B4WarmED forest warming experiment: Design and implementation of four years of concurrent above- and below-ground warming at the temperate-boreal ecotone

Background/Question/Methods

Boreal Forest Warming at an Ecotone in Danger (B4Warmed) is a manipulative experiment addressing the potential for projected climate warming to alter tree function, species composition, and ecosystem processes at the boreal-temperate forest ecotone. The experiment includes two forested sites, near Cloquet and Ely, Minnesota, with plots in both open and closed canopy habitats, into which seedlings of 11 tree species were planted. The experimental design is a 2 (site) x 2 (habitat) x 4 (treatment) factorial, with six replicates (total of 96, 7.1m² circular plots). Treatments include three target levels of simultaneous plant and soil warming (ambient, + 1.7 °C, + 3.4 °C) all of which included soil heating cables (dummy cables in ambient plots), plus an undisturbed ambient temperature treatment. Warming treatments approximate warming anticipated during the next 75-100 years and enable assessment of non-linear responses to warming. Warming represents a continuous (24h/d) elevation above ambient of above and below ground temperature for ≈ 8 months/year (i.e. when soils are thawed). We have completed four years of warming coupled with comprehensive measurements of plant and soil responses. Because complex warming manipulations such as this are rare, this presentation focuses on the warming treatment techniques and application.

Results/Conclusions

Warming is achieved in open-air (i.e. without chambers) via feedback control that acts concurrently and independently at the plot scale to maintain a fixed temperature differential from ambient conditions. The rationale for this design is that simultaneous warming above- and belowground provides a more realistic treatment than does either in isolation.

To date, we have achieved temperature elevation near our targets of +1.7 °C and +3.4 °C. Warmed season means across 2009-2012 field seasons were +1.88°C and +3.48°C for soil temperature at 10-cm depth. Aboveground means were +1.63°C and +3.27°C from infrared radiometer measurements of canopy surface temperature. Year to year variation in mean soil temperature elevations ranged from 1.74°C to 1.99°C and 3.00°C to 3.65°C in the two warming treatments, and variation in mean aboveground temperatures increases ranged from 1.35°C to 1.99°C and 2.85°C to 3.79°C.  Aboveground heating efficacy in open canopy areas was reduced over time as canopy complexity created by growing vegetation has required additional energy to heat. Aboveground and belowground measurement data show the established warming treatments emulate observed diurnal, seasonal and annual patterns in temperature. Aboveground treatment stability is greater during the nighttime than daytime and in closed versus open canopy sites.