Understanding recruitment failure in tropical tree species: Insights from a tree-ring study

Abstract

Many tropical tree species have population structures that exhibit strong recruitment failure. While the presence of adult trees indicates that appropriate regeneration conditions occurred in the past, it is often unclear why small individuals are absent. Knowing how, when and where these tree species regenerate provides insights into their life history characteristics. Based on tree age distributions inferences can be made on past forest dynamics and information is obtained that is important for forest management. We used tree-ring analyses to obtain tree ages and reconstruct >200years of estimated establishment rates in a sparsely regenerating population of Afzelia xylocarpa (Fabaceae), a light-demanding and long-lived canopy tree species. We sampled all 85 Afzelia trees >5cm diameter at breast height (dbh) in a 297-ha plot in a seasonal tropical forest in the Huai Kha Khaeng (HKK) Wildlife Sanctuary, western Thailand. The age distribution of the sampled Afzelia trees revealed two distinct recruitment peaks centred around 1850 and 1950. The presence of distinct age cohorts provides a strong indication of disturbance-mediated recruitment. Additionally we found three lines of evidence supporting this interpretation. (1) Similarly aged trees were spatially aggregated up to ∼500m, a scale larger than single tree-fall gaps. (2) High juvenile growth rates (5–10mmdbhyear−1) of extant small and large trees indicate that recruitment took place under open conditions. (3) A significant positive correlation between tree age and local canopy height indicates that trees recruited in low-canopy forest patches. Likely causes of these severe canopy disturbances include windstorms and ground fires, which are common in the region. In addition, successful establishment seems to be favoured by wetter climate conditions, as the estimated establishment rate was correlated to the Palmer Drought Severity Index (PDSI). Thus, the co-occurrence of canopy disturbance and favourable climatic conditions may provide a window of opportunity for Afzelia establishment. Our results indicate that forest patches with occurrence of large Afzelia trees have undergone high-severity canopy disturbance prior to establishment, suggesting that these disturbances have shaped forests at HKK. Tree-ring analyses provide a powerful tool to understanding tropical tree establishment patterns. Rare, high-severity canopy disturbances may play a key role in the regeneration of long-lived tropical canopy tree species with recruitment failure, potentially in interaction with climate variability to determine variation in establishment success over decades or centuries.