Above ground factors mediate the suppression of an invasive grass in tropical reforestation

Background/Question/Methods

In abandoned agricultural areas in the tropics, the high light conditions and low nutrient availability that follow intense agricultural practices can hinder natural succession and favor colonization by exotic species.  In Panama, the exotic C4 grass Saccharum spontaneum now dominates many abandoned pastures.  Efforts to restore forest to these areas focus on establishing fast-growing tree species to shade out the Saccharum and facilitate colonization by other tree species.  Across 40 small, mono-specific restoration plots including 16 canopy tree species, shade from planted trees strongly reduced Saccharum density.  In addition, among plots with dense tree canopies, Saccharum density was even more reduced when the overstory species was a legume.

As part of a series of experiments that evaluate above- and below-ground mechanisms to explain how legumes inhibit Saccharum, I placed Saccharum tillers in nursery bags full of potting soil in the understory of restoration plots of 3 legume and 3 non-legume overstory species (n=270 tillers).  After 2 months I compared Saccharum performance under legume and non-legume canopies, in the absence of below ground competition or soil nutrient effects.  I expected that removing below-ground interactions should eliminate the effect of legumes on Saccharum performance, but that Saccharum would perform better in plots with more open canopies.

Results/Conclusions

Both survival and below-ground biomass of Saccharum increased with increasing tree canopy openness.  However, contrary to expectations, both survival and below-ground biomass were significantly lower for Saccharum in the understory of legumes, even though the grasses were isolated from below-ground interactions.  There were no significant effects of canopy openness or legume/non-legume functional groups on stem height or diameter, number of tillers produced, or above-ground biomass.  One hypothesis to explain the observed effects is that legume overstory trees may possess allelo-chemicals that suppress the growth of Saccharum when it occurs in the understory of legumes.  Future studies will evaluate this potential mechanism.