COS 256-1 - Disentangling the interactions between abiotic and biotic factors on successful coastal shrub establishment

Abstract: Woody plant encroachment into grasslands has rapidly increased in southeastern US coastal areas over the last few decades. Previous studies at the Virginia Coast Reserve, a Long-Term Ecological Research site, show that the native shrub, Morella cerifera, is encroaching into grasslands due to warming winter temperatures. A minimum dune elevation of ~2m is correlated with established shrub presence, but the role of dune elevation is unknown for preparing the microenvironment for successful shrub establishment and growth. Dunes protect interior areas through limiting sediment movement and saltwater, affecting the biotic and abiotic environment. On the south end of Hog Island, Virginia, new sediment accretion has resulted in dunes varying in elevation, providing a unique opportunity to examine how the abiotic and biotic environment change as dunes build. In this study we determine the abiotic and biotic factors that vary with dune elevation and determine controls on successful shrub establishment and growth. We quantified grass density, species composition, air temperature above the soil surface, soil chlorides, in low-lying areas behind dunes of three different height classes (1m, 1.5m, < 2m). Where seedlings were most abundant, we examined the relationship with biotic and abiotic factors and number of seedlings, seedling size, and growth rate.

Dune height influenced grass cover, stem density, shrub seedling presence, and temperature. Minimum and maximum temperatures were highest in areas behind 1m dunes while total vegetative cover was >5 times lower in the same areas relative to taller dunes. Spartina patens was the dominant grassland species, but abundance did not differ by dune height. Unexpectedly, soil chlorides were highest behind >2m dunes in the only area where salt-sensitive shrub seedlings occurred. Shrub seedling number was negatively related to grass cover with no seedlings observed in cover >95%. Grass cover reduced light availability with low light levels (< 280 umol m² s^-1) observed in high density grass plots. Where shrubs were present, herbaceous species richness was higher. It is unknown if this is due to light availability or other competitive interactions with high grass density (primarily Spartina patens). Dune height modifies the abiotic environment, allowing for higher grass cover and moderated temperatures and successful establishment behind 2m tall dunes. Where seedlings occur, high grass cover limits establishment of Morella cerifera. Limitations to shrub establishment behind 1.5 m dunes is still unknown. Our work can inform future predictions of shrub growth as sediments shift and dunes evolve at larger spatial scales.