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

Abstract

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.
Conclusions
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.