Tree growth response to growing season frost and drought is shaped by species traits and competition in temperate-boreal transition zone forests

Understanding how species-specific traits and density-dependent competition modulate the effects of extreme climate events is essential for informing future forest management strategies. We analyzed the radial growth of western larch (Larix occidentalis) and Douglas-fir (Pseudotsuga menziesii var. glauca) trees growing in unmanaged old-growth stands — and western larch grown at two experimentally controlled densities — at four sites across western Montana, USA. Differences in leaf habit (deciduous vs. evergreen) and divergent water use strategies (anisohydric vs. isohydric) of western larch and Douglas-fir provided a model for linking growth–climate relationships to functional traits. We evaluated resistance, resilience, and recovery of basal area growth to drought and growing season frosts between (a) species and (b) experimental stand densities. Western larch growth exhibited a negative relationship with previous autumn frost (October freezing degree days; FDD), while Douglas-fir showed no response. Radial growth response to spring frost (May-June FDD) was negative for both species, with western larch exhibiting greater sensitivity. Both species showed reduced growth in years following drought conditions (previous July-August Palmer Drought Severity Index; PDSI), while only Douglas-fir was sensitive to contemporaneous July-August PDSI. Growth of larch trees in low-density stands was more resistant and resilient to previous summer drought and spring frost than that of trees in high-density stands. Furthermore, observations of greater soil volumetric water content in low-density stands provide a mechanistic link for drought responses. However, growth response to autumn frost was density independent. Our results suggest that early thinning has the potential to limit the negative impacts of summer drought and spring frost in sensitive species, and that species-specific physiological traits can drive divergent growth responses to extreme climate events.