n Fenologiese studie van die plantegroei van die Roodeplaatdam-Natuurreservaat

Abstract

The objective of this study, conducted in the Roodeplaat Dam Nature Reserve (RDNR), was the classification of the vegetation and the analysis of the interaction between plant and environ=mental variables. The climate of the RDNR is seasonal, characterised by warm semi-humid summers and cold, dry winters. The mean annual rainfall and temperature are 650,1 mm and 18,2°c respectively. The vegetation was classified by means of the Braun-Blanquet method into six plant communities on the basis of their floristical composition. An additional seven variations were identified. The phenology of a number of phanerophytes, chamaephytes and hemicryptophytes was studied over a three year period and it was found that certain phanerophytes exhibited activity as early as the second half of July. Most other species commence growing or flowering only in September, whilst the flowering phase of a number of species commences only in late summer or early winter. Growth activity in woody species evidently takes place when the air temperature exceeds a certain threshold, in conjunction with an increase in photoperiod. Growth and reproduction are affected by rain only after certain threshold levels of air temperature and photoperiod have been exceeded. The phenomorphological description of a number of woody species was detailed with reference to, amongst others, the presence of dolichoblasts and brachyblasts. The life-span of the leaves was used to differentiate between deciduous and evergreen species. The rates of photosynthesis and transpiration as well as leaf diffusion resistance, chlorophyll concentration, leaf water content and leaf area of a number of woody species was studied on a daily or seasonal basis. The mutual dependence as well as the relationship with climatic conditions was investigated. The threshold levels, optimum values and maximum limits of environmental factors as related to the rates of photosynthesis and transpiration of Berchemia zeyheri and ulea europaea subsp. africana was established. The daily variation in the rates of photosynthesis and transpiration are influenced predominantly in the case of B. zeyheri by photosynthetically active radiation and wind speed. The seasonal variation in the rates of photosynthesis and transpiration of B. zeyheri and Olea europaea subsp. africana appears to be independent of photosynthetically active radia=tion whilst relatively low or high rainfall and relatively high air and leaf temperature appear to be influential. There appears to be no useful application of heat units in the study of woody species. There is virtually no relationship between seasonal variation in the rate of photosynthesis and chlorophyll concentration in B. zeyheri and O. europaea subsp. africana. In addition there is a strong positive relationship between seasonal change in the rate of transpiration and leaf water content in particularly the case of B. zeyheri. In an investigation of the spectral properties of the leaves of a diversity of woody species no meaningful variation in the absorptivity of the leaves was found which indicates that the spectral properties of leaves is not important in respect of the distribution of these species.