Increasing topographic influence on vegetation structure during primary succession

Mount Pinatubo, Philippines (15.14A degrees N, 120.35A degrees E) erupted violently in 1991 to initiate significant primary succession. Aspect, the direction faced by a slope, affects patterns of vegetation at higher latitudes, but such effects remain unreported in the wet tropics. Therefore, we monitored species composition and cover in established plots during 2006, 2010, and 2013 to characterize how aspect affected primary succession. We used redundancy analysis (RDA) to assess vegetation change in response to time and environmental factors. Vegetation cover increased from 153 to 245% on north-facing slopes, and from 174 to 230% in south-facing slopes while species richness and diversity indices also increased. From 38 to 63% of the species were restricted to one aspect, depending on the year of study. Redundancy analysis demonstrated that aspect strongly affected species composition and that its effects persist. Fabaceae was concentrated on south-facing slopes, which suggested that aspect effects might be accentuated due to enhanced soil nitrogen. Vines, grasses, and forbs, all typical of habitats with greater insolation, were more abundant on south aspects, while trees and ferns were more common on the north aspects. This is the first survey of vegetation dynamics using permanent plots on new volcanic surfaces in this region. Aspect differences produced distinct insolation and moisture patterns that enhanced habitat diversity and altered species composition. This effect has not been noted in monsoon forests. Aspect may continue to initiate divergence in succession trajectories as soils and vertical canopy structure develop differentially in response to differential dominance.