Shrub thicket dynamics and underlying mechanisms relative to barrier island successional processes.

Donald R. Young. Department of Biology, Virginia Commonwealth, University, Richmond, Virginia.

For the past six years, we have been examining the spatial and temporal characteristics of the woody plant communities on Hog Island of the Virginia Coast Reserve. These communities are almost entirely composed of dense thickets of the actinorhizal shrub, Myrica cerifera. Aerial photograph analysis, in conjunction with anecdotal information, has revealed that the shrub thickets rapidly expand in mesic swales located in the island interior. Moreover, thicket expansion may be related to island accretion patterns. However, the successional dynamics of island shrub thickets have not been investigated. The purpose of this study was to continue annual measurements of spatial variations in shrub growth and to initiate a new longterm study focusing on the initial establishment of island shrub thickets. Results from these studies will provide insight towards island successional processes.

In 1990, three sites were established on northern, accreting portion of Hog Island. These sites represent differences in soil age, ocean exposure and in thicket "life stages". Leading Edge, the youngest site, is behind the foredune and many individual shrubs are closing to form a thicket. Mid-Island is located in the central, most stable portion of the island and the shrub thicket is well defined. Bay Side, with the oldest soils, is located adjacent to the fringe marsh. Here, the shrubs are senescent, with frequent large gaps in the thicket. In 1992, a fourth site, Eroding, was added to quantify the direct effects of storm related erosion on a mature thicket.

In mid-October, when aboveground biomass peaks in the shrub thickets. New shoot growth was randomly sampled (n=100) from each of the four sites. In 1994, the Leading Edge had the highest dry mass in new shoot growth, followed by Mid-Island, Bay Side and Eroding (Table 1). Further, with the exception of Leading Edge, new shoot growth was reduced at the other three sites relative to values in all previous years. There has been a decrease in growth at all sites since 1990, the peak year (Table 1). Spatial differences most likely are related to ocean exposure and a natural aging process in the thickets. Variations through time are probably related to a complex relationship of meteorological parameters, including northeaster storms and summer drought.

For the second study objective, five 5X10 m plots were established in a grassy swale (Colonizing Site) on the ocean side of the primary dune. Numerous shrub seedlings are common and this area most likely represents the potential development of a future shrub thicket, if this portion of the island continues to accrete towards the ocean. Three woody species were present in the plots, Myrica cerifera, Baccharis halimifolia, and Iva frutescens. Myrica cerifera was most abundant. Approximately 85% of the seedlings were less than 20 cm tall, nearly 13% were between 20 and 40 cm in height, while only 2% were greater than 50 cm (Fig. 1). The data indicate that colonization must occur repeatedly over several years. The plots will be resampled every year to determine seedling survivorship and growth rates so that the shrubs can be aged. Manipulative experiments are planned for the Colonizing Site to test mechanistic hypotheses (i.e. abiotic and biotic facilitation) to explain the successful shrub establishment and rapid formation of formation of Myrica cerifera thickets.

Table 1.

Estimates of annual dry mass production (g/shoot) for Myrica cerifera shrub thickets at four sites on Hog Island.


Site		    1990	     1992	    1993	   1994
			         (post-storm)     (drought)      (drought)

Leading Edge	13.67 +/- 1.71	8.94 +/- 1.13	6.51 +/- 1.15	6.73 +/- 1.11

Mid-Island	10.22 +/- 1.82	6.44 +/- 1.03	5.53 +/- 0.54	4.01 +/- 0.54

Bay Side 	 6.14 +/- 1.22	4.74 +/- 0.73	3.00 +/- 0.46	1.69 +/- 0.19

Eroding		      ---	1.27 +/- 0.18	0.85 +/- 0.09	1.16 +/- 0.13