Storms and Biodiversity

Storms off the coast at the VCR are tabulated each month. Annual averages from 1885 are shown here. CED readers will note that the long term history of storm frequency is the inverse of that found at the Konza Prairie. On average between 4 and 7 of these storms produce inland overwashes of beach sands. Vegetation varies considerably in its tolerance of storm erosion, sand burial and salt water flooding. A shore parallel zonation in species composition and species richness results from these frequent disturbances.

Storms and Biodiversity Coastal storms push sand and saline water across the coastal strand. Coastal vegetation is scoured away or buried during such overwashes across the coast. Many coastal plant species vigorously encroach seaward. The boundary between the landward pushing active sand zone and the seaward encroaching vegetation is an ecotone dynamic in time. This ecotone is easily seen on aerial photographs. At the VCR we use metric aerial photography to record the coordinates of this ecotone at 50 m intervals along the coast. Our period of photographic coverage for Hog Island, two islands south of Cedar Island, begins in 1949 and we have images every 8 years or so. Hog Island is about 7 km long and we used 140 transects along this island in our disturbance and vegetation studies.

We know the direction and mean rate of change of this ecotone and we know the standard deviation of this mean. We have calculated the probability that any point on Hog Island will be subjected to the disturbance of sand overwash and burial from the beach. We have mapped this probability of storm disturbance to a resolution of 2 meters. With some luck and a lot of hard work the VCR LTER will be around for the next minimum in storm frequency (expected around the year 2020 AD. We can then look west where Konza should be enjoying a new maximum in storm frequency.

In 1985, Carol McCaffrey classified ground cover on false color infrared photographs and then extensively ground-truthed these analyses. She also completed extensive species inventories of each land cover class that she identified on the photographs. We have added the McCaffrey maps to VCR GIS vegetation layer we have tabulated the occurrence of 95 species in each of 10 disturbance probability classes. Each class spans a 0.1 probability increment for storm overwash. Using these data, Fahrig, Hayden and Dolan (1993) tabulated the occurrence of 95 species in each of the 10 disturbance probability increments.

We found four types of spectra of occurrence in disturbance classes. Of the 95 species of plants included in the study 39 were largely restricted to very low disturbance probability areas (0 to .2). Thirty four species had their mean occurrence in probability zone 0.5 with a probability range of .3 to .8. Eighteen species had their occurrence at 0.5 probability level and higher. Only 4 species were largely restricted to probability levels 9 and 1.0. Contrary to expectations, the four species that were successful in areas likely to be disturbed each year were all perennials and all had rhizomatous and clonal growth forms. While above ground biomass is generally lost during disturbance, these plants exhibit a substantial capacity to resprout and emerge through the surface if the depth of burial is not too great.

We were also able to determine the fraction of the 95 species of plants included in our study that occur in each disturbance class. This fraction is like a standardized index of biodiversity. With this estimate, we plotted both disturbance probability and plant biodiversity across a typical transect across Hog Island. In general biodiversity increases as disturbance probability declines. While we did not estimate the probability of flooding from the lagoons onto the back side of the islands this saline water hazard accounts for the lowered biodiversity in this zone of the island.

There is always a danger of taking the results of a study like our disturbance/vegetation and assuming it is always this way. Since the disturbance in question arises from coastal storm it is useful to consider the temporal history of the number of these storms. We tabulated the monthly number (1885-1992) of storms offshore and to the northeast of the VCR (see illustration above). The period of our study was the storminess of this century. McCaffrey's work on the vegetation mapping was within this very storm period. Since the 1960 storm frequency has declined substantially. There is little doubt that the storm climate change reported at the VCR and at KNZ are related and part of a hemispheric change in the circulation of the atmosphere. However, the impacts of these related climate changes are really rather different at the VCR and KNZ LTER sites.

Citations:

McCaffrey, C.A. 1975. The major vegetation communities of the Virginia Coast Reserve. In: Virginia Coast Reserve Study: Ecosystem Description. Report of the nature Conservancy. Washington. D. C. p. 385-437.

Fahrig, L., B. Hayden and R. Dolan. 1993. Distribution of Barrier Island Plants in Relation to Overwash Disturbance: A Test of Life History Theory. J. of Coastal Research 9(2):403-412.