Effects of native and non-native grasses on woody species regeneration in a subtropical dry forest

Background/Question/Methods

Invasive non-native species can reduce native species diversity, alter ecosystem structure and produce barriers to restoration.  Within many tropical dry forests, changes in native community composition and species diversity have been attributed to invasive non-native C₄ grasses. These species often induce a grass/fire cycle and can alter ecosystem characteristics.  In some dry forest landscapes, native grasses with similar ecological characteristics to non-natives are also present.  Little is known about how native and non-native grasses may differ in their effects on woody species diversity or seedling regeneration.   We surveyed randomly selected patches dominated by either non-native (Megathyrsus maximus)   or native (Uniola virgata) within the Guánica Tropical Dry Forest, Puerto Rico, to assess the effects of grass type and patch size on woody species diversity, stem diameter and density. We then conducted a field experiment to assess possible facilitation or inhibition effects of grasses on woody seedlings. We planted seedlings of three native woody species either under or away from the canopy of large clumps of the two grass species and assessed seedling survivorship for six months. 

Results/Conclusions

Overall woody stem density did not differ between native and non-native grass patches, but basal area tended to be higher in non-native grass patches due to the presence of Leucaena leucocephala, a fast-growing non-native tree. Native woody species richness was linearly related to grass patch size, but the relationship was positive for native grass patches and negative for non-native grass patches.  Large native grass patches contained more native woody species than did small patches, while large non-native grass patches contained fewer native woody species than did small non-native grass patches.  However, native woody species richness per stem was greater in non-native grass patches and was unaffected by patch size.  Planted seedlings near the edges of native or non-native grass clumps had higher percent survival than seedlings planted in bare exposed soil in the same sites. Between the two grass species, woody seedling survival was higher near native grass clumps than near non-native grass clumps, perhaps due to differences in phenology and root development that produced a more suitable microclimate near the native grass clumps. These results suggest that native grasses may be used during dry forest restoration to mitigate the negative effects of non-native grasses.