The influence of trees on resource availability in their understories has been well documented for decades. Yet influences beyond the canopy edge have rarely been studied including for invasive trees. The spatial extent of influence can have important implications for landscape scale productivity, nutrient limitation and plant composition. We evaluated the influence of the invasive N-fixing tree Morella faya, on N status of plants in two habitat types where the species co-occur in seasonal environments in Hawaii. We asked, (1) how do tissue N and 15N of co-occurring species change with distance to this N fixer? and (2) how does grassland response to N or P addition vary with proximity to M. faya? We assessed static patterns of plant tissue N content relative to established M. faya individuals across two habitats (a savanna and a woodland) to assess question 1, and an N addition experiment in the savanna sites to address question 2.
We found that all species sampled showed elevated N when growing near the canopy edge compared to 5 meter away from sampled M. faya trees but that delta 15N of these co-occuring species did not strongly reflect values of M. faya and did not vary consistently between adjacent shrubs and those 5 meters away from M. faya trees. We further found that the tree influence on % N of neighbors was apparent at least 4 meters beyond the canopy edge for 3 of the 4 resident species sampled. Finally, in the savanna, we found a weak response to N addition for grass-dominated plots close to M. faya trees but a strong response for plants >3 meter away from M. faya individuals. Results were similar for N+P together but not P alone. Our results suggest N limitation of the savanna has been alleviated near to M. faya trees. They call into question the simple division of space in savannas into ‘near tree’ (or near shrub) versus and “away from tree” microenvironments since tree influences can extend many meters. It is also clear that trees may drive increased N in plant tissue and thus potentially enhance productivity and reduce N limitation well beyond their edges. Thus tree invasions have landscape implications even when trees are at low density.