Plant succession and biomass dynamics following logging and burning in Watersheds 1 and 3, Andrews Experimental Forest, 1962 to Present

There has been long history of geomorphic, hydrological, and ecological study in the experimental watersheds of HJA, which include WS1 and WS3 and the neighboring old-growth basin, WS2. These include comparative studies of sediment yield, stream chemistry, and water flow following whole-basin logging of WS1 and patch-cut logging of WS3, and long-term studies of post-harvest forest development. The latter, initiated in 1962, are among the longest-running and most intensive studies of secondary succession in forests of North America. They are based on two types of permanent plots (understory and overstory), spatially linked to facilitate analyses of overstory-understory interactions. Plots in WS1 and WS3 occur within transects within units of the watersheds. Understory quadrats were established in 1962, prior to harvest, and have been sampled periodically since the watersheds were cut and broadcast burned. They have been used to study how species’ life histories, pre-disturbance composition, disturbance intensity, and overstory development contribute to the long-term dynamics of the understory. These are 2 m by 2 m quadrats where canopy cover, ground conditions, growth forms, species cover and biomass have been monitored. Overstory tree plots were established nearly two decades later (1979-1980) to characterize the early stages of forest development, including the recruitment (ingrowth), growth, and mortality of conifers and hardwoods. These are circular plots (radius=8.92 m) where live trees greater than or equal to 1.37 m tall are measured for diameter at breast height or basal diameter. Remeasurement of these plots occurs on 5 or 6 year intervals.HJA实验流域（experimental watersheds）拥有悠久的地貌学、水文学与生态学研究历史，其覆盖WS1、WS3及邻近的原始林流域WS2。相关研究涵盖针对WS1全流域皆伐、WS3斑块状择伐后的产沙量、溪流化学特征与水流动态的对比分析，以及采伐后森林发育的长期监测项目。后者始于1962年，是北美地区持续时长最长、研究程度最深入的森林次生演替（secondary succession）研究之一。该研究依托两类永久样地：林下层样地与林上层样地，通过空间联动设计以支撑林上-林下相互作用的相关分析。WS1与WS3内的样地布设于流域单元内的样带之中。林下样方（understory quadrats）于采伐前的1962年设立，并在流域完成伐木与全面火烧（broadcast burning）后持续开展定期采样。此类样方规格为2米×2米，监测指标包括冠层盖度、地表状况、生长型、物种盖度与生物量，其被用于探究物种生活史、扰动前群落组成、干扰强度以及林上层发育如何共同驱动林下植被的长期动态。林上层样地则于近二十年后的1979-1980年设立，旨在表征森林发育的早期阶段，包括针叶树与阔叶树的幼树补充（ingrowth）、生长与死亡动态。此类样地为圆形样地（半径8.92米），对高度≥1.37米的活立木测量其胸径（diameter at breast height）或基径。该类样地每5至6年开展一次重测。