Neighbourhood-mediated shifts in tree biomass allocation drive overyielding in tropical species mixtures

Variations in crown forms promote canopy space-use and productivity in mixed-species forests. However, we have a limited understanding on how this response is mediated by changes in within-tree biomass allocation. Here, we explored the role of changes in tree allometry, biomass allocation and architecture in shaping diversity-productivity relationships (DPRs) in the oldest tropical tree diversity experiment. We conducted whole-tree destructive biomass measurements and terrestrial laser scanning. Spatially explicit models were built at the tree level to investigate the effects of tree size and local neighbourhood conditions. Results were then upscaled to the stand level, and mixture effects were explored using a bootstrapping procedure. Biomass allocation and architecture substantially changed in mixtures, which resulted from both tree-size effects and neighbourhood-mediated plasticity. Shifts in biomass allocation among branch orders explained substantial shares of the observed overyielding. By contrast, root-to-shoot ratios, as well as the allometric relationships between tree basal area and aboveground biomass, were little affected by the local neighbourhood. Our results suggest that generic allometric equations can be used to estimate forest aboveground biomass overyielding from diameter inventory data. Overall, we demonstrate that shifts in tree biomass allocation are mediated by the local neighbourhood and promote DPRs in tropical forests.

树冠形态的差异促进了混交林冠层空间利用和生产力。然而，我们对于这种响应如何受到树木内部生物量分配变化的调节了解有限。在此，我们在最古老的热带树木多样性实验中探究了树木异速生长、生物量分配和结构变化在塑造多样性 - 生产力关系（DPRs）中的作用。我们进行了整树破坏性生物量测量和地面激光扫描。在树木层面建立了空间显式模型，以研究树木大小和局部邻域条件的影响。然后将结果提升到林分层面，并使用自助法程序探究混合效应。在混交林中，生物量分配和结构发生了显著变化，这是树木大小效应和邻域介导的可塑性共同作用的结果。枝条等级间生物量分配的变化解释了所观察到的超产现象的很大一部分。相比之下，根冠比以及树木基面积和地上生物量之间的异速生长关系受局部邻域的影响较小。我们的结果表明，可以使用通用的异速生长方程从直径调查数据估算森林地上生物量超产量。总体而言，我们证明了树木生物量分配的变化是由局部邻域介导的，并促进了热带森林中的多样性 - 生产力关系。