Design and performance of combined infrared canopy and belowground warming in the B4WarmED (Boreal Forest Warming at an Ecotone in Danger) experiment

Abstract

Conducting manipulative climate change experiments in complex vegetation is challenging, given considerable temporal and spatial heterogeneity. One specific challenge involves warming of both plants and soils to depth. We describe the design and performance of an open-air warming experiment called Boreal Forest Warming at an Ecotone in Danger (B4WarmED) that addresses the potential for projected climate warming to alter tree function, species composition, and ecosystem processes at the boreal-temperate ecotone. The experiment includes two forested sites in northern Minnesota, USA, with plots in both open (recently clear-cut) and closed canopy habitats, where seedlings of 11 tree species were planted into native ground vegetation. Treatments include three target levels of plant canopy and soil warming (ambient, +1.7°C, +3.4°C). Warming was achieved by independent feedback control of voltage input to aboveground infrared heaters and belowground buried resistance heating cables in each of 72-7.0 m(2) plots. The treatments emulated patterns of observed diurnal, seasonal, and annual temperatures but with superimposed warming. For the 2009 to 2011 field seasons, we achieved temperature elevations near our targets with growing season overall mean differences (∆Tbelow ) of +1.84°C and +3.66°C at 10 cm soil depth and (∆T(above) ) of +1.82°C and +3.45°C for the plant canopies. We also achieved measured soil warming to at least 1 m depth. Aboveground treatment stability and control were better during nighttime than daytime and in closed vs. open canopy sites in part due to calmer conditions. Heating efficacy in open canopy areas was reduced with increasing canopy complexity and size. Results of this study suggest the warming approach is scalable: it should work well in small-statured vegetation such as grasslands, desert, agricultural crops, and tree saplings (<5 m tall).

Identifying environmental drivers of greenhouse gas emissions under warming and reduced rainfall in boreal–temperate forests

“…A free‐air warming experiment was established at two field sites, in northern Minnesota, USA, in two different habitat conditions (closed canopy or open canopy) at each site. The B4WarmED experiment was established in 2008, with one site located in the Cloquet Forestry Center (CFC), in Cloquet (46°40′46″N, 92°31′12″W) and the second site located approximately 150 km further north, in the Hubachek Wilderness Research Center (HWRC), in Ely (47°56′46″N, 91°45′29″W) (Reich et al., ; Rich et al., ). Briefly, both sites were situated in 40–60‐year‐old mixed aspen‐birch‐fir forests scattered with pine, spruce and other species, representing the transition from temperate to boreal biomes.…”

Comment on “The whole-soil carbon flux in response to warming”

“…In an alpine meadow with clipping to simulate animal grazing under 4°C surface air warming, for instance, the magnitude of temperature elevation decreased by 88% and 77%, respectively, at 60-and 100-cm soil depths (5). In a boreal forest under 3.4°C soil warming at a depth of 10 cm, the magnitude of temperature elevation declined by 40% and 53%, respectively, at 75-and 100-cm soil depths (6). These previous findings imply that Hicks Pries et al's experimental warming approach may cause higher temperature elevations in deeper soil layers (e.g., <50 cm) than expected under air warming, thus overestimating the whole-soil CO 2 production.…”

Precipitation, not air temperature, drives functional responses of trees in semi‐arid ecosystems

“…; Rich et al . ) limiting our current understanding on woody species responses to warming. Overall, it seems that rising temperatures caused by climate change could alter ecosystem functions if foliar temperature exceeds an optimum temperature for net carbon gain.…”