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.
Estimates of annual dry mass production (g/shoot) for
Myrica cerifera shrub thickets at four sites on Hog Island.
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Site 1990 1992 1993 1994
(post-storm) (drought) (drought)
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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
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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.