Multidecadal stability in tropical rain forest structure and dynamics across an old-growth landscape

Have tropical rain forest landscapes changed directionally through recent decades? To answer this question requires tracking forest structure and dynamics through time and across within-forest environmental heterogeneity. While the impacts of major environmental gradients in soil nutrients, climate and topography on lowland tropical rain forest (TRF) structure and function have been extensively analyzed, the effects of the shorter environmental gradients typical of mesoscale TRF landscapes remain poorly understood. To evaluate multi-decadal performance of an old-growth TRF at the La Selva Biological Station, Costa Rica, we established 18 0.5-ha annually-censused forest inventory plots in a stratified-random design across major landscape edaphic gradients. Over the 17-year study period, there were moderate differences in stand dynamics and structure across these gradients but no detectable difference in woody productivity. We found large effects on forest structure and dynamics from the mega-Niño event at the outset of the study, with subdecadal recovery and subsequent stabilization. To extend the timeline to >40 years, we combined our findings with those from earlier studies at this site. While there were annual to multiannual variations in the structure and dynamics, particularly in relation to local disturbances and the mega-Niño event, at the longer temporal scale and broader spatial scale this landscape was remarkably stable. This stability contrasts notably with a current hypothesis of increasing biomass and dynamics of TRF, which we term the Bigger and Faster Hypothesis (B&FHo). We consider possible reasons for the contradiction and conclude that it is currently not possible to independently assess the vast majority of previously published B&FHo evidence due to restricted data access.

近几十年来，热带雨林景观是否发生了方向性变化？要解答这一问题，需要随时间推移并跨越森林内部环境异质性，追踪森林结构与动态变化。尽管学界已针对土壤养分、气候与地形等主要环境梯度对低地热带雨林（TRF）的结构与功能产生的影响开展了大量分析，但中尺度热带雨林景观中典型的短尺度环境梯度的效应仍鲜有研究。为评估哥斯达黎加拉塞尔瓦生物站一处老龄热带雨林（TRF）的数十年动态表现，我们采用分层随机设计，跨越主要景观土壤梯度，设立了18块面积为0.5公顷、每年开展一次清查的森林样地。在为期17年的研究周期内，各梯度间的林分动态与结构存在中等程度差异，但木本生产力未出现可检测到的差异。研究伊始发生的超级厄尔尼诺事件对森林结构与动态产生了显著影响，该影响历经十年内的恢复过程后趋于稳定。为将研究时间跨度延长至40年以上，我们将本次研究结果与该站点此前的相关研究结果进行了整合。尽管森林结构与动态存在年际至多年际的波动（尤其与局部干扰及超级厄尔尼诺事件相关），但在更长时间尺度与更广空间尺度下，该景观表现出显著的稳定性。这一稳定性与当前一项关于热带雨林生物量与动态增长的假说形成鲜明对比，我们将该假说命名为‘更大更快假说（Bigger and Faster Hypothesis, B&FHo）’。我们探讨了这一矛盾的潜在成因，并得出结论：由于数据获取受限，当前无法独立评估此前已发表的绝大多数‘更大更快假说’相关证据。