Within the most extensive stretch of coastal barriers, Virginia barrier islands represent a unique opportunity to study vegetation responses to climate drivers with little direct anthropogenic disturbance. In this portion of the mid-Atlantic coast, shoreline erosion rates are highly variable and sea-level rise is 3-4 times higher than the global average. Over the past 32 years, barrier islands within the Virginia Coast Reserve (VCR) Long-Term Ecological Research site have lost over 25% of terrestrial upland area, island width has been reduced, yet salt-sensitive woody vegetation has expanded across the landscape. Our objectives were to evaluate the consequences of shrub expansion on diversity and productivity, and the relative importance of shrub presence on microclimate. We hypothesized that shrub expansion into grassland is self-reinforcing through microclimate modification, reducing variability and temperature extremes within thickets, especially during the coldest months of the year. We include long-term trends in temperature for the region. Species composition and leaf-area index (LAI) were assessed annually during three growing seasons across three landscape types: grassland, transition, and shrub plots. Ground and air temperature within plots were measured hourly over one year.
Results/Conclusions
Expansion of shrubs reduced species diversity while enhancing annual net primary productivity from 470 ± 147 to 846 ± 36 g m-2. Diversity was highest in the transition zones, but was reduced over three years as shrub cover increased from 70 to 85%. Grassland had the lowest LAI (1.3 ± 0.4) compared to shrub zones (4.6 ± 0.5) and experienced higher variability in maximum and minimum temperatures over the entire year. Shrubs reduced extreme temperature events in summer by 16 ⁰C and in winter by 2 ⁰C. Reduction in minimum temperatures may prevent catastrophic failure of hydraulic conductivity below -16 ⁰C. Variability in shrub temperature exhibited a higher influence of one day on the next, supporting the moderated microclimate hypothesis, which was not evident in grassland plots. These results coupled with warming temperatures over the last 60 years may explain the success and prolific growth of shrubs across the landscape. Expansion of woody vegetation has effects on the landward migration of barrier islands by reducing rollover, affecting resilience to extreme events and changing climate. In this highly dynamic region, shrub expansion may have significant consequences across the landscape and impact the future of barrier islands.