Allelopathy: A tool for invader suppression and forest restoration

Background/Question/Methods

Forest restoration is the re-introduction of native species to naturally or anthropogenically disturbed areas to re-establish the natural habitat.  Early successional species, especially invasive exotic species, can inhibit or delay succession to the desired restoration endpoint.  Restoration of areas occupied by undesired species requires mechanical or chemical management to aid the restoration process, and this can be costly.  To address these problems, restoration ecologists have focused on identifying traits of native tree species that allow them to establish, grow rapidly, and out-compete exotic competitors. However, few studies have considered how allelopathic interactions may aid restoration, the same way it has been applied in agriculture and agroforestry as a useful tool for sustainable weed management.  In Panama, we conducted three greenhouse studies using leaves from tree species used for tropical forest restoration to determine whether litter from these trees could inhibit the growth of three problematic exotic grass species (Saccharum spontaneum, Panicum maximum, and Pennicetum purpurea). Survival, change in height, total above ground biomass, below ground biomass, and total biomass were compared between individuals grown with leaves from target tree species (treatment) and those with no leaves added (control).

Results/Conclusions

In an experiment comparing litter from three legume trees to three non-legume species, we found that legumes had a significantly more negative effect on Saccharum survival (Z= 3.88; df=5; p<0.001) and a marginally significant effect on shoot growth (F=2.95; df=1,4; p=0.09) than non-legumes. In another experiment with 17 different species of trees, plants with litter from legume trees showed marginally significantly lower above ground biomass (F=3.45; df=1,15; p=0.07) and below ground biomass (F=3.45; df=1,15; p=0.07), than those with non-legume litter but there was no difference in mortality in this study (Z=-0.58; df=16; p=0.56). An experiment with Panicum and Pennicetum with four tree species suggested that the effect of litter might vary substantially among response species, even if these are all taken from the same functional group (large, tropical C4 grasses). Panicum was little affected by the presence of leaves from legume and non-legume overstory trees, however these same trees facilitated survival and below ground biomass accumulation in Pennisetum. Species-specific responses to the presence of overstory trees suggest that evaluation of target understory species response to candidate tree species is necessary in order to make the appropriate management decision. These studies suggest the potential for incorporating allelopathy into the consideration of forest restoration strategies.