Grasses as nurse plants?: Effects of native Uniola virgata and non-native Megathyrsus maximus on woody species regeneration in degraded subtropical dry forests

Background/Question/Methods

The effects of non-native plants on the recruitment of native woody species in degraded tropical dry forests is not well understood. Such forests succumb to agricultural and urban development and are exposed to novel disturbance regimes, such as fires.  After abandonment such degraded sites are usually colonized by non-native plants, especially C₄ grasses.  Restoration strategies in such degraded sites range from active to passive restoration efforts, with the former being energy and economically intensive and the latter more economical. Passive restoration tools include but are not limited to the use of nurse plants. To determine the propensity of native and non-native grasses to act as nurse plants in degraded dry forest in Puerto Rico, we measured the survivorship of planted seedlings of three native woody species Jacquinia berteroi, Coccoloba krugii and Erythroxylum areolatum in response to different microclimatic conditions.  Seedlings were planted under the edge of clumps of native Uniola virgata and non-native Megathyrsus maximus grass, as well as in adjacent open patches of bare soil.  In all planting locations, soil moisture and light measurements were taken during wet and dry seasons to determine if environmental factors could explain the emerging survivorship pattern.  

Results/Conclusions

After six-months, highest survivorship was for Erythroxylum areolatum seedlings, followed by Coccoloba krugii seedlings and Jacquinia berteroi seedlings. Across the three target species highest seedling survivorship tended to be under native U. virgata grass edges, followed by non-native grass M. maximus edges. Survivorship was generally poor in the bare soil spot areas. Positive association between low seedling mortality rates and higher soil water content across the six-month period was observed for seedlings planted under U. virgata. Our light measurements found seasonal change in the relative difference in photosynthetically active radiation in bare soil vs. grass edge within M. maximus grass patches, but no such differences were found for U. virgata grass patches. This suggests that U. virgata produces more consistent shade conditions across seasons, which may contribute to seedling survival. Future restoration efforts in degraded sites could use U. virgata grass clumps as a starting point to which ameliorate the environmental stress that seedlings face during this critical stage.