Ecological manipulation of Psidium guajava to facilitate secondary forest succession in tropical forests.
Psidium guajava L. has been documented as an exotic invasive species in many parts of the world, but little is known about its interactions with native woody species during secondary forest succession in tropical forests. Its invasion and interactions with native species in different stages of secondary forest succession were assessed in Kakamega Rainforest in western Kenya. The study covered three forest blocks each with five different forest types, namely: open fields, young secondary forest, middle-aged secondary forest, old-growth secondary forest and disturbed primary forest, which served as the control. Open fields that were subjected to frequent clearing to control the spread of Psidium guajava remained under a thicket of the species two decades later. On the other hand, open fields where Psidium guajava was ignored, either due to lack of resources or sheer neglect, transformed into young secondary forest stands within a decade. The transformation increased woody species richness from 2.0±0.0 to 5.0±0.0 ha-1, and the Shannon diversity index from 0.30±0.33 to 1.10±0.01. It reduced the dominance of Psidium guajava from 80.5±22.7 to 62.26±0.84% and changed the canopy structure. The change in canopy structure led to the mortality of Psidium guajava stems in the sub-canopy and understory layer, which significantly reduced its stem density from 1,111±313 to 639.4±45 stems ha-1. The pattern was repeated in middle-aged secondary forest stands with woody species richness increasing to 26.0±8.2 ha-1, and Shannon index to 2.72±0.32. Psidium guajava's dominance and stem density decreased further to 30.44% and 400.57 stems ha-1, respectively, due to mortality attributed to shading by native tree species. In the old-growth secondary forest, only snags of Psidium guajava were recorded. The species was not represented in the disturbed primary forest. The results indicate that Psidium guajava facilitates secondary forest succession by allowing shade-tolerant native tree species to recruit and grow in its shade. It is thereafter eliminated when the native species close the forest canopy. The species can be ecologically manipulated to facilitate post-disturbance forest regrowth and thereafter removing it when the forest canopy begins to close.