Coexistence of tropical forest tree species along the demographic buffering spectrum

Organisms have evolved diverse adaptive strategies to cope with environmental fluctuations. Slow-growing long-lived species tend to exhibit low temporal variability in population growth (strongly buffered demographically), whereas fast-growing short-lived species optimize growth in favorable years (weakly buffered). These patterns set up the expectation that differentiation in demographic buffering may reduce disparities in long-term fitness among species, enhancing the potential for coexistence in variable environments. Yet, this expectation has never been empirically tested for trees. Here, we quantified differences in long-term population growth among 204 co-occurring tropical trees spanning a life-history spectrum from strongly to weakly buffered. We found predictable variation in long-term population fitness for species at low densities, pointing to demographic differentiation as a key for coexistence in fluctuating environments. Simulated increases in temperature, precipitation, and drought variability led to divergent impacts on species' fitness and increased fitness differences. Together, these findings provide a novel perspective on the mechanisms that underpin the astounding tree diversity in tropical forests.

生物演化出多样的适应策略以应对环境波动。生长缓慢、寿命较长的物种种群增长的时间变异性通常较低（种群动态缓冲（demographic buffering）作用较强），而生长迅速、寿命较短的物种则会在适宜年份最大化种群增长（种群动态缓冲作用较弱）。上述模式预示，种群动态缓冲作用的分化或许会缩小物种间长期适合度的差异，进而提升波动环境中物种共存的可能性。然而，这一假说尚未在树木类群中得到实证检验。本研究对204种同域分布的热带树木进行了量化分析，这些物种覆盖了从强缓冲到弱缓冲的完整生活史谱系。研究发现，低密度物种种群的长期适合度存在可预测的变异，表明种群动态分化是波动环境下物种共存的关键机制。模拟的气温、降水及干旱波动加剧，会对不同物种的适合度产生差异化影响，并进一步扩大适合度差异。综上，本研究为解析热带森林中惊人的树木多样性维持机制提供了全新视角。