Population size structure of trees in a semi‐arid African savanna: Species differ in vulnerability to a changing environment and reintroduction of elephants

Abstract

Abstract The size structure of a plant population provides a snapshot of potential population trend and weak inference on past history. Size structure of 24 tree species was sampled in a savanna reserve in order to assess their potential vulnerability to local extirpation following the reintroduction of elephants. Regression of number per size class based on an expected reverse‐J structure for a healthy population was undertaken in order to classify trend of each species. On account of the weak realism of this expected structure in semi‐arid environments, log‐normal mixture models were also fitted to individual observations in order to identify states, whose distribution, size and shape can indicate potential population persistence. Population structure of 12 species approximated a reverse‐J, indicating potential future growth. Although regressions of seven species were not significant, suggesting vulnerable populations, mixture models of all except one showed multiple states, indicating variable regeneration and recruitment over time. A single state of adult plants indicates that Sclerocarya birrea was vulnerable to local extirpation if adult mortality escalates. Some apparently declining populations displayed multiple states of established individuals suggesting temporally varying regeneration and recruitment, attributed mainly to rainfall variation, but which were judged as likely to persist. Poor recent regeneration of most species was attributed to well below‐average rainfall for the preceding decade. Different savanna tree species apparently ensure persistence based on individualistic responses to population perturbations, but persistence of some following the reintroduction of elephants may be threatened if certain size classes experience escalated mortality. Subsequent monitoring has supported this concern.