Fahey Robert T, Fotis Alexander T, Woods Kerry D
Ecol Appl. 2015 Apr;25(3):834-47. doi: 10.1890/14-1012.1.
The regrowing forests of eastern North America have been an important global C sink over the past 100+ years, but many are now transitioning into late succession. The consequences of this transition are unclear due to uncertainty around the C dynamics of old- growth forests. Canopy structural complexity (CSC) has been shown to be an important source of variability in C dynamics in younger forests (e.g., in productivity and resilience to disturbance), but its role in late-successional forests has not been widely addressed. We investigated patterns of CSC in two old-growth forest landscapes in the Upper Peninsula of Michigan, USA, to assess factors associated with CSC and its influence on productivity and disturbance resilience (to moderate-severity windstorm). CSC was quantified using a portable below-canopy LiDAR (PCL) system in 65 plots that also had long-term (50-70+ years). inventory data, which were used to quantify aboveground net primary productivity (ANPP), disturbance history, and stand characteristics. We found high and variable CSC relative to younger forests across a suite of PCL-derived metrics. Variation in CSC was driven by species composition and size structure, rather than disturbance history or site characteristics. Recent moderate severity wind disturbance decreased plot-scale CSC, but increased stand-scale variation in CSC. The strong positive correlation between CSC and productivity illustrated in younger forests was not present in undisturbed portions of these late-successional ecosystems. Moderate severity disturbance appeared to reestablish the positive link between CSC and productivity, but this relationship was scale and severity dependent. A positive CSC-productivity relationship was evident at the plot scale with low-severity, dispersed disturbance, but only at a patch scale in more severely disturbed areas. CSC does not appear to strongly correlate With variation in productivity in undisturbed old-growth forests, but may play a very important (and scale/severity-dependent) role in their response to disturbance. Understanding potential, drivers and consequences of CSC in late-successional forests will inform management focused on promoting complexity and old-growth conditions, and illustrate potential inipacts of such treatments on regional C dynamics.
在过去100多年里，北美东部不断再生的森林一直是全球重要的碳汇，但现在许多森林正过渡到演替后期。由于老龄森林碳动态存在不确定性，这种转变的后果尚不清楚。冠层结构复杂性（CSC）已被证明是年轻森林碳动态变化的一个重要来源（例如在生产力和对干扰的恢复力方面），但其在演替后期森林中的作用尚未得到广泛探讨。我们在美国密歇根州上半岛的两个老龄森林景观中调查了CSC模式，以评估与CSC相关的因素及其对生产力和干扰恢复力（对中度至重度风暴）的影响。使用便携式冠层以下激光雷达（PCL）系统在65个样地中对CSC进行了量化，这些样地也有长期（50 - 70多年）的库存数据，用于量化地上净初级生产力（ANPP）、干扰历史和林分特征。相对于年轻森林，我们在一系列基于PCL得出的指标上发现了较高且多变的CSC。CSC的变化是由物种组成和大小结构驱动的，而不是干扰历史或立地特征。近期的中度严重风干扰降低了样地尺度的CSC，但增加了林分尺度CSC的变化。年轻森林中CSC与生产力之间明显的正相关在这些演替后期生态系统未受干扰的部分并不存在。中度严重干扰似乎重新建立了CSC与生产力之间的正相关，但这种关系取决于尺度和严重程度。在低严重度、分散干扰的样地尺度上，CSC与生产力之间存在明显的正相关关系，但在受干扰更严重的地区仅在斑块尺度上存在这种关系。在未受干扰的老龄森林中，CSC似乎与生产力变化没有强烈关联，但在其对干扰的响应中可能发挥非常重要的（且取决于尺度/严重程度）作用。了解演替后期森林中CSC的潜在因素、驱动因素和后果，将为旨在促进复杂性和老龄森林条件的管理提供信息，并说明此类处理对区域碳动态的潜在影响。
