Even modest climate change may lead to major transitions in boreal forests

The sensitivity of forests to near-term warming and associated precipitation shifts remains uncertain(1-9). Herein, using a 5-year open-air experiment in southern boreal forest, we show divergent responses to modest climate alteration among juveniles of nine co-occurring North American tree species. Warming alone (+1.6 degrees C or +3.1 degrees C above ambient temperature) or combined with reduced rainfall increased the juvenile mortality of all species, especially boreal conifers. Species differed ingrowth responses to warming, ranging from enhanced growth in Acer rubrum and Acer saccharum to severe growth reductions in Abies balsamea, Picea glauca and Pinus strob us. Moreover, treatment-induced changes in both photosynthesis and growth help explain treatment-driven changes in survival. Treatments in which species experienced conditions warmer or drier than at their range margins resulted in the most adverse impacts on growth and survival. Species abundant in southern boreal forests had the largest reductions ingrowth and survival due to climate manipulations. By contrast, temperate species that experienced little mortality and substantial growth enhancement in response to warming are rare throughout southern boreal forest and unlikely to rapidly expand their density and distribution. Therefore, projected climate change will probably cause regeneration failure of currently dominant southern boreal species and, coupled with their slow replacement by temperate species, lead to tree regeneration shortfalls with potential adverse impacts on the health, diversity and ecosystem services of regional forests.

Automated summary

The sensitivity of forests to near-term warming and precipitation shifts remains uncertain. A 5-year open-air experiment in the southern boreal forest showed that different North American tree species had divergent responses to climate alteration. Warming and reduced rainfall increased the mortality of all species, especially boreal conifers. Different species had different growth responses to warming, with some experiencing enhanced growth while others had severe growth reductions. Treatment-induced changes in photosynthesis and growth explained the changes in survival. Species that experienced conditions warmer or drier than their range margins had the most adverse impacts on growth and survival. Climate change is likely to cause regeneration failure of currently dominant southern boreal species, leading to tree regeneration shortfalls. This will have potential adverse impacts on the health, diversity, and ecosystem services of regional forests.