Does the high density of Psychotria shrubs in the understory influence the natural regeneration of trees?

Abstract

Shrubs of the Psychotria L. (Rubiaceae) genus can occur like a dominant species in the understory of subtropical forests. This study aims to understand the ecological conditions of Psychotria dominance, specifically Psychotria leiocarpa Cham. & Schltdl. and Psychotria carthagenensis Jacq. in the understory, and its relation with natural tree regeneration. We sampled 162 plots of 1 m2 in an ecotonal Semi-deciduous Seasonal Forest remnant (243.28 ha) in the Pampa biome, in Southern Brazil. The analysis considered three habitat-selective regimes which occur in the remnant (road-edge, eucalyptus-edge, and forest-interior), and environmental variables (air temperature, relative humidity, canopy opening, inclination, solar exposition, altitude, and ground cover), according to which relationships between Psychotria and regenerating trees could be established in a different way. We assumed the hypothesis that the dominance of Psychotria shrubs in the understory may influence the natural regenerating trees and differ in relation to habitat-selective regimes and in function of environmental variables. No evidence of effective ecological dominance was found. The results indicated regular regeneration density for regenerating trees and for Psychotria shrubs. P. leiocarpa was predominant in the regeneration community, followed by regenerating trees and P. carthagenensis. In addition, P. carthagenensis demonstrated an aggregated behavior, forming few groups with high density. The environmental variables influenced the composition and/or abundance of the regeneration community and were more intense than the habitat-selective regimes. The habitat-selective regimes mostly acted in P. leiocarpa, which presented less density at the road-edge. Our results demonstrate that Psychotria shrub densities do not decrease the natural regeneration of trees, thus suggesting that P. leiocarpa and P. carthagenensis are multifunctional species which act in the understory as filters which favor ecological succession, even in different habitat-selective regimes. Therefore, they can be a key tool for ecological restoration and conservation, acting as core species in strategies to promote succession processes and ecological enrichment.