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（Ecosystem Management Emulating Natural Disturbances，EMEND）项目旨在验证以下假说：相较于传统皆伐作业，不同水平的保留活立木（green tree retention）策略更能维持并保护森林生物多样性。本研究针对四种北方混交林覆盖类型，设置了一系列部分保留采伐（包括皆伐、10%~75%保留率）以及未采伐对照样地，在采伐后第2、5、10年对地表活动蜘蛛（epigaeic spiders）进行调查，以评估生物多样性变化情况。研究共采用陷阱诱捕器（pitfall traps）采集到隶属于220个物种的56371只成体蜘蛛。森林结构受到的长期影响与采伐强度呈正相关，这些结构变化显著影响了蜘蛛的物种丰富度、种群数量、物种组成以及类群恢复情况。独特的蜘蛛类群与干扰程度密切相关，尤其是在保留率≤50%的部分采伐样地中，即便在采伐后10年，此类样地的蜘蛛类群仍以开阔生境物种为优势类群。保留率最高（75%）的处理样地中的蜘蛛类群与未采伐对照样地更为相似，但在所有覆盖类型的所有采伐方案中，均未观测到类群结构向采伐前状态发生显著恢复。尽管保留采伐在早期对蜘蛛类群表现出积极影响，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，样地蜘蛛类群与未采伐对照的相似度反而更低，且在采伐后第10年仅观测到极轻微的类群恢复。森林生物多样性的保留水平随时间推移逐渐下降，尤其是在针叶林样地以及保留率为10%~50%的处理组中。总体而言，保留采伐相较皆伐作业能更好地保留生物多样性并提升景观异质性，但不同处理的响应存在明显梯度，基于本研究数据，无法推荐适用于保护工作的保留率阈值。此外，研究结果表明，保留采伐方案应根据森林覆盖类型进行调整。本研究发现，低保留率策略可缓解混交林演替早期阶段的阔叶林中的采伐影响，但在演替后期的针叶林中，仅高保留率才能降低采伐带来的影响。尽管蜘蛛类群在10年内的短期响应支持保留采伐的应用，但相较于传统采伐方式，要明确此类措施真正的保护价值，还需要更长时间尺度的评估，且后续研究应更关注生物多样性的恢复而非采伐后的短期留存。