The importance of plant life form on spatial associations along a subtropical coastal dune gradient

Abstract

Questions Does the spatial association between isolated adult trees and understorey plants change along a gradient of sand dunes? Does this association depend on the life form of the understorey plant? Location Coastal sand dunes, southeast Brazil. Methods We recorded the occurrence of understorey plant species in 100 paired 0.25 m2 plots under adult trees and in adjacent treeless sites along an environmental gradient from beach to inland. Occurrence probabilities were modelled as a function of the fixed variables of the presence of a neighbour, distance from the seashore and life form, and a random variable, the block (i.e. the pair of plots). Generalized linear mixed models (GLMM) were fitted in a backward step‐wise procedure using Akaike's information criterion (AIC) for model selection. Results The occurrence of understorey plants was affected by the presence of an adult tree neighbour, but the effect varied with the life form of the understorey species. Positive spatial association was found between isolated adult neighbour and young trees, whereas a negative association was found for shrubs. Moreover, a neutral association was found for lianas, whereas for herbs the effect of the presence of an adult neighbour ranged from neutral to negative, depended on the subgroup considered. The strength of the negative association with forbs increased with distance from the seashore. However, for the other life forms, the associational pattern with adult trees did not change along the gradient. Conclusions For most of the understorey life forms there is no evidence that the spatial association between isolated adult trees and understorey plants changes with the distance from the seashore, as predicted by the stress gradient hypothesis, a common hypothesis in the literature about facilitation in plant communities. Furthermore, the positive spatial association between isolated adult trees and young trees identified along the entire gradient studied indicates a positive feedback that explains the transition from open vegetation to forest in subtropical coastal dune environments.

The role of terrestrial bromeliads in determining the spatial organization of plant life forms in a tropical coastal forest

“…We did not observe discernable effects of stress gradients on the organization of forest understory communities in this coastal system. Likewise, Castanho et al (2012) found no evidence that spatial life form associations change with increasing distance from seashore. Our study showed that the findings for sand dunes (Castanho et al 2012) and shrub vegetation (Pimentel et al 2007) also apply to the forest formations in the restinga.…”

“…It seems that the restinga forest and the beach vegetation ) are capable to buffer the effects of wind and salt spray. Distance from seashore did not affect spatial associations and nurse effects in restingas (Castanho et al 2012;Castanho & Prado 2014). Moreover, in open shrub formations of the restinga, distance from seashore was not associated with floristic composition or structural parameters of the vegetation.…”

“…In coastal systems, distance from seashore is a good proxy for abiotic stress (Castanho et al 2012). In our study, the distance from seashore to the nearest transect was about 170 meters.…”

“…Th e benefi t from nurse plants may not change with increasing distance from seashore (Castanho & Prado 2014). Moreover, distance from seashore did not aff ect the outcome of associations between diff erent life forms, such as trees, herbs, climbers or shrubs (Castanho et al 2012). Brancalion et al (2009) found that terrestrial bromeliads were mainly present in open canopy areas, where they negatively aff ected palm seedlings.…”

“…Brancalion et al (2009) found that terrestrial bromeliads were mainly present in open canopy areas, where they negatively aff ected palm seedlings. Knowledge on how terrestrial bromeliads organize in space in response to canopy cover with increasing distance from seashore (a proxy for stress conditions, sensu Castanho et al 2012) and how they associate with other life forms can provide a better understanding of the interplay between plant associations and light environment in restinga forests. Th is has not yet been quantitatively tested under a spatially explicit approach.…”