Vegetation Composition and Structure in Frequently Burned Northern Andean Shrublands

The northern Andes is a biodiversity hotspot. However, historical land use and the current increase in wildfires have profoundly altered the landscape. There is an urgent need to understand how successional processes and wildfires operate to support biodiversity management in this region. We studied the effect of wildfire frequency and time since the last fire on vegetation regeneration patterns in a heavily transformed northern Andean landscape, which was once dominated by Quercus humboldtii forests, the only native oak species in South America. Using the Iguaque mountains (Colombian Andes) fire map, we selected areas with different fire frequencies (three and five fires in thirty years) and the time since the last fire (one to six years) to study post-fire vegetation structure and composition. Post-fire communities were dominated by resprouting herbs and small shrubs, and their composition was closer to subpáramo vegetation than to Andean forests. Indeed, forest species were virtually absent from burned sites, as were obligate seeders. The richness, density and cover of herbaceous species recovered soon after the fire. In contrast, the richness and density of small shrubs increased one to six years after the last fire. The same trend, although not statistically significant, was observed for the density, frequency and basal area of large shrubs and trees. Fire frequency did not affect the composition and structure of the herbaceous layer. Conversely, there was a general trend towards a reduction in species richness, cover and the proportion of obligate resprouters of woody species, which was only significant for the basal area of large shrubs and trees. Exotic species were largely herbaceous and were dominated by Andropogon lehmannii and Melinis minutiflora . Their cover increased from the first to the sixth year after fire, reaching an average of 19.1 and 5.2% respectively, but was reduced by fire frequency to 1.5 and 1.2%. In contrast, Pteridium aquilinum cover increased in recently burned sites, reaching an average of 19.2%. Our results show that current wildfire regimes in the Iguaque Mountains favor the persistence of fire-tolerant herbs and shrubs, compromise tree recruitment and arrest forest succession. Therefore, new integrated fire management strategies are needed to protect remnant forest and establish oak forests in burned shrublands.