Colder winter minimum temperatures did not decrease the next-year growth of mountain pine beetle tree-mortality area in four western US regions, 1999-2025

Question: do cold winter extremes decrease the next-year growth of mountain pine beetle (Dendroctonus ponderosae) tree-mortality area when survey effort, outbreak size and host depletion are controlled? Approach: we wrote the plan before we opened the data. We gridded USFS Insect and Disease Detection Survey maps (1997-2025; Regions 1, 2, 4 and 6) onto 0.5-degree cells and used only cells with at least 25% survey-footprint cover in both years. We compared log growth of mortality area from survey year t to t+1 with the coldest daily minimum (CPC Tmin) of the brood winter, with cell and region-by-year fixed effects, outbreak size, the previous five years of mortality, and CRU summer rain and heat. Intervals come from a bootstrap over survey years. Result: in 3,986 cell-year pairs (378 region-cells, 26 year pairs), a 10 C colder winter minimum changed log growth by +0.007 (95% interval -0.283 to +0.208). This excludes a halving of growth. The coefficient was negative in only 2 of 4 regions and in 1 of 2 periods. A lead-winter placebo (+0.030) was as large as the test winter, and a threshold model (minimum at or below -30 C) gave +0.039 (-0.309 to +0.329). Spruce budworm, a negative control, also gave no clear effect. The model detected density dependence and host depletion. A secondary term for October-November cold was negative (-0.45; -0.90 to -0.01), but it was positive in one region and its placebo was also negative, so it is only a hypothesis. Limitations: gridded air temperature is not phloem temperature; aerial survey methods changed; the test covers year-to-year variation, not the long-term release of outbreaks into formerly cold places. Code, plan, derived grids, results and figures are bundled; CRU data stay external.