Fire threatens the diversity and structure of tropical gallery forests

Gallery forests are widespread in most tropical savanna landscapes creating a mosaic of grass-dominated systems with tree components and forest patches. Interactions among water, nutrient availability, and wildfires have influenced the structure, patterns, and processes of these landscape systems and have shaped today’s biodiversity in the savanna–forest transitions of the Orinoco basin in Colombia and Venezuela. However, savanna fires have become larger and more frequent in these areas. In order to investigate the impacts of fire on gallery forests, we established 18 forest plots of 0.1 ha within three different fire exposures: (1) fire affected forests along the edge (<100 m) from open savanna to gallery forest (burned edge), (2) fire affected closed forests (burned interior; >100 m from the edge), and (3) no evidence of fire (unburned). We identified responses of the vegetation to fire (through natural regeneration) by analyzing the compositional and structural changes and quantifying fire-related functional traits for the most abundant tree species. We surveyed 128 species, corresponding to 77 genera and 35 families. Tree height is over 10.70 m (± 2.72) in unburnt forests. Dead tree presence in burnt edges (21.76% ± 24.3) almost doubles that of burnt interior (13.08% ± 15.73). The aboveground biomass (AGB) of unburnt forests (176.9 Mg/ha) was more than double that of the interior burnt forests (74.4 Mg/ha) and four times higher that AGB in the edge burnt sites (41.3 Mg/ha). Of the most abundant species, 62% were classified as fire-sensitive species, 14% were fire survivors and 24% were resistant to fires and mostly present in communities with lower diversity. Although fire-tolerant species in general presented thicker barks, higher wood densities, and higher leaf dry matter content, no clear pattern of fire response was associated only with functional traits. Thus, overall fire is resulting in strong compositional changes, with more fire-resistant species, which are species that establish in disturbed areas and encouraging grasses, resulting in savanna encroachment into gallery forests. Consequences on the long-term should be monitored.