Soil nutrient availability and reproductive effort drive patterns in nutrient resorption in Pentaclethra macroloba

The removal of nutrients from senescing tissues, nutrient resorption, is a key strategy for conserving nutrients in plants. However, our understanding of what drives patterns of nutrient resorption in tropical trees is limited. We examined the effects of nutrient sources (stand-level and tree-level soil fertility) and sinks (reproductive effort) on nitrogen (N) and phosphorus (P) resorption. We evaluated resorption efficiency (percentage of original nutrients removed during senescence) and resorption proficiency (indicated by senesced-leaf nutrient concentrations) in a symbiotic N-fixing tree species, Pentaclethra macroloba, common to tropical forests in Costa Rica. Although tree-level soil P alone did not drive patterns in nutrient resorption, P efficiency and proficiency declined with increasing tree-level soil P when reproductive status was also considered. Nutrient resorption declined with increasing tree-level soil P in trees that were actively fruiting or that experienced high seedfall the year prior to sampling. Trees with greater short- and long-term reproductive demands had lower seneseed-leaf N and P concentrations than trees with smaller reproductive demands indicating that trees increase resorption proficiency in response to phenological demand. P. macroloba is the dominant tree species in this tropical ecosystem. Thus, source-sink relationships will feed back on nutrient cycling in these forests.