Graduate Thesis Or Dissertation
 

Succession and certain adaptive features of plants native to the sand dunes of the Oregon coast

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  • The vegetation of the Oregon coast is characterized by a variety of stages of plant succession which are repeated widely throughout the sand dune area. During the years from 1960 to 1963 the gradation of upland sand dune succession was studied on 48 plots within a 150 mile section of the Oregon coastline. An effort was made to describe these various sand dune communities and to relate them according to their successional sequence. Both plot and plotless techniques were used to obtain data for frequency, cover, density and dominance. This latter factor was expressed by basal area at breast height for the tree species. Some of the physical factors of the salt spray community were examined such as salt spray tolerance, salt spray as a nutrient source, response of plants to low nutrient levels in beach sand, moisture and sand movement. Vascular plants were collected and identified. The results may be summarized as follows: Successional stages characteristic of the upland areas of the coastal sand dunes may be represented by nine different communities. According to the ordination these communities grade from the primary, herbaceous plants to the near climax forest tree species. In this ordination single values represented stands. The value of the ordination was checked by the manner in which the distribution of the individual species is related to the gradient. Some plants were uniquely distinct for a given community while other species graded from one community to another. The distribution of the plant species appeared to confirm the validity of the more objective ordination. The response of coastal vegetation to effects of salt spray is seen in a pruning effect where plants are subjected to the drier, salt-laden northwest winds of summer months. The salt tolerance of herbaceous plants on the raw dunes is greater than those plants growing in the more sheltered areas along the sides of the more exposed dunes. Shrubs and trees are often quite tolerant to the salt spray except for the new vegetative parts of the plant in the spring of the year. Various concentrations of sea water were applied to certain plants characteristic of the early stages in succession to determine the possibility of sea water serving as a nutrient source. Holcus was the only species which appeared to show a favorable response to the treatments. Among the plants tested, Hypocheris showed the greatest reduction in growth, with Convolvulus and Senecio showing a gradual reduction of growth with increasing concentrations of sea water spray. The possibility of sea water as an aid in growth in certain plants of the sand dune environment is suggested. Plants of Senecio sylvaticus from the coast and from a valley location were tested for their response to low nutrient levels when grown in beach sand. Coastal Senecio generally grew better than the valley Senecio when six major mineral nutrients were supplied, and when these nutrients were eliminated one by one both in beach sand and in water cultures. Based on this study there is the suggestion that there are two physiological races of Senecio. Such adaptations may also exist in other species common to the coastal sand dunes and also found on other types of soils in western Oregon. Moisture was found to be limiting to plants of the secondary vegetation of the sand dunes during the dry summer months, but not limiting in the primary vegetation of the dunes. The plants of the primary stage generally seem able to endure longer periods of moisture stress than those of the secondary stage. Certain plants of the primary stage in succession have some morphological features which suggest certain adaptive advantages with respect to evaporation stress. In many areas there is little net gain or loss of sand on the dune surface during the course of a year. However, large amounts of sand are moved and deposited at the ends of the winter dunes. An accumulation of sand in a local area causes a swift change in the composition of vegetation. Sites receiving fresh sand on the top of secondary vegetation change rapidly, with only primary plants invading or surviving. Sand movement appears critical in seedling germination and establishment, with the plants from the secondary stages in succession showing a more deleterious effect from deep covering of seeds than plants from the primary stage.
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