Little recovery of the residual stand after mountain pine beetle disturbance in old stands in the northern Rocky Mountains, Alberta, Canada.

In early 2000s, long-distance wind dispersal of mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) resulted in massive outbreaks in the northern Rocky Mountains, Alberta, Canada, outside of the beetle’s natural range. We analyzed data from permanent plots measured for up to 15 years after MPB outbreaks in fire-origin, mature lodgepole pine (Pinus contorta var. latifolia Engelm.)-dominated stands scattered over an area of ∼ 75,000 km². We evaluated stand level stocking, regeneration, and ingrowth of three species groups (pine, shade-tolerant conifers, and broadleaves) in stands where > 50 % of pine basal area was killed. Using multiple linear regression analysis, we examined the relationship between stand and climatic covariates and basal area growth, density of regeneration, and ingrowth into the sapling size class at 9–15 years post-outbreak. Results showed that total live basal area for all species combined in trees with height ≥ 1.3 m occupies on average < 50 % of the pre-MPB levels and is unchanged between both post-MPB measurements at 19.5 m² ha⁻¹. Ongoing pine mortality led to negative net growth, which exceeded the positive net growth in other conifers and resulted in zero net change for the whole stand. Post-MPB ingrowth and regeneration rates were close to zero for pine but increased slightly in other conifers and broadleaf species. Regression analysis revealed a negative relation between post-MPB growth and quadratic mean diameter of the reconstructed pre-MPB total basal area, while initial basal area and composition post-MPB varied among species. Our findings support the hypothesis that, post-MPB, mature pine dominated stands stagnate due to ongoing pine mortality, the maturity of remnant overstory, and a lack of adequate understory and regeneration that can accumulate sufficient growth at short-to mid-term post-outbreak. Our conclusions contrast other research, suggesting that outbreaks leave degraded residual stands with declining pine overstory and that transition to vigorous productive mixed stands is impossible in the absence of stand-replacing disturbance or silvicultural investment.