B4WarmED forest warming experiment: Effects of warming on seedling growth of co-ocurring temperate and boreal species

Background/Question/Methods

Uncertainty remains about biotic responses to climate change. The Boreal Forest Warming at an Ecotone in Danger (B4WarmED) experiment addresses the potential for projected climate warming to alter many aspects of forest function at the boreal-temperate forest ecotone; herein we focus on potential impacts on tree species composition through effects on juvenile growth and survival. Our overarching hypothesis is that warming will enhance the growth and survival of seedlings of temperate species at the cold edge of their range but reduce growth and survival of seedlings of boreal species at the warm edge of their range. To test this hypothesis, 96 7.1m² plots with above- (via infrared lamps) and below-ground (via buried cables) warming were established in open and closed canopy conditions at  two sites in northern Minnesota with three target  levels (ambient, + 1.7 °C, + 3.4 °C) and an undisturbed ambient treatment. Across all plots we planted ≈11,000 seedlings of 10 temperate and boreal tree species and an important woody invader. Here we present data on tree seedling growth over the first three years (2009-2011) of experimental warming.  This includes total leader height, leader growth and stem diameter annually, plus seasonal phenology of leader growth

Results/Conclusions

There was high survivorship of planted seedlings across all treatments. We found significant effects of warming on height and diameter growth that differed among species. Linear declines in growth with warming were detected in two boreal conifers, Abies balsmea and  Picea glauca, and the temperate conifer Pinus strobus. Three other boreal species ,Populus tremuloides, Pinus banksiana and Betula papyrifera, showed modest change with warming with greatest growth at +1.7°C.

 In contrast, temperate deciduous species, Acer rubrum, Acer saccharum, Quercus rubra and Quercus macrocarpa, and the deciduous invader Rhamnus cathartica, increased growth with +1.7°C and also further (but non-linearly) increased growth at +3.4°C. Species differed in the direction and magnitude of growth response, even though all had a longer growing season in warmed plots.  We found that despite earlier leafing, boreal conifers ceased shoot extension earlier in warmed plots, perhaps one reason for lower height growth in these species. Our results provide general support for our overarching hypothesis but also highlight the individualistic nature of species responses to warming.