Leaf Gas Exchange in the Clearcut Site at Harvard Forest 2010-2012

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（clearcutting）会导致林分（stand）生产力大幅下降。尽管这类干扰程度极强，但既往研究发现，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。明显原因之一为恢复过程中逐年增加的叶面积。而另一项较不直观的潜在原因，则可能是自然更新过程中物种组成与分布的变异性。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，在多大程度上源自1）叶面积的整体扩张，以及2）林冠光合能力的提升——该提升可能由物种组成变化，或物种内生理性状的漂移所导致。本研究发现，采伐后多年间的GEP提升，显然源自叶面积的扩张，而非植被组成的变化。在该早期演替环境中存在的多种植被类型，其叶片生理属性极为相似，这掩盖了物种相对丰度的显著变化。将尺度升级后的叶室法（leaf-chamber）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具有高度一致性。本文所提出的研究方法阐明了：在高度动态的环境中，如何将叶室法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，从而对生态系统过程随时间的变化得出确定性推论。