CAPE LOOKOUT Vegetation and Ecological Processes on Shackleford Bank, North Carolina NPS Scientific Monograph No. 6

As mentioned earlier, during the recent history of the island the elimination and desecration of habitats through man's misuse of land initiated or at the least vastly accelerated the devastation of the original vegetation. This kind of destruction could be started again by developing the island into a public amusement playground. At the time of this writing, only one family lives on the island and does so only during the summer. The vegetation is damaged mainly by livestock. However, weekend intruders make campfires, carry away driftwood, and carelessly leave behind a large number of tin cans and other garbage.

There is no good method for counting the exact number of grazing animals on the island, but the highest number I encountered in the field at one time is as follows: 52 sheep, 15-20 goats, 20 cows, and 26 horses. Cows (Pl. III-24) and horses (Pl. IV-29) are owned by persons living on Harkers Island, while the goats (Pl. II-13) and sheep (Pl. II-14) are wild. The damage done by these animals is considerable, not only because they graze on the grasses but also by their roaming and trampling. Animal trails are numerous. Besides feeding on young leaves and twigs of woody species, the goats and sheep do much physical destruction on the dunes and accelerate the encroachment of sand on the forest. Therefore, if natural conditions on the island are to be restored and maintained, the livestock must be removed.

Physiological Responses of Plants

Methods

Since no study of water relations of coastal plant species has been reported in North Carolina, the water potentials of plants developing under moist root environments and extreme dry conditions above ground (i.e., under full sunlight, high temperature, and constant wind in the summer on highly reflective sands) were measured. The effects of salt water, salt spray, drought, shade, and nutrients on the growth of selected dune species were also investigated.

The water potential of 16 species was measured with the Shardakoy dye method (Knipling 1967). Leaf samples were taken between 1:30 p.m. and 2:30 p.m. on a sunny day at least 3 days after rain in both summer and winter. Twenty measurements were taken for each species and the daily patterns of several species were observed also.

Four species growing on the dune proper, horseweed, sandgrass, seabeach evening primrose, and ground cherry, were selected and young plants with soils were collected from the field in 10-cm diameter plastic pots. The size of plants was kept as uniform as possible and special care was taken to minimize the damage during the transplantation. All plants were used for each of the following treatments every 3 days: sea water, 1:3 diluted sea water, salt spray, shaded with canvas, solution containing N, K, and P (Hoagland and Arnon 1938), solution containing K only, drought, and a control plant watered with tap water. Dates of death were recorded for each plant that received treatments of sea water, 1:3 diluted sea water, salt spray, and drought. For the nutrition treatments as well as the control, the height and dry weight of above-ground portions of the plants were measured after 2 months.

Results

Water potential: There is a wide range of variation in water potential among species, ranging from -5 bars to -18 bars (Fig. 11a). A variation of 4 bars is quite common within a species, possibly due to the difference of individuals, of various habitats, and in different months. However, a 2-bar difference in one species measured on the same day from different individuals is not uncommon. The water potential is lower in woody species, namely, red cedar, yaupon, live oak, and wax myrtle, than in herbaceous species: seashore elder (semiherbaceous), sea oats, seaside spurge, saltmeadow cordgrass, silver-leaf croton, horseweed, camphor weed (Heterotheca subaxillaris), seabeach evening primrose, wild bean, seaside pennywort, sandgrass, and sandspur. Also, the variation of water potential is larger in woody species than in herbaceous ones. Among herbaceous species, those which occur frequently on fore dunes or on dry sites have a little lower water potential, such as seashore elder, sea oats, beach spurge (Euphoria polygonifolia), saltmeadow cordgrass, and silver-leaf croton, Therefore, there seems to be a correlation between the water potential of plants and the nature of habitats.

Fig. 11. (top) Water potentials of various species. Vertical lines refer to the range of water potentials of corresponding species recorded in the early afternoon while the cross bars refer to the average values of 20 measurements for each species; (bottom) changes of water potentials of different species from 8a.m. to 8p.m.

The water potentials of all species in the early morning are quite close (Fig. 11b). Approaching midday, water potentials decrease; again, much lower water potentials are detected among the woody plants. Since the water potentials of woody species drop to -14 to -15 bars while the lowest water potentials of the herbaceous plants are only -5 to -10 bars, the difference is quite clear. Because there is always abundant water about 30 cm below the soil surface, the decrease in water potential in the afternoon apparently is due to the lack of absorption under the stress of excessive transpiration on leaves rather than to the lack of water supply in the soil. In other words, under high light intensity, excessive heat, and constant wind, the rate of transpiration from leaves exceeds the water absorption by roots. Nevertheless, the water potentials of all species studied usually recover overnight.

Effect of salt spray, salt water, and drought: In regard to the accumulated number of dying individuals vis-a-vis the number of days after detrimental treatments (Fig. 12), horseweed appears to be the species most susceptible to salt spray, followed by seabeach evening primrose, ground cherry, and sandgrass. Evening primrose (Oenothera) and ground cherry have dense epidermal hairs, while sandgrass has a wax layer on the leaf surface. Since salt water kills plants of all these species in 10-13 days (Fig. 12), none of them should be considered as true halophytes. Plants of all species studied, except ground cherry, died on the 15th day after treatment with 1:3 diluted sea water. Apparently their resistance to diluted sea water is only slightly better than to the full-strength sea water. The longest survivor, ground cherry, probably depends on the merit of its thick fleshy rhizome.

Fig. 12. Top eight graphs show the results of an experiment in which horseweed, ground cherry, seabeach evening primrose, and purple sandgrass were subjected to seawater, diluted sea water (1:3), salt spray, and drought stress. Accumulated number of dead individuals is plotted against the date of death. The bottom two graphs show the effects of nutrients and shade on plant growth. White bars represent horseweed; black bars represent sandgrass. Left graph shows the effect on plant height; right graph shows the effect on standing crop in even-dry weight.

Generally speaking, all species have slight resistance to drought, the death date for the last individual of each species ranges from 19 to 26 days after the initial watering (Fig. 12). One may note that the planting pot is only 10cm in diameter and contains no more than 500 g of sand.

Effects of nutrient solutions and shade on plant growth: Only two species, horseweed and sandgrass, were studied (bottom two graphs of Fig. 12). Plants of both species show a greater increase in height under all treatments than that of the control. The increase is greatest in the complete nutrient solution treatment, the shaded treatment is next, and the one treated only with K solution shows the least increase. However, concerning the net gain in dry weight, the adding of NPK solution shows prominantly the highest increase, while the net gains of the shaded plant and the plant treated with K only are substantially reduced and not significantly different from that of the control. There seems to be no improvement in plant growth by just adding K to the water. Therefore, potassium alone is not a limiting factor to the growth of these dune species. However, nitrogen and phosphorus are shown to be extremely important in enhancing plant growth. Finally, the height increase under shaded condition does not indicate a higher productivity, but mainly etiolation.