PARENT SESSION
Oral Session #3: Conservation Ecology: Threatened and Endangered Species.
Presiding: J. Lawler
Monday, August 5. 8:00 AM to 11:30 AM. Graham Meeting Room, TCC.
The influence of water availability on the distribution of Widdringtonia cedarbergensis.
February, Edmund*,1, 1 Department of Botany, Cape Town, South Africa
ABSTRACT- The Clanwilliam cedar (Widdringtonia cedarbergensis Marsh) is an endangered species entirely restricted to the mountain fynbos habitats of the Cedarberg mountains of the Western Cape Province, South Africa. This study is an attempt to gain a better understanding of the water use of this endangered species. The guiding hypothesis of the study is that plants growing in different geomorphological positions at an altitude between 900 and 1400 metres in the Cedarberg have access to different sources of water viz. stream, soil, rain and ground water. The belief is that the spatial and temporal distribution of these different water supplies combined with protection from fire, as in the degree of rockyness of the landscape, are the key variables that both directly and indirectly influences the demographic behaviour of trees. The suggestion is that if the preferred habitat for cedars were the observed rocky outcrops above 800 m then they would have adapted both growth habit and water uptake to the annual soil moisture deficits inherent within a Mediterranean ecosystem. Three different methods are used to determine the physiological responses of W. cedarbergensis to available water. These methods include Xylem Pressure Potentials, Dendrobands and Stable Hydrogen Isotope analysis. The study site located at Welbedacht in the Cedarberg Wilderness area is from 1270 to 1370 metres above sea level. Six trees were sampled monthly for isotopic analysis, while twelve trees were fitted with dendrobands. Predawn xylem pressure potentials were measured for both summer and winter. The results indicate that the cedars are not water stressed at any time of the year. Rather, they are using a deep water source independent of both stream and rain. Therefore, it is probably fire rather than water supply that dictates the distribution of this species in the landscape.
KEY WORDS: Widdringtonia cedarbergensis, Water, Isotopes