Major shifts in vegetation often have notable consequences for habitat function. Woody vegetation has expanded beyond historic ranges due to increased temperatures, altered precipitation patterns, and changed grazing pressures; woody expansion occurs globally in prairies, tundra, and coastal grasslands. With shrub encroachment, the invading woody species can act as an ecosystem engineers, altering habitat and ecosystem function. The evergreen shrub, Morella cerifera, has expanded > 40% on the Virginia Coast Reserve (VCR) barrier islands in the past 30 years due to warming temperatures. Shrub expansion on these islands has significant consequences for nutrient availability and cycling, microclimate modification, and species composition. Our objective was to experimentally test the microclimate amelioration of M. cerifera and determine effects on soil characteristics and species composition. We removed 10x10 m of fully thicketized shrubs and compared to long-term shrub and grassland plots. We assessed hourly temperatures, water table depth, edaphic characteristics, and species composition in all plots. Understanding legacy effects of shrub encroachment on the VCR Long Term Ecological Research site can add to the greater knowledge on woody species invasion and landscape dynamics.
Results/Conclusions
Plots with shrub thicket removed experienced higher variability in temperature relative to shrub plots throughout the year with maximum temperatures ~18 oC warmer and minimum temperatures ~3 oC cooler. These effects were strongest in the summertime. Water table depth during the summer in shrub thickets was 12 ± 0.5 cm higher than the clear-cut plots, likely due to the significantly higher transpiration of shrubs. Expansion of shrubs reduced diversity, but experimental removal did not result in conversion to grassland; rather, forbs had higher cover. Although species diversity did not differ between grassland and clear-cut plots, the underlying species composition was distinctly separate. Nitrogen content was high in shrub and clear-cut plots. Because M. cerifera acts as an ecosystem engineer, drastically changing the nitrogen and water availability in soil, the relic habitat results in restored grass zones with similar functional groups of plants but markedly different species and cover. Barrier islands are on the forefront of many different climate change drivers; species composition changes and resources dynamics have broad consequences for coastal protection. This study contributes to a wider understanding of shrub encroachment, particularly on a high-speed landscape.