Species interactions during succession in the western Cascade Range of Oregon, 1990 to present

The factors that contribute to plant species establishment and decline following disturbance determine the rates and patterns of successional change of a system. In this long-term field experiment, we test the commonly held assumption that competition for space or resources by dominant species determines the outcome of succession. Specifically, we examine the population- and community-level consequences of removing one or more potentially dominant species from the post-disturbance community after clearcut logging and burning of a mature/old-growth Douglas-fir forest. Experimental treatments include: (1) removal of early-seral annual, Senecio sylvaticus, or perennial, Epilobium angustifolium—or both—to test the influences of these early-seral dominants on subsequent community development; (2) removal of all species except Senecio or Epilobium, to test whether the decline of these early-seral dominants is driven by competitive displacement; or (3) removal of shade-tolerant forest species that dominate subsequent stages of succession—Rubus ursinus or Berberis nervosa plus Gaultheria shallon—to test the influences of these long-lived perennials on understory development. The experiment is a randomized block design comprising eight removal treatments plus a control replicated in each of 25 blocks. Removal (reduction in competition) is achieved by removing seedlings or vegetative stems annually from a treatment area of 2.5 x 2.5 m. Sample plots (1 x 1 m) centered within these are used to estimate cover of all vascular plant species and, for the first 8 yr of the experiment, stem density and height, facilitating estimates of above-ground biomass. Pre-harvest data were collected in 1990, logging/burning occurred in 1991, and removal treatments and post-treatment sampling were initiated in 1992. Six of the nine experimental treatments were terminated between 1996 and 1998, with loss of early-seral Senecio and Epilobium from the system. The remaining three treatments (removal of Rubus, removal of Berberis plus Gaultheria, and the control) have been sampled annually since 1992. All regeneration of trees within the experimental area has occurred naturally. In September 2018, the study site was subjected to a controlled burn to limit spread of the Terwilliger Fire from the north. In 2019, plots were assessed for burn severity. Species' removals were terminated, but cover measurements continue in the plots.干扰后驱动植物物种定植与消亡的因子，决定了生态系统演替变化的速率与格局。本长期野外实验旨在验证一项广为接受的假设：优势物种对空间或资源的竞争决定了演替的最终结果。具体而言，我们在经皆伐与焚烧处理的成熟/老龄花旗松（Douglas-fir）林的扰动后群落中，移除一种或多种潜在优势物种，以此探究其在种群与群落层面的后续影响。 实验处理包括：(1) 移除早期演替一年生植物林地千里光（Senecio sylvaticus）、多年生植物柳兰（Epilobium angustifolium），或同时移除二者，以检验这些早期演替优势种对群落后续发育的影响；(2) 移除除林地千里光或柳兰之外的所有物种，以验证这类早期演替优势种的消亡是否由竞争排挤驱动；(3) 移除在演替后续阶段占据优势的耐阴森林物种——茅莓（Rubus ursinus）、细叶小檗（Berberis nervosa）以及越橘状白珠树（Gaultheria shallon），以检验这些长寿多年生植物对林下植被发育的影响。 本实验采用随机区组设计，包含8种移除处理与1个对照组，在25个区组中均设置重复。通过每年在2.5×2.5米的处理区域内移除幼苗或营养茎，实现降低竞争压力的实验处理。以该区域中心的1×1米样方来估算所有维管植物物种的盖度，并在实验开展的前8年测定茎密度与株高，以此推算地上生物量。 实验前的采伐基准数据采集于1990年，1991年完成皆伐与焚烧作业，1992年正式启动移除处理与处理后采样工作。1996至1998年间，9项实验处理中的6项被终止，且系统中已无早期演替的林地千里光与柳兰种群。剩余3项处理（移除茅莓、移除细叶小檗加越橘状白珠树，以及对照组）自1992年起每年进行采样监测。实验区域内的所有树木均为天然更新。 2018年9月，研究样地实施了受控焚烧作业，以遏制特威利格大火（Terwilliger Fire）从北侧蔓延。2019年，对样地的火烧烈度进行了评估。物种移除作业已终止，但样地的盖度测量工作仍在持续开展。