Live fast, die young: estimating size-age relations and mortality pattern of shrubs species in the semi-arid Karoo, South Africa

We present a technique for estimating size-age relations and size-dependent mortality patterns of long-lived plants. The technique requires two sets o f size data of individual (non-marked) plants that should be collected with a time-lag of several years in the same area of a study site. The basic id ea of our technique is to assume general (three parameter) families of size -dependent functions which describe growth and mortality that occurred betw een the two data gathering events. We apply these growth and mortality func tions to the size data of the early data set and construct predicted size-c lass distributions to compare it, in a systematic way, to the size-class di stribution of the later data set. In a next step we calculate the size-age relations from the resulting growth functions, which yield the smallest dif ference between observed and predicted size-class distribution. Applying th is technique to size data of five dominant shrub species at the Tierberg st udy site in the semiarid Karoo, South Africa produced new insight into the biology of these species which otherwise cannot be obtained without frequen t measurements of marked plants. We could relate characteristics of growth behavior and mortality, for certain subgroups of the five species, to the l ife-history attributes evergreen vs. deciduous, succulent vs. woody, and ea rly reproductive vs. late reproductive. The results of our pilot-study sugg est a broad applicability of our technique to other shrublands of the world . This requires at least one older record of (individual) shrub-size data a nd performance of resampling.