Donald R. Young, Dept. of Biology, Virginia Commonwealth University Guofan Shao and John H. Porter, Dept. of Environmental Sciences, University of Virginia
Most of the barrier island along the Atlantic Coast of the United States have been developed for recreation, leaving few remaining in a natural state even though the ecology of these systems is poorly understood. An exception is the Virginia Coast Reserve, where the Nature Conservancy has protected a dozen islands and several expansive marshes. The dominant woody species on the VCR islands are bayberry (Myrica pensylvanica) and wax myrtle (Myrica cerifera) (McCaffrey and Dueser 1990). All Myrica species are characterized as rapidly growing shrubs and are considered to be important in the process of ecological succession because they contribute to substrate stability and enrich the soil with nitrogen via symbiotic fixation with the bacterium, Frankia (Morris et al., 1974, Permar and Fisher 1983). Presumably, the symbiotic fixation of nitrogen enables Myrica to be successful in the nutrient poor, sandy soils of the barrier islands (Levy 1983, 1990). Surprisingly, very little is known about the ecology of Myrica pensylvanica and M. cerifera, even though these shrubs are important in the terrestrial communities of nearly all barrier islands along the eastern coast of the United States (Duncan and Duncan 1987).
The goal of this study was to expand on the understanding of Myrica ecology. Specific objectives were: (1)quantify spatial variations in Myrica vigor on Hog Island, (2)determine the extent of soil nitrogen enhancement by Myrica on the islands of the Virginia Coast Reserve, and (3)evaluate Myrica thicket stability and dynamics.
Methods To examine spatial variations in Myrica vigor on Hog Island, three study sites were chosen for intensive sampling, each differing in thicket age and exposure to the ocean. The three sites were: Leading Edge, small widely spaced Myrica shrubs located near the ocean in a swale immediately behind the primary dune; Mid-Island, the most stable region of the island with a well developed Myrica thicket; and Bayside, located in the oldest soils of the island with Myrica thickets that are in a state of degeneration.
At each site, shrub age structure was determined by measuring the basal diameter of 200 stems randomly chosen. Branch, shoot and fruit production were measured (n=20) at four times during the growth season, after harvesting and drying material to determine dry mass. In addition, 20 root segments were excavated at each site to qualitatively assess nitrogen-fixing nodule abundance. The photosynthetic potential of leaves, twigs and fruits were determined at each site with a portable infrared gas analyzer (Data Design). Tissue nitrogen levels were determined by Kjeldahl analysis (Moore and Chapman 1986).
Islands with obvious Myrica populations, as indicated by aerial photographs, were visited to determine the potential enhancement of soil nitrogen by symbiotic fixation. On each island, 10 root segments were excavated to confirm the presence of nitrogen-fixing nodules. In addition, 10 soil samples from beneath Myrica and 10 samples from nearby sites without Myrica were collected for nitrogen analysis. Total soil nitrogen was determined by Kjeldahl analysis (Moore and Chapman 1986).
To assess the extent of Myrica thickets on the barrier islands of the Virginia Coast Reserve, aerial photographs from the January of 1990 were analyzed with a imaging processing system to determine percent cover and total area covered. Additionally, aerial photographs were used to assess Myrica expansion and stability on the northern end of Hog Island.
Results and Discussion The age structure analysis of Myrica shrubs at the three sites on Hog Island differed considerably. All shrubs were Myrica cerifera. The Leading Edge consisted of relatively small (< 1.5 m tall), widely spaced shrubs. More than 95% of the stem diameters were less than 40 cm, reflecting the relatively young age of the plants and indicating a recent colonization of the site. At the Mid-Island, a diverse age structure was represented by stem diameters from less than 10 cm to more than 200 cm. The shrubs have formed a dense thicket with no undergrowth. Approximately 45% of the stems were between 60 and 100 cm in diameter and many of the shrubs were 2 to 3 m in height. M. cerifera was similar in size and age structure at the Bayside site; however, no stems were less than 20 cm in diameter, reflecting a lack of recruitment. Much of the thicket was covered by Parthenocissus quinquefolia (Virginia creeper) and in several areas the shrubs were degenerating, resulting in gaps in the canopy. Where shrubs had recently died, there were no Myrica seedlings or root suckers present in the resulting open spaces.
The age structure differences among the three sites reflect the ages of the sandy soils. The north end of Hog Island has been accreting since about 1900 (Hayden et al. 1991). Progressively younger soils occur as one moves from the bayside to the ocean side of the island. At the Bayside site, soils are at least 80 years old, at the Mid- Island site, soils are approximately 40 years old, and the soils are less than 25 years old at the Leading Edge site. Presumably, Myrica colonization occurs sometime after the soils stabilize, forming a pseudo-chronosequence of thicket development across the island.
In addition to age structure, Myrica differed with regard to sex ratio and production characteristics across the three sites. The Leading Edge and Mid-Island sites were predominantly male, whereas the Bayside site had equal numbers of male and female plants. Fruit production, as measured by weight or number per branch, was significantly greater at the Mid-Island site. However, there was a decrease in shoot production from the Leading Edge to the Bayside sites. Measurements throughout the summer indicated that production was continually higher at the Leading Edge site. For all three sites, production was greatest from July through August when temperatures were high and frequent precipitation ensured an abundant supply of soil moisture.
The seasonal pattern in production reflects the high temperature optimum for photosynthesis that has been measured for Myrica cerifera (Young 1992). This evergreen shrub also requires considerable soil moisture and is sensitive physiologically to atmospheric humidity deficits and plant water status (Young 1992). Field observations of increased growth during high temperature / precipitation periods in the present study are consistent with these conclusions. Further, root excavations indicated a shallow distribution, with most occurring above 30 cm.
Measurements of photosynthetic characteristics did not reveal seasonal or intersite differences, consistent with previous work (Young 1992). However, positive photosynthesis was measured for fruits, in addition to leaves, but also for bark on five orders of branches. Although values for bark and fruits were more than an order of magnitude lower when compared to leaves, these structures contribute to whole plant carbon gain. An evergreen, all-green strategy provides tremendous potential for carbon gain and plant growth. In addition, all plants that were excavated had nodules. The photosynthetic potential, in conjunction with the ability to fix nitrogen via a symbiotic association, may explain the ability of Myrica shrubs to rapidly colonize the relative infertile soils of barrier islands.
Tissue nitrogen content corresponded to the similar photosynthetic potentials among three sites. For all three sites nitrogen content was about 1.72% of dry mass. Soil nitrogen levels were greatest at the Bayside site, relative to the other two sites, and there was a general decrease in soil nitrogen content throughout the season.
Young et al. (1992) quantified a similar trend in soil nitrogen for a transect across Hog Island. Soils on the older bayside of the island had the highest nitrogen levels, as did soils beneath Myrica thickets when compared to non-Myrica soils. When comparing soils beneath Myrica thickets to soils from adjacent non-Myrica sites, only three of the eight islands of the Virginia Coast Reserve with substantial Myrica populations had significantly higher nitrogen levels: Parramore, Hog and Smith. One island, Cobb had significantly lower nitrogen levels in soils beneath Myrica. Yet, Myrica pensylvanica and M. cerifera enhance soil nitrogen in other systems (Morris et al. 1974., Permar and Fisher 1983). Clearly, factor in addition to Myrica related fixation influence soil nitrogen levels on the Virginia barrier islands.
Myrica cerifera and/or Myrica pensylvanica are present on eleven of the islands of the Virginia Coast Reserve. With the exception of Wreck Island, Myrica cerifera is the most abundant of the congeners. When comparing Myrica shrub cover to total island size, a threshold of approximately 100 ha in island area appears to be necessary prior to Myrica colonization or expansion. This may be related to increase island stability with size or to the development of a substantial freshwater lens beneath the island. A freshwater lens is essential to support the water demands of the increased leaf area associated with woody species and the lens may be a controlling factor in determining the rate and direction of ecological succession on barrier islands (Hayden et al. 1991).
The current distribution of Myrica shrubs on the islands may not be in equilibrium, reflecting the dynamic geomorphology of the islands. Shrub cover on at least some of the islands may fluctuate dramatically, perhaps related to island erosion and accretion patterns. An examination of Hog Island photographs from 1949 to the present indicated that Myrica cerifera thickets have rapidly expanded on the accreting northern third of the island. In 1949, 3.5% of the north end was covered by thicket. This increased to 12.4% by 1962. Twelve years later Myrica shrubs covered 20.4% and in 1989, 36.3% of the northern third of Hog Island was covered with thickets. This represents an increase of over 900%, relative to coverage in 1949.
Study Conclusions 1. The age structure and productivity of Myrica thickets varies with position on the island and is most likely related to the stability and age of the underlying island soils.
2. Soil nitrogen levels beneath Myrica thickets are generally high relative to non-Myrica sites. The enhancement of soil nitrogen by Myrica varies among the islands of the Virginia Coast Reserve and may be related to thicket age, soil age and past history of the island.
3. Each island of the Virginia Coast Reserve is unique with regard to physical structure and shrub thicket characteristics. Myrica cerifera and Myrica pensylvanica are present on eleven of the islands of the Virginia Coast Reserve. The current island shrub cover varies among the islands and appears to be related to island size.
4. Myrica thickets are not stable on all of the islands, with periods of thicket expansion occurring. The growth of Myrica thickets is facilitated by a capacity for symbiotic nitrogen fixation, a high photosynthetic rate, evergreen leaves and photosynthetic bark and fruit. Thicket growth may be held in check by sensitivity to moisture stress and a relatively low tolerance to salinity.
LITERATURE CITED Duncan, W.H. and M.B. Duncan. 1987. Seaside plants of the Gulf and Atlantic Coasts. Smithsonian Institution Press, Washington, D.C. Hayden, B.P., R.D. Dueser, J.T. Callahan, and H.H. Shugart. 1991. Long-term research at the Virginia Coast Reserve. BioScience 41: 310-318. Levy, G.F. 1983. A study of vegetational dynamics on Parramore Island, Virginia. Castanea 48: 32-36. Levy, G.F. 1990. Vegetation dynamics of the Virginia barrier islands. Virginia Journal of Science 41: 300-306. McCaffrey, C.A. and R.D. Dueser. 1990. Plant associations of the Virginia barrier islands. Virginia Journal of Science 41: 282- 299. Moore, P.D. and S.B. Chapman. 1986. Methods in plant ecology. Blackwell Scientific, London. Morris, M., D.E. Eveleigh, S.C. Riggs, and W.N. Tiffney. 1974. Nitrogen fixation in the bayberry (Myrica pensylvanica) and its role in coastal succession. American Journal of Botany 61: 867- 870. Permar, T.A. and R.F. Fisher. 1983. Nitrogen fixation and accretion by wax myrtle (Myrica cerifera) in slash pine (Pinus elliottii) plantations. Forest Ecology and Management 5: 39-46. Young, D.R. 1992. Photosynthetic characteristics and potential moisture stress for the actinorhizal shrub, Myrica cerifera (Myricaceae), on a Virginia barrier island. American Journal of Botany 79: 2-7. Young, D.R., E. Sande, and G.A. Peters. 1992. Spatial relationships of Frankia and Myrica cerifera on a Virginia, USA barrier island. Symbiosis 12: 209-220.
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