Greater Shrub Root Production Under Warming and Elevated Carbon Dioxide Is Not Distributed More Deeply
Authors
Sören E. Weber* (weberse@ornl.gov), Joanne Childs, John Latimer, Colleen M. Iversen, Paul J. Hanson, Melanie Mayes, Daniel Ricciuto
Institutions
Oak Ridge National Laboratory, Oak Ridge, TN
URLs
Abstract
Peatlands store a disproportionate amount of terrestrial soil carbon (C) relative to their global spatial extent (roughly 44% vs. 3%). The accumulation and stability of this C is impacted by the amount, timing, and depth distribution of plant root production. These aspects of plant root growth are in part determined by plant functional type (PFT), which are likely to be differentially sensitive to increased atmospheric carbon dioxide (CO2) and temperatures. Common vascular PFTs in boreal bogs are: coniferous trees and ericaceous shrubs with shallowly, and non-woody sedges + forbs with deeply, distributed roots. The team estimated the effects of elevated CO2 and whole-ecosystem warming (WEW) on root production in the Spruce and Peatland Response Under Changing Environments (SPRUCE) experiment in an ombrotrophic boreal bog in northern Minnesota, U.S. To date shrub productivity has increased dramatically in response to warming, both aboveground and belowground. The team hypothesized this response was due to: (1) shrub rooting depth tracking depressed water tables; and (2) longer growing seasons for shrub roots. WEW treatments ranged from 0 to +9 °C, in 2.25 °C steps above ambient temperatures.
These WEW treatments are crossed by ambient and elevated (+500 ppm) [CO2] (n=10). Root production, timing and depth distribution were estimated from minirhizotron images collected throughout the year from 2015 to 2021. Shrub root production increased with WEW, and this response was stronger with elevated CO2. Growing season length for shrub roots did not respond consistently to WEW nor elevated CO2. Rather, on average, shrubs with +6.75°C WEW (both ambient and elevated CO2) grew their roots later in the year (c. Oct. 1) compared to all other plots (c. Aug. 7). Shrub roots grew more shallowly, rather than more deeply, with WEW. Researchers attribute this response in part to the lack of water table depression with WEW, but also to the distribution of nutrients.