UC Santa Cruz

The potential for tropical restoration legume and non-legume trees to suppress the invasive C4 grass Saccharum spontaneum in Panama

Justin A. Cummings

Saccharum spontaneum L. subsp. spontaneum is a large, exotic C4 grass species that invades abandoned agricultural fields in many tropical countries and can prevent natural forest regeneration. In Panama, studies on restoration of areas dominated by S. spontaneum have focused on identifying trees that establish readily after S. spontaneum removal. Few studies have evaluated how planted trees can suppress subsequent regeneration of S. spontaneum.

This dissertation focused on how legume and non-legume trees planted for restoration alter light availability and soil chemistry, and the influence of those effects on S. spontaneum suppression. In addition, I explored the potential use of agricultural legumes to suppress S. spontaneum through nutrient competition. Finally, I examined potential for using allelopahty as a tool for forest restoration and exotic species management both through a review of the literature and green house experiments.

I evaluated the response of S. spontaneum along a gradient of understory light conditions in single species plots of trees planted for restoration. S. spontaneum regeneration after planting restoration trees was most limited by light availability. Legume trees produce deeper shade than non-legume trees, and were more efficient at suppressing S. spontaneum. In addition, legume overstory species suppressed S. spontaneum abundance beyond the expected reductions associated with shade. S. spontaneum is nitrogen and phosphorus co-limited in our system in the absence of competition. However, although we find trends towards higher soil nitrogen and lower phosphorus under legume trees, greenhouse and field experiments provided weak evidence for both phosphorus competition and allelopathy to play roles in S. spontaneum suppression.

Overall, results emphasize the value of incorporating legume trees into restoration plantings where invasive S. spontaneum suppression is desired. Restoration practitioners should consider establishing mixed species stands dominated by legume tree species when reforesting areas dominated by S. spontaneum to expedite the reforestation process.