Abstract

Belowground climate change responses remain a key unknown in the Earth system. Plant fine-root response is especially important to understand because fine roots respond quickly to environmen-tal change, are responsible for nutrient and water uptake, and influence carbon cycling. However, fine-root responses to climate change are poorly constrained, especially in northern peatlands, which contain up to two-thirds of the world's soil carbon. We present fine-root responses to warming between +2 degrees C and 9 degrees C above ambient conditions in a whole-ecosystem peatland experi-ment. Warming strongly increased fine-root growth by over an order of magnitude in the warmest treatment, with stronger re-sponses in shrubs than in trees or graminoids. In the first year of treatment, the control (+0 degrees C) shrub fine-root growth of 0.9 km m(-2) y(-1) increased linearly by 1.2 km m(-2) y(-1) (130%) for every degree increase in soil temperature. An extended below -ground growing season accounted for 20% of this dramatic in-crease. In the second growing season of treatment, the shrub warming response rate increased to 2.54 km m(-2) degrees C-1. Soil mois-ture was negatively correlated with fine-root growth, highlighting that drying of these typically water-saturated ecosystems can fuel a surprising burst in shrub belowground productivity, one possible mechanism explaining the "shrubification" of northern peatlands in response to global change. This previously unrecognized mech-anism sheds light on how peatland fine-root response to warming and drying could be strong and rapid, with consequences for the belowground growing season duration, microtopography, vegetation composition, and ultimately, carbon function of these globally relevant carbon sinks.