More realistic warming by including plant feedbacks: A new field-tested control method for infrared heating

Abstract

Infrared heating is a technique that allows warming experiments to be conducted in open-air conditions. Several improvements have been made since the method was introduced two decades ago, but none of these succeeded in reconciling controllability with an adequate consideration of vegetation responses that can counteract or reinforce warming. In an earlier conceptual study, we proposed a new infrared heater control method based on energy balance calculations that offers both custom-adjustable target temperatures and unconstrained vegetation responses. Here, we refine that concept, and test the control method under field conditions.Although quantitatively assessing the performance of the new control system is not straightforward as no pre-set canopy temperatures are used (because the vegetation is allowed to influence these), the test results suggest that our system functioned as envisaged. We adopted a scenario of 2°C air warming and gradual drying. Warming of the canopy was +1.4°C in a first phase with plentiful water, but increased to +3.5°C at the end of the test when the soil water content approached the wilting point. In other words, the amount of water lost via evapotranspiration was higher when the vegetation could afford to transpire freely (high soil water availability), counteracting the warming, but lower when water became scarce, reinforcing the warming. Such a response would also occur naturally, while these vegetation-mediated differences would be erased by the classical, fixed ΔT control.The successful test under field conditions demonstrates that this alternative infrared heater control method is ready to be applied in both new and existing set-ups. The advantages over traditional control methods would especially benefit studies that combine warming with precipitation manipulation by explicitly taking the interdependence between the water and the energy balance into account.