Questions: To what extent are the distributions of tropical rain forest tree ferns (Cyatheaceae) related to environmental variation, and is habitat specialization likely to play a role in their local coexistence?
Location: Lowland rain forest at La Selva Biological Station, Costa Rica.
Methods: Generalized linear (GLM) and generalized additive (GAM) logistic regression were used to model the incidence of four tree fern species in relation to environmental and neighbourhood variables in 1154 inventory plots regularly distributed across 6 km2 of old-growth forest. Small and large size classes of the two most abundant species were modelled separately to see whether habitat associations change with ontogeny.
Results: GLM and GAM model results were similar. All species had significant distributional biases with respect to microhabitat. Environmental variables describing soil variation were included in the models most often, followed by topographic and forest structural variables. The distributions of small individuals were more strongly related to environmental variation than those of larger individuals. Significant neighbourhood effects (spatial autocorrelation in intraspecific distributions and non-random overlaps in the distributions of certain species pairs) were also identified. Overlaps between congeners did not differ from random, but there was a highly significant overlap in the distributions of the two most common species.
Conclusions: Our results support the view that habitat specialization is an important determinant of where on the rain forest landscape tree ferns grow, especially for juvenile plants. However, other factors, such as dispersal limitation, may also contribute to their local coexistence.
Nomenclature: Moran & Riba (1995).