Tropical restoration strategies often include planting native tree species to overcome two major restoration barriers: (a) dispersal limitation and (b) establishment limitation. Dispersal is enhanced by the resources (food, shelter, etc.) these trees provide that attract dispersal agents; seedling establishment is enhanced by how the trees modify the microhabitat's abiotic conditions (e.g., seedlings protected from desiccation by shade that eliminates weedy species). In a restoration plantings experiment in Los Tuxtlas, Veracruz, Mexico, however, even the treatments planted with wide-canopy trees have been invaded by the Asian Swordtail fern (Nephrolepis brownii). This experiment (established Aug 2006) is composed of 24 30x30m fenced plots separated by 35 m of actively grazed pasture divided into three treatments: (a) 8 plots planted with animal-dispersed species, (b) 8 plots planted with wind-dispersed species, (c) and 8 plots left unplanted. Within each plot, I established 16 1x1m quadrats where I evaluated the recruiting plant community, canopy cover, and the density and size of ferns to determine: (a) What is the extent of the invasion? (b) Does fern density and size vary by treatment? and (c) What is the effect of fern density on plant recruitment?
Results/Conclusions
The Asian swordtail fern has successfully infiltrated all 24 of the restoration plots. The treatments differ in how extensively they have been invaded by ferns and other weeds; the control plots are plagued by weedy grasses, lianas, and ferns more so than the planting treatment plots. Treatment had a significant effect on frond density and frond length. Pairwise comparisons reveal that the control plots have significantly higher frond density than the animal-dispersed (p < 0.001) and the wind-dispersed treatments (marginal significant difference, p = 0.103). Average frond length in the control plots was higher than either of the planting treatments (p < 0.001). Wind-dispersed plots had longer fronds than the animal-dispersed, but the difference is marginally significant (p = 0.075). Frond density was positively correlated with frond length (r = 0.605, p < 0.001). Species richness was negatively correlated with frond density (r = 0.216, p < 0.001). Together these results reveal that the unplanted control treatment has: the greatest fern density, the longest ferns, and lowest plant recruitment. However, the lack of recruitment is due in part to dispersal limitation in these unplanted plots; future experiments are in development to conclusively determine how the invasive ferns limit plant recruitment.