Litterfall in the Clearcut Site at Harvard Forest 2012

Clearcutting a forest ecosystem can result in a drastic reduction of the stand’s
productivity. Despite the severity of this disturbance type, past studies have found that the
productivity of young regenerating stands can quickly rebound, approaching that of mature
undisturbed stands within a few years. One of the obvious reasons is increased leaf area with
each year of recovery. However, a less obvious reason may be the variability in species
composition and distribution during the natural regeneration process. The purpose of this study
was to investigate to what extent the increase in GEP, observed during the first four years of
recovery, in a naturally regenerating clearcut stand was due to 1) an overall expansion of leaf
area, and 2) an increase in the canopy’s photosynthetic capacity stemming from either species
compositional shifts or drift in physiological traits within species. We found that the
multi-year rise in GEP following harvest was clearly attributed to the expansion of leaf area
rather than a change in vegetation composition. Sizeable changes in relative abundance of
species were masked by remarkably similar leaf physiological attributes for a range of
vegetation types present in this early successional environment. Comparison of upscaled
leaf-chamber to eddy-covariance-based light-response curves revealed broad consistency in both
maximum photosynthetic capacity and quantum yield efficiency. The approaches presented here
illustrate how chamber- and ecosystem-scale measurements of gas exchange can be blended with
species-level leaf area data to draw conclusive inferences about changes in ecosystem processes
over time in a highly dynamic environment.

对森林生态系统实施皆伐作业，会导致林分生产力大幅下降。尽管该干扰类型的影响程度极强，但过往研究发现，更新幼龄林分的生产力可快速恢复，数年内即可接近未受干扰的成熟林分水平。其中一个直观原因为：随恢复进程推进，叶面积逐年增加。而另一个较隐蔽的潜在原因，则可能是天然更新过程中物种组成与分布的动态变化。本研究旨在探究：在天然更新的皆伐林分中，恢复前四年观测到的总生态系统生产力（Gross Ecosystem Productivity, GEP）提升，在多大程度上源于两个因素：1）叶面积的整体扩张；2）林冠光合能力的增强——而该增强可归因于物种组成的转变，或是物种种内生理性状的漂移。研究结果表明，采伐后总生态系统生产力的多年持续提升，明显源于叶面积的扩张，而非植被组成的变化。在该早期演替环境中，多种植被类型的叶片生理性状极为相似，这掩盖了物种种相对丰富度的显著变化。通过将叶室法尺度上推得到的光响应曲线，与基于涡度相关法的光响应曲线进行对比，发现二者在最大光合能力与量子产额效率上均具有高度一致性。本研究提出的研究方法，展示了如何将叶室与生态系统尺度的气体交换测量数据，结合物种水平的叶面积数据，从而在高度动态的环境中，对生态系统过程的长期变化得出确定性推论。