Grow or die: A 49-year growth history of a Japanese warm-temperate tree species

The growth trajectories of trees are not fully understood due to their long lifespan. We characterized the population dynamics of the canopy tree Castanopsis cuspidata (Thunb.) Schottky in a Japanese warm-temperate forest over 49 years (1966–2015). Our study was initiated approximately 50 years after our study site was clear-cut. The forest had a closed canopy for the first 23 years of the study. Strong typhoons in 1991 and 1993 seriously damaged the forest, and since then the forest has been recovering from these disturbances. The diameter distribution of this species was bell-shaped in 1966, suggesting that the trees emerged simultaneously after the clear-cut in the 1910s, and the recruitment of trees has remained unchanged since then. The lack of recruitment of C. cuspidata before the typhoon disturbance supports this conclusion. Assuming that the C. cuspidata trees in 1966 were cohorts that were established soon after the clear-cut, the size differences reflect differences in growth rate, with small trees corresponding to slow growers and large trees corresponding to fast growers. Before the typhoon, slow growers had low survival, and the mortality rate of fast growers was low. Many fast growers were uprooted or snapped by strong winds by the typhoons. However, their mortality rate did not differ from that of slow growers because many slow growers were killed by large fallen trees. The growth of some slow-growing survivors increased after the typhoon, which allowed them to rapidly reach the canopy. Therefore, the typhoon altered the distribution of canopy trees among slow and fast growers. Survivors experienced faster growth than trees that died during the census period, suggesting that growth rate provides a robust indicator of future survival. Before the typhoon, the survival of fast growers was higher than that of slow growers. This suggests that fast growers disproportionally contribute to reproduction compared with slow growers. However, no recruited tree was observed in this subperiod, suggesting that fast growers made no contribution to reproduction. Fast growers might not play a more significant demographic role than slow growers in this species.由于树木寿命极长，其生长轨迹尚未被完全阐明。本研究针对日本暖温带森林中的冠层树种栲树（Castanopsis cuspidata (Thunb.) Schottky），对其种群动态展开了为期49年（1966–2015年）的跟踪调查。本研究启动时，研究地块的皆伐作业已完成约50年。研究前23年，该森林冠层完全闭合。1991年与1993年的强台风对该森林造成了严重破坏，此后森林持续从这些干扰中恢复。 1966年，该物种的径级分布呈钟形，表明其个体均在1910年代的皆伐事件后同步萌发定植，且自彼时起种群补充量始终维持稳定。台风干扰前未观测到该物种的种群补充，这一结果进一步佐证了上述结论。假设1966年现存的该栲树个体均为皆伐后不久建立的同龄群，则个体间的尺寸差异反映了生长速率的差异：小径级个体对应慢生长个体，大径级个体对应快生长个体。 台风发生前，慢生长个体的存活率较低，而快生长个体的死亡率相对更低。强风导致大量快生长个体被连根拔起或茎干折断。但两类个体的死亡率并无显著差异，这是因为许多慢生长个体被倒伏的大树砸死。部分存活的慢生长个体在台风后生长速率显著提升，得以快速进入冠层。因此，台风改变了冠层树种中快慢生长个体的分布格局。 相较于普查期内死亡的个体，存活个体的生长速率更快，这表明生长速率可作为未来存活率的可靠预测指标。台风发生前，快生长个体的存活率高于慢生长个体，这意味着相较于慢生长个体，快生长个体对种群繁殖的贡献不成比例地更高。但该子阶段未观测到任何新补充的个体，表明快生长个体并未对种群繁殖作出任何贡献。综上，该栲树物种中，快生长个体在种群动态中的作用可能并不比慢生长个体更为重要。