered temperatures during frost events, such that forest sites were up to 5 degrees C warmer than nearby sites occupied by open shrub savanna. Consequently, in forest, woody plants were not damaged, but in savanna, there was extensive dieback of trees and shrubs. Within savanna, frost had disproportionately large effects on small individuals, likely due to colder temperatures near the ground and the lower thermal mass of thin stems. Across species, frost tolerance was strongly correlated with latitudinal range limit, revealing the importance of minimum temperature as a species filter at the regional scale. Counterintuitively, savanna species, which are adapted to open habitats where frost is more likely, were more vulnerable to cold than were forest species and experienced more than twice the amount of leaf damage as co-occurring forest species. 4. Synthesis. Frost reinforces the effects of fire on vegetation structure by preferentially damaging trees in open environments and generating high loads of dead, flammable fuels. However, frost does not currently occur with sufficient frequency in the region to maintain open savanna by itself. Nevertheless, it occurs with sufficient frequency and severity that it likely acts as an environmental filter and evolutionary selective factor for some of the most frost-sensitive species.