Structural outcomes of climate adaptation treatments in two contrasting northern temperate forests

Forest managers require climate adaptation strategies that are regionally relevant and translatable into planning processes. Adaptation frameworks, such as the resistance, resilience, transition framework, can guide the development of these strategies. However, there are limited examples of how these concepts can be operationalized with concomitant estimates of changes in forest structural complexity and diversity, which may support adaptive capacity. To address this knowledge gap, two operational-scale, replicated experiments were studied to understand how application of the resistance, resilience, transition framework influences stand structure in two contrasting northern forests: mixed pine and mesic hardwoods. We found that treatments corresponding to each adaptation approach (resistance, resilience, transition) manifested differently in each forest type. In mixed-pine, there were greater differences in structural diversity metrics among treatments; there were fewer differences among treatments in the mesic hardwood forest, which had comparatively greater pre-treatment structural variation. In both forests, our analyses demonstrate that management strategies associated with greater emphasis on change and adaptation (i.e., resilience and transition) created a quantifiably more heterogeneous arrangement of structural elements across treatment units, supporting shifts in stand-scale understory resource patterning. These results underscore the importance of accounting for the influence of pre-treatment stand conditions on outcomes of adaptation treatments and that stand-wide averaging of plot measurements may serve to obscure stand-scale diversity metrics. The variation in structural conditions across stands that we found is expected to contribute to forest response to novel disturbances by providing multiple recovery pathways. Based on these findings, application of adaptive silvicultural treatments can generate varying levels of structural heterogeneity at the stand-scale which in turn can confer adaptation potential; however, ecological memory strongly influences post-management stand conditions.