Peatland shrub roots increase resource acquisition with warming

Biogeochemical cycling and properties of ecosystems depend, in part, on the assimilation of carbon and uptake of mineral nutrients and water by plants. Belowground, these processes can be studied with the root economic spectrum (RES) which describes plant resource acquisition strategies along a spectrum of fast (e.g., longer and thinner roots) to slow acquisition (e.g., shorter and thicker roots). However, plant-mediated controls on ecosystem functions have often been studied aboveground, leaving the belowground processes largely understudied. This is especially true for water-saturated wetlands, such as peatlands, where drivers of root resource acquisition strategies may be very different than in upland soils. In order to reliably predict peatland plant responses to environmental change and the subsequent shifts in biogeochemical cycles, we need a better understanding of plant fine root trait plasticity under climate warming. 

 

We investigated RES trait-environment linkages at a peatland whole-ecosystem climate change experiment (SPRUCE, Minnesota, USA). We collected shrub and tree (spruce and larch) fine root samples from root ingrowth cores installed in enclosures (n=10) with warming (+0 ℃ to +9 ℃) and elevated CO₂ (ambient and +500 ppm above ambient) treatments, and plots without enclosure (n=2), over five years (2014-2017 and 2022-2023). We obtained root economic trait data, such as specific root length (SRL) and root tissue density (RTD), from the samples using a root scanning software, as well as root chemistry using an isotope-ratio mass spectrometer. In addition, we collected root exudation rate data from trees and shrubs in each enclosure in 2022. To estimate the contribution of increased soil temperature and elevated CO₂ on RES traits, we will build linear mixed effect models for each root trait (response variables) with soil temperature, soil moisture and elevated CO₂ treatment as fixed effects and year and microtopography (hummock and hollow) as random effects. 

 

Preliminary results suggest that shrubs respond to warming by shifting to a stronger resource acquisition strategy, as indicated by increasing SRL by soil temperature and slightly decreasing RTD. Tree SRL did not change along the warming and elevated CO₂ treatments, but RTD seems to decrease, particularly in the 9 ℃ warming treatment, in general indicating a slow resource acquisition strategy. We also found indications of an increasing nutrient-mining strategy with warming for shrubs, where increasing root exudation rates in higher soil temperature may lead to increasing plant-available N and increased root N uptake.