Fine-scale spatial heterogeneity shapes compensatory responses of a subalpine forest to severe bark beetle outbreak

Context Growth releases of individuals that survive disturbances are important compensatory response mechanisms that contribute to ecological resilience. However, the role of fine-scale spatial heterogeneity in shaping compensatory growth responses is poorly understood for many broad-scale disturbances. Objectives We quantified how fine-scale spatial structure affects individual and aggregate tree growth leading up to and following a severe mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak. We asked: (1) How does individual tree growth vary with tree- and neighborhood-scale characteristics? (2) How do within-stand aggregate growth and overstory recruitment vary with neighborhood-scale characteristics? Methods We used a spatially explicit long-term monitoring dataset of a subalpine lodgepole pine (Pinus contorta var. latifolia) forest (in Colorado, USA) in which every tree >= 5 cm diameter was measured and mapped prior to (1989, 2004) and following (2018) a severe MPB outbreak (2003-2011). We used spatial regression to characterize drivers of growth. Results Overall, we found strong evidence for post-outbreak compensatory responses across spatial scales. Neighborhood characteristics shaped both individual and aggregate growth, with the magnitude of growth strongly mediated by pre-outbreak neighborhood structure and neighborhood mortality. Variation in tree-scale growth, combined with the spatial arrangement of surviving trees, resulted in highly variable emergent patterns of aggregate growth and recruitment. Conclusion Our findings highlight the importance of fine-scale landscape configuration in shaping forest resilience. Quantifying compensatory responses in a spatially explicit framework at different scales is critical for modeling post-disturbance forest dynamics, which is increasingly important as climate warms and forest disturbance regimes change.

Automated summary

This study quantified the impact of fine-scale spatial structure on individual and aggregate tree growth. The findings highlight the importance of fine-scale landscape configuration in shaping forest resilience.