Shrub growth and demography in arid Karoo, South Africa: inference from wood rings

Pteronia pallens(Asteraceae) produced annual growth rings, the width of which were influenced by water availability, particularly in the winter growing season. Ring widths (mean 0•83 mm, S.D. 0•46 mm) are within the range reported for other desert shrubs. Field measurements of basal diameter were significantly correlated with the number of growth rings, but accuracy of age estimates was limited by the longitudinal splitting of the stems. Longevity estimates from basal diameters suggest thatP. pallenscan live for 150 years. Age class frequency distributions ofP. pallensat the study sites showed two peaks over the period 1920 to 1990. Rainfall explained only 33% of the variation in recruitment. Other factors, such as the reduction of competition by drought or grazing, may have had a greater influence on recruitment patterns.