Scale-dependent effects of alternative forest management on structural connectivity in Sweden

Maintaining forest connectivity is vital for supporting the movement of forest-dwelling species and for sustaining the multiple ecosystem services and benefits forests provide to humanity. Forest Area Density (FAD) is the required metric for monitoring forest connectivity under the European Union Nature Restoration Regulation (EU NRR). FAD is a scale-dependent structural index, meaning that assessments of connectivity can change depending on the spatial scale of observation. Promoting biodiversity conservation under the EU NRR may involve a reduction in clear-cutting (CC) and the adoption of alternative, uneven-aged management strategies. Here, we simulate clear-cutting, and three alternative management strategies (AM1-AM3) in two contrasting Swedish regions. The simulations are designed to reflect the aggregated nature of harvesting blocks in CC management, and the more dispersed nature of harvesting blocks in alternative strategies. We calculate FAD relative to baseline (ΔFAD) at 11 observation scales (10–1,000 ha) and model the effects of management strategy, observation scale, and region. We show that, at fine observation scales, dispersed harvesting strategies consistently produce more negative ΔFAD than aggregated clearcuts. This reverses at observation scales of approximately 570-620 ha, as dispersed strategies result in gradually less negative ΔFAD. This is a mathematical property of the FAD metric itself, independent of the region. Our findings reveal an important tension between FAD as a structural connectivity metric and the ecological expectations of uneven-aged forestry.维持森林连通性，是支撑林栖物种迁移活动、维系森林为人类提供的各类生态系统服务与福祉的关键前提。森林面积密度（Forest Area Density, FAD）是欧盟《自然恢复法规》（EU Nature Restoration Regulation, EU NRR）中用于监测森林连通性的法定指标。FAD属于尺度依赖型结构指数，即连通性评估结果会随观测空间尺度的差异发生变化。 落实欧盟《自然恢复法规》的生物多样性保护目标，往往需要削减皆伐（clear-cutting, CC）作业规模，并转而采用替代性异龄林经营策略。本研究针对两个特征迥异的瑞典区域，分别模拟了皆伐作业与三种替代性经营策略（AM1–AM3）的实施效果。模拟设计旨在体现皆伐经营中采伐斑块的聚集特性，以及替代性策略下采伐斑块的分散特性。 研究团队在11个观测尺度（10–1000公顷）下计算了相较于基线的FAD变化量（ΔFAD），并构建模型分析了经营策略、观测尺度与区域因素的影响效应。结果显示，在精细观测尺度下，分散式采伐策略产生的ΔFAD始终比聚集式皆伐作业更为负向。当观测尺度约为570–620公顷时，这一趋势发生反转：分散式采伐策略带来的ΔFAD负向程度逐渐减弱。该现象是FAD指标本身的数学属性，与研究区域无关。 本研究的发现揭示了作为结构连通性指标的FAD，与异龄林经营的生态学预期之间存在的重要张力。