Phosphorus allocation to and resorption from leaves regulate the residence time of phosphorus in aboveground forest biomass on Mount Kinabalu, Borneo

1. The residence time of phosphorus (P) in trees is a consequence of plant adaptation to P deficiency, with longer P residence time on soils with low P availability. P residence time has been studied at the leaf or canopy level but seldom at the whole-tree level. Whereas P residence time at the leaf or canopy level is largely determined by leaf longevity and the resorption of P before leaf abscission, P residence time at the whole-tree level will also be influenced by differences in P allocation to different plant parts because leaves and woody organs have distinct longevities. 2. We estimated the residence time of P in aboveground tree biomass (AGB) as the ratio of P mass (i.e. leaves plus wood) to the annual flux of P via litterfall (i.e. fine litter plus coarse woody debris) for seven tropical rain forests with different soil P availabilities on Mount Kinabalu, Borneo. We analysed the effects of P allocation to and resorption from leaves on P residence time along a soil P gradient. 3. P residence time (2.7–9.8 years) was approximately one fifth of biomass residence time (AGB/annual litterfall mass; 19.8–48.8 years). This was due to a disproportionately greater relative allocation of P to leaves (P mass in leaves/P mass in AGB; 0.11–0.46), which had a smaller fraction of biomass (leaf biomass/AGB; 0.02–0.05) but a shorter longevity (1.0–1.8 years). 4. The relative allocation of P to leaves was often high on low-P soils, and P residence time was expected to be short. By contrast, the resorption rate of P from leaves was also high on low-P soils, which extended P residence time with P deficiency. Consequently, P residence time was nearly constant across the forests. 5. The short residence time of P relative to biomass indicates that P residence time depends largely on relative P allocation among plant organs. Similar P residence times among sites were maintained because greater P allocation to leaves on low-P soils was effectively offset by higher P-resorption efficiency.1. 树木体内磷（P）的滞留时间（residence time of phosphorus）是植物适应磷缺乏的结果：土壤有效磷（P availability）含量越低，磷滞留时间越长。此前针对磷滞留时间的研究多集中于叶片或冠层尺度，极少涉及整树尺度。叶片或冠层尺度的磷滞留时间主要由叶片寿命（leaf longevity）及叶片脱落（leaf abscission）前的磷回收（resorption）过程决定，而整树尺度的磷滞留时间还会受到磷在不同植物器官间分配（P allocation）差异的影响——这是因为叶片与木质器官（woody organs）的寿命存在显著差异。 2. 我们针对婆罗洲（Borneo）基纳巴卢山（Mount Kinabalu）上7个不同土壤有效磷含量的热带雨林样地，以树木地上生物量（aboveground tree biomass, AGB）的总磷质量（即叶片与木质器官的磷质量之和）与凋落物（litterfall）年通量（即细凋落物（fine litter）与粗木质残体（coarse woody debris）的磷通量之和）的比值，估算了树木地上生物量的磷滞留时间。同时，我们分析了沿土壤有效磷梯度（soil P gradient）下，叶片的磷分配与磷回收对磷滞留时间的影响。 3. 磷滞留时间为2.7~9.8年，约为生物量滞留时间（biomass residence time，即地上生物量与年凋落物生物量的比值，19.8~48.8年）的五分之一。这一现象源于磷在叶片中的相对分配比例偏高（叶片磷质量/地上生物量总磷质量：0.11~0.46）：叶片生物量仅占地上生物量的小部分（叶片生物量/地上生物量：0.02~0.05），但其寿命更短（1.0~1.8年）。 4. 在低有效磷土壤上，叶片的磷相对分配比例通常更高，此时磷滞留时间理应更短；但与此同时，低有效磷土壤上的叶片磷回收效率（P-resorption efficiency）也更高，这一过程会因磷缺乏而延长磷滞留时间。最终，各样地的磷滞留时间基本保持一致。 5. 相较于生物量滞留时间，磷滞留时间更短这一结果表明，磷滞留时间在很大程度上取决于植物器官间的相对磷分配。由于低有效磷土壤上更高的叶片磷分配，被更高的叶片磷回收效率所有效抵消，因此各样地的磷滞留时间得以维持在相近水平。