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Are Chiloe's forests immune to plant invasions: A test of three models.
Windham, Lisamarie1, Kaufman, Sylvan2, Armesto, Juan3, 1 2 3
ABSTRACT- Mature forests of Isla de Chiloe have remained effectively uninvaded by exotic understory plants despite pressure from populations of plant species known to be problematic in analagous forests of North America and Europe. Here we report environmental conditions and the presence of exotic species in 30 permanent transects across different forest edges and 9 permanent plots in forests of different successional stages. Of more than 40 species introduced to Chiloe, none were observed within forests, and only two, Himalayan blackberry (Rubus discolor) and scotch broom (Cytisus scoparius), were found within 20 meters forest edges. The diverse (>100 native species encountered), dense structure of vegetation, especially a native, invasive bamboo (Chusquea quila), may reduce sites and access for exotic recruitment in forests. Although resources were not correlated with the presence of exotic species, edge plots were the most depauperate in light and soil nitrogen (especially nitrate) and forest plots, though generally resource-poor, were extremely patchy in soil nitrogen and light availability. With historical surveys, habitat comparisons, and field and greenhouse experiments, we are testing 3 hypotheses to explain the current lack of exotic plants within these forests. First, the forests are not immune and populations of exotic plants are simply at an early stage of population growth. Second, the native understory community effectively controls fluctuating resources (light and/or nutrients). Third, low nitrate:ammonium ratios prevent the invasion of nitrophilic understory plants.
KEY WORDS: forest succession, nitrate reductase activity, Cytisus scoparius, Chusquea quila