The regulating role of Mimosa acantholoba var. eurycarpa in tropical dry forest succession: stem twisting as a successional accelerating mechanism

Abstract

Several Fabaceae species dominate tropical dry forest (TDF) early succession thanks to their mechanisms to cope with the harsh environmental conditions prevailing at these successional stages. Mimosa acantholoba var. eurycarpa is one of these dominant species. We studied its dominance effect with special attention to the consequences of a stem deteriorating process here referred to as ‘stem twisting’, which ultimately causes stem breakage and leads to crown de-structuring and possibly premature tree death. Using 16 years of dynamic data gathered in permanent secondary forest plots spanning a broad age range in a Mexican TDF, we evaluated the effect of this species on understorey vegetation attributes. The density of M. acantholoba var. eurycarpa reduced understorey plant density. Almost half (47%) of the trees of this species underwent stem twisting at some point in succession, with the twisting probability being directly related to increments in tree size (crown area and basal area). The death risk of twisted trees (adjusted for crown area, number of stems, and basal area) was three-fold that of non-twisted trees. With age, M. acantholoba var. eurycarpa trees collapse because of their twisted stem condition, altering forest structure and opening the canopy. This phenomenon releases the growth of late-successional species in the understorey, thus accelerating successional development. These results indicate that stem twisting is a key driver of successional speed, given its high occurrence among adult trees of the study species. We conclude that M. acantholoba var. eurycarpa is a keystone taxon capable of controlling the successional pathway in TDF, first through its dominance in early succession and later via its massive death due to stem twisting.