Nucleation by isolated emergent native trees can push back the shade-intolerant invader Broussonetia papyrifera (L.) L'Hér. ex Vent.

Abstract

The facilitatory role that isolated trees play in the regeneration of degraded areas that are not under the cover of invasive exotic plants is well recognized. In contrast, the influence of isolated trees in restoring sites invaded by exotic plants has hardly been studied. Such studies are needed because conditions that favor native tree regeneration under isolated trees, such as microclimate amelioration, increased nutrient levels, and facilitated dispersal, may also be favorable to certain species of exotic plants. Using transects, we studied species richness, abundance and resilience (as indicated by re-sprouting) of trees regenerating underneath the crowns of 15 isolated native trees that are emergent over a near continuous canopy of paper mulberry Broussonetia papyrifera in a degraded area of Mabira Forest, Uganda. Data collected beneath emergent tree crowns were matched by control data from the area immediately beyond the crowns. Native tree species abundance and richness were significantly higher beneath than beyond crowns; while the opposite was true of the density of B. papyrifera seedlings and saplings. Species richness was significantly higher beneath than beyond crowns for late-, but not early-succession trees. Polynomial regressions of species richness and abundance showed that B. papyrifera abundance increased significantly from stem bases to crown edges and leveled-off beyond crowns; while the density of native tree species was constant from stem bases to crown edges and decreased thereafter. Native tree species richness peaked at crown edges. The proportion of re-sprouts assigned to cutting of native tree species was significantly higher beyond than beneath crowns. We attribute the higher abundances and species richness of native species underneath crowns to both facilitation by isolated trees and the apparent shade-intolerance of B. papyrifera. Significantly higher levels of re-sprouting by native tree species could be an indication that the regeneration underneath crowns is less resilient to disturbance than that beyond, or that less resilient species occur beneath crowns. Overall, the results suggest that in sites invaded by mulberry, isolated trees have potential to accelerate recovery of native tree species while suppressing mulberry. Peaking of species richness at crown edges is not a commonly reported finding for isolated trees and needs to be further investigated.