Data from: Ten-year responses of ground-dwelling spiders to retention harvest in the boreal forest

The Ecosystem Management Emulating Natural Disturbances (EMEND) project tests the hypothesis that varying levels of green tree retention maintain and retain forest biodiversity better than conventional clear-cutting. We studied epigaeic spiders to assess biodiversity changes two, five and ten years following a range of partial retention harvests (clear-cut, 10-75% retention) and unharvested controls in four boreal mixedwood cover-types. A total of 56, 371 adult spiders representing 220 species was collected using pitfall traps. Lasting effects on forest structure were proportional to harvest intensity. These changes strongly influenced spider richness, abundance and species composition, as well as assemblage recovery. Distinctive assemblages were associated with disturbance level, especially with partial harvests (≤50% retention), and these were dominated by open-habitat species even ten years after harvest. Assemblages were more similar to those of controls in the highest (75%) retention treatment, but significant recovery toward the structure of pre-disturbance assemblages was not detected for any prescription in any cover-type. Although early responses to retention harvest suggested positive effects on spider assemblages, these are better explained as lag effects after harvest because assemblages were less similar to those of unharvested controls five years post-harvest, and only minor recovery was observed ten years following harvest. Retention of forest biodiversity decreased over time, especially in conifer stands and the lower (10-50%) retention treatments. Overall, retention harvests retained biodiversity and promoted landscape heterogeneity somewhat better than clear-cutting; however, there was a clear gradient of response and no retention ‘threshold’ for conservation can be recommended on the basis of our data. Furthermore, results suggest that retention harvest prescriptions should be adjusted for cover-type. We show that low retention ameliorated impacts in broadleaved forests characteristic of earlier stages in mixedwood succession, but only higher retention was associated with less impact in successionally older conifer forests. Although these short-term responses (10 years) of spider assemblages support use of retention harvests, understanding the true conservation merit of these practices, relative to conventional approaches, requires evaluation over longer time scales, with work more focused on recovery of biodiversity than on its preservation after harvest.

模拟自然干扰的生态系统管理（EMEND）项目旨在验证这一假说：相较于传统皆伐作业，不同强度的保留活立木（green tree retention）策略能更有效地维持并保护森林生物多样性。本研究针对地表活动蜘蛛（epigaeic spiders）展开调查，以评估在4种北方混交林覆盖类型中，实施一系列部分保留采伐（包括皆伐、10%~75%保留率采伐）以及设置未采伐对照样地后，第2、5、10年的生物多样性变化情况。研究人员通过陷阱法（pitfall traps）共采集到56371只成年蜘蛛，隶属于220个物种。森林结构受到的持续影响与采伐强度呈正相关关系。这些变化显著影响了蜘蛛的物种丰富度、个体数量、物种组成以及群落恢复进程。具有独特特征的蜘蛛群落与干扰强度密切相关，尤其是在保留率≤50%的部分采伐样地中，即便在采伐10年后，这些样地的蜘蛛群落仍以开阔生境物种为优势类群。保留率最高（75%）的处理组中，蜘蛛群落与未采伐对照样地更为相似，但在任意覆盖类型的任意采伐方案下，均未观测到群落结构向干扰前状态发生显著恢复的迹象。尽管保留采伐在初期对蜘蛛群落表现出积极影响，但这一现象更适合被解释为采伐后的滞后效应：采伐后第5年，蜘蛛群落与未采伐对照样地的相似度反而更低，且直至采伐后第10年，群落恢复程度仍十分有限。森林生物多样性的保留水平随时间推移逐渐下降，尤其在针叶林林分以及保留率为10%~50%的处理组中这一现象更为显著。总体而言，相较于传统皆伐，保留采伐策略在保留生物多样性与提升景观异质性方面表现稍优；但本研究结果显示，生物多样性响应存在明确的梯度变化，基于本研究数据无法推荐适用于生物保护的保留率阈值。此外，研究结果表明，采伐保留方案应依据林分覆盖类型进行调整。我们发现，低强度保留采伐可缓解混交林演替早期阶段典型阔叶林所受的干扰影响，但仅高强度保留采伐才能降低演替后期针叶林所受的干扰程度。尽管蜘蛛群落的这些短期（10年）响应结果支持保留采伐策略的应用，但相较于传统采伐方式，要明确这类措施的真实保护价值，仍需在更长时间尺度下开展评估，且后续研究应更侧重于生物多样性的恢复而非采伐后的短期留存。