Data and code from: 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 seasonal post-fire climate conditions influence western larch (Larix occidentalis) regeneration across its range in the northwestern US. We destructively sampled 1651 seedlings from 57 sites within 32 fires that burned at moderate or high severity between 2000 and 2015, and which 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 wildfire, at sites within 25 m of reproductively mature western larch. Wildfire-related factors had the highest relative influence on post-fire recruitment (87%), whereas post-fire seasonal climate had less influence (13%). Annual recruitment probability increased with growing season actual evapotranspiration, to a maximum 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 decreasing modestly at ‘warmer-and-drier’ sites. The strong influence of wildfire-related factors highlights the potential for management decisions to promote western larch. For instance, facilitating prescribed or managed wildfire with moderate- to high-severity patches will generate conditions suitable for natural regeneration, provided sufficient seed sources. Additionally, our findings support monitoring for natural regeneration or directing outcomes by planting within the first five years after fire, consistent with current management practices.