Selective logging and fire as drivers of alien grass invasion in a Bolivian tropical dry forest

Abstract

Logging is an integral component of most conceptual models that relate human land-use and climate change to tropical deforestation via positive-feedbacks involving fire. Given that grass invasions can substantially alter fire regimes, we studied grass distributions in a tropical dry forest 1–5 yr after selective logging, and experimentally tested the effect of forest fire on populations of invasive grasses. In unlogged forests and in microhabitats created by selective logging we found a total of four alien and 16 native grass species. Grasses covered 2% of unlogged and 4% of logged forest, with grass cover in logged forest concentrated in areas directly disturbed by logging; log landings and roads had relatively greater grass cover (37% and 17%, respectively) than did skid trails (10%) and felling gaps (8%). Total grass cover and grass species richness increased with canopy openness and were greatest in sites most severely disturbed by logging. The grass flora of these disturbed areas was composed mostly of native ruderal species (e.g., Digitaria insularis, Leptochloa virgata), a native bamboo ( Guadua paniculata), and Urochloa ( Panicum) maxima, a caespitose C 4 pasture grass introduced from Africa. Urochloa maxima formed monodominant stands (up to 91% cover and 2–3 m tall) and grew on 69% of log landings and 38% of roads. To better understand the potentially synergistic effects of logging and fire on the early stages of grass invasion, we tested the effect of a 12-ha experimental fire on U. maxima populations in a selectively logged forest. Three years after the fire, the area covered by alien grass in burned forest increased fourfold from 400 m 2 (pre-fire) to 1660 m 2; over the same period in a logged but unburned (control) area, U. maxima cover decreased from 398 m 2 to 276 m 2. Increased canopy openness due to fire-induced tree mortality corresponded with the greater magnitude of grass invasion following fire. Selective logging of this dry forest on the southern edge of the Amazon Basin promotes alien grass invasion; when coupled with fire, the rate of invasion substantially increased. Recognition of the grass-promoting potential of selective logging is important for understanding the possible fates of tropical forests in fire-prone regions.