Plant species abundance and phylogeny explain the structure of recruitment networks

Established plants can affect the recruitment of young plants, filtering out some and allowing the recruitment of others, with profound effects on plant community dynamics. Recruitment networks (RNs) depict which species recruit under which others. We investigated whether species abundance and phylogenetic distance explain the structure of RNs across communities. We estimated the frequency of canopy-recruit interactions among woody plants in 10 forest assemblages to describe their RNs. For each RN, we determined the functional form (linear, power or exponential) best describing the relationship of interaction frequency with three predictors: canopy species abundance, recruit species abundance and phylogenetic distance. We fitted models with all combinations of predictor variables, from which we simulated RNs. The best functional form of each predictor was the same in most communities (linear for canopy species abundance, power for recruit species abundance and exponential for phylogenetic distance). The model including all predictor variables was consistently the best in explaining interaction frequency and showed the best performance in predicting RN structure. Our results suggest that mechanisms related to species abundance are necessary but insufficient to explain the assembly of RNs. Evolutionary processes affecting phylogenetic divergence are critical determinants of RN structure.

Interaction of land management and araucaria trees in the maintenance of landscape diversity in the highlands of southern Brazil

In the southern Brazilian highlands, pre-Columbian societies created domesticated landscapes through the use and management of forests, including nurse Araucaria angustifolia trees, a common conifer in these regions. Nowadays, local smallholders still use traditional practices, such as burning, to promote vegetation for cattle grazing in highland grasslands. Even though burning is normally of small extent and low frequency, such management can slow down natural forest expansion and contribute to the maintenance of grasslands, by opposing the facilitative effect of nurse araucaria trees. To comprehend the interplay between human cultural management, species interactions and the environment, it is important to better understand how these relations affect diversity and composition. Our goal was to investigate how land management, biotic interactions and abiotic factors affect saplings species richness, abundance and composition. We hypothesized that (1) land management would decrease sapling richness and abundance and change sapling composition, (2) nurse araucaria trees would increase species richness and abundance and change sapling composition, and (3) the interactive effect between land management and nurse araucaria trees would shape sapling richness, abundance and composition. Data were collected in unmanaged and managed conditions, both beneath araucaria crowns and in nearby treeless areas. Our results indicate that abundance and species composition are affected by land management and araucaria crown influence. The highest values of sapling abundance were found beneath crowns in unmanaged areas. Species composition changed between all assessed combinations of land management and crown influence. Our study demonstrates the major roles of land management and facilitation in structuring communities, despite the effects of rock and grass cover. Moreover, our results clarify patterns and processes that may emerge in natural highland grasslands, such as the conversion of grasslands into forests and the loss of cultural landscapes when the main local management actions are excluded.