Clark David B, Clark Deborah A, Oberbauer Steven F, Kellner James R
Department of Biology, University of Missouri-St. Louis, St. Louis, Missouri, United States of America.
Department of Biological Sciences, Florida International University, Miami, Florida, United States of America.
PLoS One. 2017 Oct 5;12(10):e0183819. doi: 10.1371/journal.pone.0183819. eCollection 2017.
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？要回答这个问题，需要跟踪森林结构和动态随时间的变化以及森林内部环境异质性的情况。虽然土壤养分、气候和地形等主要环境梯度对低地热带雨林（TRF）结构和功能的影响已得到广泛分析，但中尺度TRF景观典型的较短环境梯度的影响仍知之甚少。为了评估哥斯达黎加拉塞尔瓦生物站一片原始TRF的数十年表现，我们在主要景观土壤梯度上采用分层随机设计建立了18个0.5公顷的年度森林清查样地。在17年的研究期内，这些梯度上的林分动态和结构存在适度差异，但木材生产力没有可检测到的差异。我们发现研究开始时的超级厄尔尼诺事件对森林结构和动态有很大影响，随后经历了十年内的恢复并趋于稳定。为了将时间线延长到40年以上，我们将我们的研究结果与该地点早期研究的结果相结合。虽然结构和动态存在年度到多年的变化，特别是与局部干扰和超级厄尔尼诺事件有关，但在更长的时间尺度和更广泛的空间尺度上，这片景观非常稳定。这种稳定性与当前关于TRF生物量增加和动态变化的假设形成了显著对比，我们将其称为“更大更快假设”（B&FHo）。我们考虑了产生这种矛盾的可能原因，并得出结论，由于数据获取受限，目前无法独立评估绝大多数先前发表的B&FHo证据。
