Shrub encroachment is a global phenomenon leading to altered microclimate and habitat function. Woody vegetation can act as an ecosystem engineer, increasing structural complexity of a habitat, and altering changes in diversity, and nutrient dynamics. On the Virginia Coast Reserve Long-Term Ecological Research landscape, Morella cerifera has expanded range >40% despite loss of land area to sea-level rise and erosion. Driven by increasing temperatures and positive feedbacks between microclimate and physiology, expansion of Morella is supported by nitrogen-fixing Frankia in an otherwise low-nutrient environment. Our objective was to experimentally test how microclimate modification coupled with supplemental nitrogen input enhances grass density and alters seed germination success. We fertilized plots at 10 g m-2 y-1 at three distances from a mature shrub – on the edge of the canopy, 50 m, and 100 m and seeded with 100 M. cerifera seeds. Grass height, density, and leaf area index (LAI) were measured before and at the end of the growing season. Seed germination was monitored, soil composition was assessed using ion exchange resin bags, and specific root length (SRL) and plant tissue nitrogen was measured at the end of the season. Quantifying indirect facilitation further contributes to our understanding of shrub encroachment globally.
Results/Conclusions
Temperature is influenced by grass density and proximity to shrubs. Maximum temperatures were altered by distance from shrub across all three seasons; the closest plots were cooler by ~6 oC in the spring ~8oC in the summer and fall. Nitrogen treatment affected maximum temperature in the spring by reducing temperature by ~5 oC relative to control plots. LAI was highest in plots closest to shrubs (p <0.0001). LAI did not differ across nitrogen treatments though grass density was increased in fertilized plots (p <0.0001). Nitrogen in unfertilized plots along transects decreased with distance from shrub to almost 0% at 100 m. Higher N in soil increases SRL while distance from shrub had a higher influence of SRL in control plots. Lab studies demonstrate that seed germination is influenced by grass density, which is currently being assessed in the field. Morella cerifera may accelerate in range expansion through indirect facilitation on grasses due to microclimatic control and the legacy effects of the nitrogen input into the system.