Leaf-Litter Depth, Not Canopy Openness, Mediates the Occurrence of an Invasive Shrub From the Forest Edge to Interior

Continental tropical forests with intact canopies and densely shaded understories are often perceived to be particularly resistant to biological invasions, yet canopy disturbances leave them vulnerable to gap colonization by invasive plants. The Neotropical shrub, Miconia crenata, is an example of a successful invader in intact continental tropical forests but is generally well known to be restricted to forest trails and edges. However, existing studies that quantify the decline in M. crenata from forest edge to interior rarely compare the relative importance of direct edge effects with indirect edge effects mediated by processes that vary with distance from forest edge. To address this gap, we aim here to identify the most relevant processes influencing M. crenata invasion in the secondary tropical forests of Singapore. Using a path model on field observations collected using an adaptive cluster sampling methodology, we found that the occurrence of M. crenata did not change directly with distance from forest edge. Instead, the decreased occurrence of M. crenata in forest interiors was indirectly mediated by thicker leaf litter, which is detrimental to the small seeds of M. crenata. While there was a direct positive relationship between canopy openness and M. crenata occurrence, canopy openness was not a mediator for relationships with distance to forest edge. These findings suggest that environmental filtering of germinants is an important determinant of the invasion success of M. crenata, and that litter retention could be an effective management against M. crenata even in smaller fragments where canopy gaps have extended far into the forest interior.