Spread of common native and invasive grasses and ruderal trees following anthropogenic disturbances in a tropical dry forest
Introduction: A fundamental challenge to the integrity of tropical dry forest ecosystems is the invasion of nonnative grass species. These grasses compete for resources and fuel anthropogenic wildfires. In 2012, a bulldozer from the Puerto Rico Electric Power Authority cleared a 570-m trail from a state road into a mature dry forest section of Guánica Forest to control a wildfire. We monitored colonization by a non-native invasive grass (Megathyrsus maximus), a highly invasive tree (Leucaena leucocephala), and a native grass (Uniola virgata), as well as natural regeneration, along the bulldozer trail. We determined whether bulldozing facilitated colonization by these species into the forest and the extent of spread.
Results: Distance from propagule source and temporal variations strongly influenced colonization by our three focal species. Megathyrsus maximus invaded along the trail from source populations by the state road. The establishment of new colonies of M. maximus seedlings went as far as 570 m inside the forest (i.e., at the end of the bulldozer trail), but we found most new colonies within 270 m of the road. Leucaena leucocephala exhibited a similar spreading pattern. Before disturbance, Uniola virgata was distributed widely across the forest, but the highest densities were found in areas near the latter portion (> 401 m) of the bulldozer trail. Subsequently, the species formed new clumps along more than half of the trail (250 to 570 m), apparently colonizing from undisturbed patches nearby.
Conclusions: Bulldozing facilitated the invasion of non-native vegetation. The projected community assemblage will be more fire-prone than before since M. maximus carries fire across the landscape better than U. virgata, emphasizing the capacity of invasive plant colonization to alter local ecological processes after only a single wildfire and bulldoze event. Our results provide a valuable baseline for short-term vegetation response to anthropogenic disturbances in tropical semi-deciduous dry forests.