Exploring mark-resighting-recovery models to study savannah tree demographics

Abstract

Despite their sessile nature, juvenile trees in savannah ecosystems are not always easy to encounter. Here, the remedy of choice in animal studies, capture-recapture modelling, is evaluated with respect to plants. During 7 censuses, 4,145 juvenile trees of 8 dominant species were monitored. Using models with joint analysis of live and dead encounters, resighting probabilities averaged 0.88±0.15 for seedlings and 0.92±0.10 for resprouts, while dead recovery probabilities averaged 0.71±0.25 for all age-classes. Ad hoc methods which did not consider encounter probabilities yielded biased survival estimates compared to the mark-resighting-recovery approaches. This occurred even for high encounter probabilities, so the use of capture-recapture models is recommended whenever the plant encounter probability is < 1. Finally, survival probabilities estimated by models based only on live or on dead data might be different and less precise than ones estimated based on combined data. This highlights the advantages of models with joint analysis of live and dead encounters even when site fidelity is 1.