Establishment of invasive plant species in canopy gaps on Robinson Crusoe Island

Abstract

To design preventive and management measures it is crucial to identify the mechanisms that facilitate an invasion process. We evaluated how microsites affect establishment success of the alien small tree Aristotelia chilensis, the alien climber Rubus ulmifolius, and endemic tree species on Robinson Crusoe Island forest. Seedling and sapling densities were estimated in 65 canopy gaps in two forest stands (PEY and Villagra) along a canopy forest-gap gradient, and in three microsites (mound, pit, and trunk) of canopy gap-maker trees. Two statistical models were applied to evaluate the effects of the position in the gap (center, edge, and close forest) and substrate microsites in relation to topographic and structural variables on invasive and endemic species establishment. The density of invasive species was higher in the center of canopy gaps and forest edges than under forest canopy. Invasive plants were present in 86.5 % (n = 32 gaps in PEY) and 89.8 % (n = 25 forest gaps) in Villagra. A higher seedling and sapling density of endemic rather than invasive species was found in canopy-gap center and gap edges. However, trunks, mounds, and pits were dominated by invasive species. We found different responses in seedlings, saplings, and juvenile stages in relation to the explanatory variables studied and between the two stands. The decomposition state of gap-maker trees, which was used as a proxy of gap age, was the only variable that showed the same tendency in the two forest stands and for both invasive and endemic seedlings. In this case, we found that the densities of invasive and endemic seedlings were lower in older gaps, but sapling densities were higher in older gaps. Based on the results we cannot conclude a general pattern of seedling, sapling, and juvenile establishment in forest gaps for either invasive species, but in some gaps the invasion seems to start in trunks, mounds, and pits. We recommend focusing invasive species control on mounds and pits in some forest stands; and in general, we recommend controlling invasive saplings, more than seedlings or juveniles.