Level and spatial pattern of overstory retention impose tradeoffs for regenerating and retained trees

Variable retention (VR) has been adopted globally as an alternative to more intensive forms of regeneration harvest. By retaining live trees within harvest units, VR seeks balance among the commodity, ecological, and aesthetic values of managed forests. Achieving these multiple, often competing objectives requires an understanding of how level and spatial pattern of retention shape the abundance, growth, and mortality of regenerating and retained trees. Using long-term (18-19 yr) data from a regional-scale VR experiment, we explore the individual and interactive effects of retention level (15% vs. 40% of initial basal area) and pattern (dispersed vs. aggregated) on the post-harvest dynamics of forests of differing structure and seral composition. Level and pattern of retention imposed tradeoffs for the density and growth of regenerating trees (>0.1 m tall, <5 cm dbh) and ingrowth (trees attaining 5 cm during the study). Greater retention led to greater density of late-seral regeneration, but lower density of early-seral ingrowth, and slower growth of late-seral ingrowth. Dispersed retention enhanced the density of early- and late-seral regeneration (compared to aggregated treatments), but reduced the growth of early-seral ingrowth. We also observed tradeoffs for retained trees. Lower retention enhanced the growth of smaller trees (<25 cm dbh)—particularly in dispersed settings—but reduced the survival of larger trees, which were more susceptible to windthrow. Greater retention reduced the growth, but enhanced the survival of smaller trees. Pattern imposed similar tradeoffs, with dispersed retention enhancing growth, but reducing survival of small trees. Finally, level and pattern resulted in tradeoffs for productivity of regenerating vs. retained-tree cohorts. Ingrowth productivity was greater at lower retention and in aggregated treatments; retained-tree productivity was greater at higher retention and in dispersed treatments. Our results provide a unique, long-term perspective on the sensitivity of tree regeneration, growth, and mortality to key structural elements of VR systems. Strong responses to level and pattern of retention produce tradeoffs for different ecological or resource objectives. Balancing these objectives may require the combined use of aggregates, dispersed retention, and clearings, to mimic the spatial heterogeneity of habitats, physical structures, and resource conditions that are produced by natural disturbances.可变保留采伐（Variable retention，VR）已在全球范围内被采纳，作为高强度更新采伐的替代方案。通过在采伐单元内保留活立木，VR旨在实现经营森林的商品价值、生态价值与美学价值之间的平衡。实现这些往往相互冲突的多重目标，需要理解保留水平与空间格局如何塑造更新林木与保留木的丰度、生长及死亡率。本研究依托一项区域尺度VR试验的18至19年长期定位数据，探讨了保留水平（初始断面积的15%与40%）与空间格局（分散式与聚集式）对不同结构与演替组成森林的采伐后动态的单独及交互影响。保留水平与空间格局对更新林木（高度>0.1m、胸径<5cm DBH）以及进界木（ingrowth，即研究期间胸径达到5cm的林木）的密度与生长形成了权衡。更高的保留水平可提升晚演替更新木的密度，但会降低早演替进界木的密度，并减缓晚演替进界木的生长。相较于聚集式处理，分散式保留可提升早、晚演替更新木的密度，但会降低早演替进界木的生长速率。研究同时观测到保留木存在相关权衡：较低的保留水平可促进较小胸径林木（<25cm DBH）的生长——尤其是在分散式配置场景下——但会提升大胸径林木的死亡率，这类林木更易遭受风倒危害。更高的保留水平会降低较小胸径林木的生长速率，但提升其存活率。空间格局同样带来了类似权衡：分散式保留可促进生长，但会降低小胸径林木的存活率。最后，保留水平与空间格局在更新林木群与保留木群的生产力之间形成了权衡。进界木生产力在较低保留水平与聚集式处理下更高；保留木生产力则在较高保留水平与分散式处理下更高。本研究结果为林木更新、生长与死亡率对VR系统关键结构要素的敏感性提供了独特的长期研究视角。对保留水平与空间格局的强烈响应，为不同生态或资源目标带来了权衡关系。平衡这些目标或需结合使用聚集保留斑块、分散保留林木与清理空地，以模拟自然干扰所产生的生境、物理结构与资源条件的空间异质性。