Bird Conservancy of the Rockies, Fort Collins, Colorado, 80603, USA.
Rocky Mountain Region, U.S. Forest Service, Denver, Colorado, 80401, USA.
Ecol Appl. 2020 Sep;30(6):e02142. doi: 10.1002/eap.2142. Epub 2020 Jun 7.
Fire suppression has increased stand density and risk of severe, stand-replacing wildfire in lower elevation dry conifer forests of western North America, threatening ecological function. The U.S. Forest Service's Collaborative Forest Landscape Restoration Program (CFLRP) aims to mitigate impacts to ecological function, while mandating effectiveness monitoring to verify restoration success. Expected benefits include improved conditions for biodiversity, but relatively few empirical studies evaluate restoration effects on biodiversity. We applied the Integrated Monitoring in Bird Conservation Regions program to survey birds in relation to CFLRP treatments along the Colorado Front Range in 2015-2017. We employed hierarchical models to analyze species occupancy and richness at 1972 points nested within 141 1-km grid cells. Our objectives were to investigate (1) species occupancy relationships with treatments at local (point) and landscape (grid) spatial scales, (2) potential mechanisms for treatment relationships considering species and treatment relationships with forest structure and composition (i.e., habitat relationships), and (3) treatment and habitat relationships with species richness. The data supported positive and negative point-level treatment relationships, suggesting uneven species distributions between treated and untreated points. At the grid scale, however, we only found positive species relationships with percent area treated, and accordingly, grid-level species richness increased with treatment extent. Potential mechanisms for treatment relationships included treatments generating foraging opportunities for aerial insectivores by opening the canopy, improving conditions for ground-associated species by increasing herbaceous growth, and limiting opportunities for shrub-nesting species by reducing shrub cover. Landscape-scale patterns suggest CFLRP treatments can benefit avian communities by generating habitat for open-forest species without necessarily eliminating habitat for closed-forest species. Our results provide evidence for a commonly expected but rarely verified pattern of increased species richness with forest heterogeneity. We suggest restoration treatments will most benefit forest bird diversity by reducing canopy cover, encouraging herbaceous ground cover, limiting ladder fuel species, and encouraging shrub diversity in canopy openings, while maintaining some dense forest stands on the landscape.
火灾抑制措施增加了北美西部低海拔干燥针叶林的林分密度和发生严重、取代整个林分的野火的风险，从而威胁到生态功能。美国林务局的合作景观恢复计划（CFLRP）旨在减轻对生态功能的影响，同时要求进行有效性监测以验证恢复的成功。预期的效益包括改善生物多样性的条件，但相对较少的实证研究评估恢复对生物多样性的影响。我们应用鸟类保护区域综合监测计划（Integrated Monitoring in Bird Conservation Regions program），在 2015-2017 年期间，根据 CFLRP 处理措施，对科罗拉多前岭的鸟类进行了调查。我们采用层次模型，在 1972 个点内嵌套 141 个 1 公里格网单元，分析了物种占有和丰富度。我们的目标是调查（1）在局部（点）和景观（格网）空间尺度上，物种占有与处理措施的关系；（2）考虑物种和处理与森林结构和组成的关系（即生境关系）的处理关系的潜在机制；（3）处理和生境与物种丰富度的关系。这些数据支持了在点尺度上处理措施与物种占有之间的正相关和负相关关系，这表明在处理和未处理的点之间，物种的分布不均匀。然而，在格网尺度上，我们只发现了与处理面积百分比呈正相关的物种关系，因此，格网水平的物种丰富度随着处理范围的增加而增加。处理关系的潜在机制包括通过打开树冠为食虫鸟类创造觅食机会，通过增加草本植物的生长来改善与地面有关的物种的条件，以及通过减少灌木覆盖来限制灌木筑巢物种的机会。景观尺度上的模式表明，CFLRP 处理措施可以通过为开阔林物种生成栖息地来使鸟类群落受益，而不一定需要消除对封闭林物种的栖息地。我们的研究结果为一种通常预期但很少得到验证的模式提供了证据，即随着森林异质性的增加，物种丰富度也会增加。我们建议，通过减少树冠覆盖、鼓励草本地面覆盖、限制林下层物种、并在树冠空隙中鼓励灌木多样性，同时保持景观上的一些密集林分，使恢复处理措施最有益于森林鸟类的多样性，同时保留一些密集林分。
