Conservative Spruce Outperforms Acquisitive Larch Under Warming and CO2.

Predicting boreal forest growth is hindered by limited experimental tests on the compounding effects of climate change and atmospheric CO2. We integrated regional tree-ring data with a peatland warming and CO2 manipulation experiment, including 45 trait measurements, to assess growth responses of two widespread boreal conifers, Larix laricina and Picea mariana, to temperature, vapor pressure deficit (VPD), soil moisture, and elevated CO2. L. laricina growth was consistently more climate-sensitive than P. mariana, increasing strongly with soil moisture and declining slightly with VPD at the regional scale, but responding positively to VPD and temperature-not soil moisture or CO2-at the experimental peatland. L. laricina's acquisitive strategy enhanced water and nutrient uptake, sustaining growth but reducing hydraulic safety. By contrast, P. mariana's conservative strategy maintained water-use efficiency and hydraulic resilience without compromising growth. Our results challenge projections of widespread CO2 fertilization in boreal forests and highlight conservative conifers as potential winners under warming and drying, sustaining growth while maintaining low mortality risk. This dynamic could reshape projections of the boreal carbon sink and its role in global climate feedback.