The current fire regime in northern Andean shrublands hinders tree recruitment and arrests forest succession

Abstract

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. In the past, Andean Forests were common in this region. Quercus once dominated humboldtii, the only Quercus species native to South America, which was frequent in these forests. Using the fire map of the Iguaque Mountains (Colombian Andes), 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. Resprouting herbs and small shrubs dominated post-fire communities. Their composition was closer to subpáramo vegetation (greater richness and density of herbaceous and shrub species than tree species) than to Andean forests. Indeed, forest species were virtually absent from burned sites, as were obligate seeders. Fire frequency did not affect the composition and structure of the herbaceous layer. Conversely, we found a substantial reduction in species richness and cover of woody species with the increase in fire frequency, a reduction of the basal area of large shrubs and trees, and a trend towards a decrease in the proportion of obligate tree resprouters. The richness, density, and cover of herbaceous species was high the first year after the fire and still showed a small increase until year 6, as the number of obligate resprouters decreased. The basal area of woody species barely increased between years 1 and 6, but the richness and density of shrubs and trees increased gradually during this period. Exotic species were largely herbaceous and dominated by Pteridium aquilinum and Andropogon lehmannii. Their cover was not affected by fire frequency or time since the last fire, although its composition changed. While the cover of Pteridium aquilinum was not affected by fire frequency, it decreased from year 1 to year 6. 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. New integrated fire management strategies are needed to protect remnant forests and establish oak forests in burned shrublands.