B4WarmED forest warming experiment: Species geographic distributions predict photosynthetic responses of local ecotypes to climate warming

Background/Question/Methods

Given rapid climate warming it is critical to assess how local ecotypes of important species will respond, and whether the broader distributions of species indicate anything about the capacity of local ecotypes to do so.  One example involves photosynthetic carbon gain in boreal forest species, which we tested in B4WarmED (Boreal Forest Warming at an Ecotone in Danger), a manipulative open-air warming experiment in open and closed canopy habitats at two sites in northeastern Minnesota. The experiment includes 72 7.1 m² plots and uses infrared heat lamps and soil heating cables to heat continuously during the growing season. Our goal was to compare responses of photosynthesis to in situ warming for local ecotypes of 10 temperate and boreal tree species (and one woody invader) grown under two temperature regimes (ambient and warmed +3.4 °C both belowground and aboveground). We aimed to determine whether local ecotypes of all species respond equally to warming or whether boreal species with more northerly distributions respond more negatively than temperate species. Light-saturated photosynthetic rates in morning or early afternoon were measured in 2009, 2010, and 2011 in situ from a subsample of the 11,600 seedlings planted in 2008.

Results/Conclusions

A total of >4,400 measurements were across species, treatments, sites, and time. Over three growing seasons, warming by 3.4 °C decreased photosynthesis of the common boreal species but increased photosynthesis in locally uncommon temperate species. The six southern boreal species (spruce, fir, pines, birch, and aspen) performed poorly with warming (-15% on average), while the temperate maples and oaks, and the exotic buckthorn – showed enhanced photosynthesis (by +15 % on average). Response to warming was more positive for species with more southerly abundance (≈200 km scale) and/or range limits. Despite large differences in distributions, all 11 species co-occur in northern Minnesota; thus in theory if fully adapted locally, local ecotypes of each should have responded similarly to warming. They did not - as the temperate species performed better with warming than the boreal species- hence key aspects of their biology are reflected in their geographic distributions. Such evidence is consistent with the notion that in the southern boreal forest, warming will degrade the competitive ability of currently dominant boreal species, especially when compared to those presently rare co-occurring species that are common in warmer neighboring regions, and suggests that broader geographic distributions do indeed predict differential response to climate warming.