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.

对森林生态系统进行皆伐作业，会导致林分生产力出现大幅下降。尽管该干扰类型的影响极为严重，但既往研究发现，自然更新的幼龄林分生产力可快速恢复，数年内即可接近未受干扰的成熟林分水平。其中一个显而易见的驱动因素为：随着恢复年限增长，林分叶面积持续扩大。而另一个较为隐蔽的驱动因素，则可能是自然更新过程中物种组成与分布的空间异质性。
本研究旨在探究：在自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）林冠光合能力的提升，而该提升要么来自物种组成的变化，要么来自物种内部生理性状的漂移。
本研究结果显示，采伐后多年间GEP的上升，显著归因于叶面积的扩张，而非植被组成的改变。在该早期演替环境中，尽管多种植被类型的物种相对丰度发生了显著变化，但这类变化被各类植被极为相似的叶片生理属性所掩盖。将尺度上推的叶室光响应曲线与基于涡度协方差（eddy-covariance）的光响应曲线进行对比后发现，二者在最大光合能力与量子产额效率上均表现出高度一致性。
本文所提出的研究方法，阐明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，进而在高度动态的生态系统中，针对生态系统过程随时间的变化得出确定性推论。