COS 64-6 - CANCELLED - Savanna woody encroachers compete more successfully with grass than non-encroaching trees

Background/Question/Methods

Woody encroachment threatens savanna systems globally. Although savanna encroachment has been widely documented and some causes identified, we do not yet understand why some woody plants are successful encroachers while others are not. Here, we test the hypothesis that tree species with higher encroachment potential compete with grasses more effectively than non-encroachers. We grew 6 tree species (2x non-encroachers, 2x possible encroachers, and 2x woody encroachers) for 16 weeks under fully factorial nutrient and grass competition treatments (with/without grasses, low/high rate of N supply, and low/high rate of P supply) to evaluate how trees would compete with grasses for essential nutrients.

Results/Conclusions

Grasses were not impacted by tree competition, but trees grew less under grass competition. Nutrient additions increased both tree and grass biomass growth across treatments, with larger responses to nitrogen than to phosphorus fertilization. Woody encroachers were more resilient to grass competition than non-encroachers. They also grew taller than non-encroachers per unit diameter, suggesting that they may be more effective light competitors at the seedling stage than non-encroachers. Although we found no significant differences in the belowground traits of encroachers versus non-encroachers, belowground traits may also matter if differences were masked by pot artifacts. Tree-grass competition for soil nutrients, encroachers’ growth strategies, and aggressive nutrient acquisition may be important in determining which species act as aggressive encroachers.