Background

Wildfires are burning more area across the western US than they have in the recent past, driving an increased need for post-fire reforestation. At the same time, trees are struggling to keep up with climate change and post-fire tree planting may present an opportunity to plant seedlings from seed sources adapted to changing conditions. To better understand how reforestation outcomes for an ecologically and economically important conifer species in the northwestern US vary with seed source and site conditions, we conducted a post-fire western larch (Larix occidentalis) planting experiment in western Montana. We ask (1) what biophysical factors influence post-fire western larch planted seedling mortality and growth? and (2) does planting seedlings from a lower-elevation seed source decrease mortality at the driest and warmest sites and/or increase mortality at the coldest sites?

Results

We planted western larch seedlings from a low-elevation and a high-elevation seed source across an elevation gradient and monitored seedling mortality and growth for 3 years. Overall seedling mortality 3 years post-planting was similar between seed sources with higher and more variable mortality in the low-elevation plots (ranging from 15 to 94% depending on seed source and plot combination; mean (sd) 60(28)%) and lower mortality in the mid-elevation and high-elevation plots (2–19%; mean (sd) 9(5.6)%). Planting seedlings from a lower-elevation seed source did not increase short-term mortality at the highest-elevation plots. Seedling mortality and growth were strongly related to site conditions, with higher mortality and lower growth associated with more southerly aspects, higher maximum temperatures, and lower soil moisture. Seedlings from the high-elevation seed source showed a stronger negative response to high maximum temperatures than seedlings from the low-elevation seed source for both mortality and growth.

Conclusions

Biophysical conditions at the planting site were the stronger drivers of seedling mortality, with seed source acting as a secondary control. Interactive effects between microclimate and seed source suggest that seedlings from the low-elevation seed source may perform better under warming conditions. For western larch reforestation, site selection may have more impact on short-term survival than planting different seed sources, although we find some evidence that seed source may mediate responses to warming. Longer-term outcomes will require additional monitoring.