Edge expansion of Araucaria forest over southern Brazilian grasslands relies on nurse plant effect

The vegetation in the southern Brazilian highlands is characterized by Araucaria forest and Campos grassland. Evidences indicate that Araucaria forest is currently expanding over grassland and that this expansion may occur by nucleation or edge dynamics. Nucleation mechanisms of Araucaria forest expansion are well documented, whereas mechanisms of expansion starting from the forest edge are not. In this study, we aimed (1) to assess how prominent is Araucaria forest expansion over Campos grassland starting from the forest edge, and (2) to discuss about the possible mechanisms underlying sapling community colonization in grassland. We conducted our sampling in 11 transects disposed from the forest edge towards the grassland. The transects were distributed in four study sites. Each transect was 50 m long and was divided into 25 plots with area of 8 m2. All forest woody sapling individuals were identified and registered. We considered distance from forest edge, presence of dead shrubs, Baccharis uncinella cover, nurse tree cover and rock cover as predictors of forest plant abundance and composition. Niche and distance contribution on the explanation of sapling composition and abundance were partitioned using, respectively, canonical correspondence analysis and multiple regressions. Cover of nurse trees explained almost 31% of total variation in sapling abundance, followed by the distance from the forest edge that explained 6.9%. Site explained 7.6% of total variation in sapling species composition, followed by distance from the forest edge, which explained 2.3%, whereas niche had a minor (2.1%) and non-significant proportion of variation explanation. Our findings show that Araucaria forest expansion over native grasslands in southern Brazil relies deeply on the nurse plant effect. Previous studies have demonstrated that Araucaria forest forms nuclei scattered in the grassland where nurse plants or nurse rocks are established. Here, we bring evidences that nurse plants are important also to the tree encroachment starting closer to the forest edge. This study provides new information on the mechanisms involved in the forest expansion over native grasslands.