COS 90-1 - Greedy grass: resource acquisition traits predict emergence, survival and growth response to microsite management in a grassland restoration

Abstract: Many perennial grasslands have been degraded, leading to low-diversity sites with diminished ecosystem services marked by the loss of native perennial grasses. One key need is to restore this critical functional group back in the system, but efforts have been met with limited success. One largely overlooked mechanism is variable establishment due to unsuitable microsites for recruitment, growth and first year survival. Microsites—fine-scale topographical environments—can vary in soil moisture and light conditions, where lower microsites may provide increased soil moisture with reduced daily and growing season variability, while higher microsites may provide more light and less competitive pressure from weeds. We hypothesized that this small-scale variation differentially affects emergence, growth and survival of native grasses in general, and that different grass species respond differently due to differences in resource acquisition traits. To test these hypotheses, we examined the effect of drill seeding in u-shaped furrows (microsite) on grass emergence and growth compared to seeding near the surface for nine grass species in the Colorado foothills, where grasslands are dominated by a mix of C3 and C4 species. We analyzed differences in emergence, survival, end-of-season growth, and season growth rate using separate generalized linear mixed-effects models. Seed and seedling traits related to water use and resource acquisition were also included in these models to determine which resource acquisition traits have the greatest response to managed microsites.

We found microsites differentially limited emergence compared to growth, resulting in balancing trade-offs in total establishment. Individuals emerged in greater numbers near the surface, despite more soil moisture in the furrows. Yet, individuals grew faster, were bigger, and survived more often when growing in furrows than on the surface. Together, these contrasting effects resulted in similar end-of-season density of seedlings for individuals seeded on the surface versus the furrow. However, species-specific responses caused variation in composition across the microsites. Despite reduced emergence in furrows overall, more acquisitive species—those with seeds having lower water needs and seedling root and leaf traits capable of more efficient resource uptake—had the greater emergence and growth in the microsite compared to more conservative species. The initial negative effect of the managed microsite on emergence, followed by an overall neutral or beneficial effect on survival and growth, attests to the varying effects microsite management has on successful restoration. Diverging responses to microsite management by different resource acquisition strategies indicates that microsite management, by increasing heterogeneity, increases diversity overall.