Acacia caven nurses endemic sclerophyllous trees along a successional pathway from silvopastoral savanna to forest

Abstract

The successional pathways linking the Acacia caven-dominated savanna habitat '' espinal '' and the closed sclerophyllous forest of central Chile have long been debated. Previously, espinal was considered an invasive degradation of closed forest that tended toward desertification, could not be restored to forest, and had little ecological value. Recent GIS (Geographical Information System) research on land-use change has, however, detected apparent regeneration of sclerophyllous forest from espinal. This suggests that there is a successional path linking espinal and sclerophyllous forest. Here, we used surveys of transects in espinals and espinal-sclerophyllous forest transitions to ask whether (1) A. caven is an invasive species or a pioneer species, (2) forest regenerates by sclerophyllous trees '' filling in '' spaces between A. caven individuals, and then shading them out (plant-plant competition), or (3) forest regenerates by plant-plant facilitation between A. caven and other species, and (4) how current and historical management and condition affect these potential successional mechanisms. We find that A. caven establishes primarily in full sunlight and is unlikely to degrade forests via invasion. We also find, for the first time, evidence that A. caven is a nurse tree to several sclerophyllous forest tree-beneficiary species. Measurements of the under-canopy microhabitat of A. caven, compared to Lithraea caustica, another possible nurse species, suggest that it provides a balance between shade and soil moisture retention, making it a regeneration site not only for directed bird-dispersed seeds but also for undirected wind-dispersed ones. Conservation and restoration of espinals, especially in drier areas, could provide capacity for future dynamic successional pathways in central Chile