Shackleford Bank was selected for this study because of: (a) a pressing need of ecological information before the island is developed as a part of the Cape Lookout National Seashore; and (b) it is a convenient site for investigating ecological processes of the maritime vegetation.
The objectives of the present study are: (a) to know the present status of the vegetation on the island; and (b) to understand the ecological factors determining the existence of such vegetation.
A classification of the plant communities on the island is proposed, with a general description for each community and a vegetation map. The successional relationship of major vegetation types is also discussed.
Geologic factors determine the unstable nature of the island and the characteristics of the substratum. Historical factors, mainly human activities, inflicted serious damage upon the vegetation in the past.
The general climate of the island is milder than corresponding inland areas. Because of adequate rainfall and the even distribution of rainy days throughout the year, no prolonged drought exists.
Although the general climate on the island is favorable for plant growth, the microclimate of the dune environment is extreme in many aspects: high temperature in the air as well as on the ground surface, wide temperature fluctuation, high light intensity due to the reflective sand, low vapor pressure deficit in the atmosphere during daytime, and active air turbulence caused by a prevailing southwest sea breeze in summer. For these reasons, only a few species with special adaptations can grow well in such harsh dune environments.
The southwest sea wind dominates in summer, but the prevailing wind shifts to the northwest or to the north in winter. The maximum wind velocity is considerable and varies little in a year. More accumulated air movement is recorded in summer than in winter on the island, possibly due to the constant southwest winds.
The wind has manifold influences on the vegetation: (a) wind-borne salt spray kills susceptible species growing close to the ocean; (b) abrasive sand blast created by strong winds causes tissue wounds on plants, which enhance the toxic effect of salt spray; (c) constant and excessive wind may induce desiccation of leaves; (d) strong wind inflicts mechanical injury on plants such as breaking branches or defoliation; (e) winter storms cause salt-water erosion and flooding to the strand vegetation; and (f) shifting sands caused by wind bury and destroy all vegetation in their way.
Although measurable sand movement was detected after a year on a dune transect, sand encroachment upon the forest has subsided and is not an imminent threat to the forest in a normal year. The amount of sand movement depends on the vegetational cover rather than the distance from the ocean.
The soils on the island have developed from medium to coarse sand and thus are well drained. The soil surface dries quickly after a rain, but ample water is always present in the subsurface layers. The soil developed on the fore dunes is characterized by coarser texture and a large amount of shell fragments. No appreciable amount of salt can be detected in dune soils. However, there is a trend of decrease in pH further away from the ocean.
The organic matter in soils varies from a rich 5% by oven-dry weight of maritime forest to less than 0.01% on the fore dunes. Dune soils are also low in nitrogen and phosphorus components. The accumulation of humus in such sandy soils is extremely slow, with no improvement in the soil structure.
A comparison between the soils on the island and inland Sandhills soils reveals a richer calcium and magnesium content in the former, possibly due to the marine origin of the coastal soils. The discrepancy in chemical properties of soils is suggested as an explanation for the absence of the Sandhills species and the disjunct distribution of red cedar (Juniperus virginiana) which is absent from the Sandhills.
Topography is regarded as an important environmental factor because of its effects on the gradients of other factors.
Livestock have done considerable damage to the vegetation on the island.
Measurements of water potential of 16 species show that they do not develop as high a water stress as that of either xerophytes or halophytes. A lower water potential is generally detected among tree species in the afternoon. Dune species under study also show some drought resistance.
The xeromorphic characteristics among dune species are due to the high stress of evapotranspiration in the air rather than the lack of water supply in the soil. Coastal dune species thus should not be considered as xerophytes in the same sense as those occurring in deserts.
Plants of dune species growing close to the ocean are generally tolerant to salt spray, but not to salt water in the soil. Hence, they cannot be classified as halophytes.
Complete nutrient solution greatly enhances the growth of dune plants. Low fertility of dune soil may be a limiting factor and explain the open vegetation on the dunes.
In addition to salt spray, the chemical properties of soil and lower limit of temperature are proposed to be effective in determining the distribution of live oak (Quercus virginiana).
It is concluded that no single factor can explain satisfactorily the occurrence of coastal vegetation. The present status of such diversified maritime vegetation on the island is the result of the interactions among various environmental, biotic, geologic, and historic factors.
Last Updated: 7-Jul-2005