Soil factors predict initial plant colonization on Puerto Rican landslides

Tropical storms are the principal cause of landslides in montane rainforests, such as the Luquillo Experimental Forest (LEF) of Puerto Rico. A storm in 2003 caused 30 new landslides in the LEF that we used to examine prior hypotheses that slope stability and organically enriched soils are prerequisites for plant colonization. We measured slope stability and litterfall 8–13 months following landslide formation. At 13 months we also measured microtopography, soil characteristics (organic matter, particle size, total nitrogen, and water-holding capacity), elevation, distance to forest edge, and canopy cover. When all landslides were analyzed together, plant biomass and cover at 13 months were not correlated with slope stability or organic matter, but instead with soil nitrogen, clay content, water-holding capacity, and elevation. When landslides were analyzed after separating by soil type, the distance from the forest edge and slope stability combined with soil factors (excluding organic matter) predicted initial plant colonization on volcaniclastic landslides, whereas on diorite landslides none of the measured characteristics affected initial plant colonization. The life forms of the colonizing plants reflected these differences in landslide soils, as trees, shrubs, and vines colonized high clay, high nitrogen, and low elevation volcaniclastic soils, whereas herbs were the dominant colonists on high sand, low nitrogen, and high elevation diorite soils. Therefore, the predictability of the initial stage of plant succession on LEF landslides is primarily determined by soil characteristics that are related to soil type.