Western larch regeneration more sensitive to wildfire-related factors than seasonal climate variability

To understand the impacts of changing climate and wildfire activity on conifer forests, we studied how wildfire and post-fire seasonal climate conditions influence western larch (Larix occidentalis) regeneration across its range in the northwestern US. We destructively sampled 1651 seedlings from 57 sites across 32 fires that burned at moderate or high severity between 2000 and 2015; sites were within 100 m of reproductively mature western larch. Using dendrochronological methods, we estimated germination years of seedlings to calculate annual recruitment rates. We used boosted regression trees to model the annual probability of recruitment as a function of (i) ‘wildfire-related factors’ including distance to seed source, satellite-derived fire severity, and time since fire, and (ii) seasonal climate conditions, including variables reflecting temperature and water availability. Most recruitment occurred within five years after wildfires, at sites within 25 m of reproductively mature western larch trees. Wildfire-related factors had the highest relative influence (87 %), while post-fire seasonal climate had less influence (13 %) on post-fire recruitment. Annual recruitment probability increased with growing season actual evapotranspiration, to a maximum of c. 275 mm, and then decreased. Annual recruitment probability decreased as growing season climatic water deficit increased. Our results suggest that recent climate trends – increased growing season water deficit and decreased actual evapotranspiration – have had variable, yet net-neutral, impacts on the climate suitability for post-fire western larch regeneration across its range. Climate suitability increased modestly at ‘cooler-and-wetter’ sites and decreased modestly at ‘warmer-and-drier’ sites. The strong influence of wildfire-related factors highlights the potential for management decisions to promote western larch in recently burned areas. Facilitating prescribed or managed wildfire with moderate- to high-severity patches will generate conditions suitable for natural regeneration, provided sufficient seed sources survive the fire. Additionally, our findings support monitoring of natural regeneration or augmenting regeneration by planting within the first five years after fire, consistent with current management practices.