Nurse rocks influence forest expansion over native grassland in southern Brazil



Questions: Do small rocky outcrops favor forest plant colonization and establishment in grasslands? If so, what are the potential mechanisms involved in this process?

Location: Araucaria forest and Campos grassland mosaic in southern Brazilian highlands (29°29′S, 50°12′W).

Methods: We collected data on the density of forest woody species in plots located on rocky outcrops and in open fields subject to different management regimes. We evaluated the influence of outcrops on the density of forest plants ≤60-cm tall, and the effects of other environmental variables and of site on plant density; we also used information on diaspore traits of the species to discuss the way in which plants disperse to the outcrops.

Results: Rocky outcrops harbored a significantly higher density of forest plants than open fields, irrespective of site effect, and rock height was the best predictor for plant density on outcrops. Diaspores of all recorded species possess characteristics associated with dispersal by birds or mammals or by both.

Conclusions: Small rocky outcrops markedly influence forest expansion by acting as perches for vertebrate dispersers and as nurse objects and safe sites for plants. Forest expansion starting in small outcrops possibly occurs as follows: perching of dispersers and increase of seed rain on rocks, promotion of better conditions of establishment for forest plants by nurse rocks, protection of plants sensitive to grazing and fire, and nucleation of forest vegetation.