Collaboration or conservation? Exudation of carbon and root economic strategies of a riparian tree species (Populus fremontii)

Background/Question/Methods

Roots release carbon (C) into forest soils, a critical pathway mediating C sequestration. Most tree roots form mycorrhizas, which may play a key role in root C economic tradeoffs as collaborators that control exudation of C from roots to soil. However, often trees are of a single ‘mycorrhizal type’, namely ecto- (EM) or arbuscular mycorrhizal (AM), making separating the effects of host species identity from mycorrhizal type on root exudation difficult. Here, we quantified C exudation from roots of Populus fremontii, a dual-associating host, grown in two common gardens established in southwestern USA. The gardens have pronounced differences in average annual temperatures, watering regimes and growing season length. Our study had three main objectives: 1) Understand differences in EM and AM colonization between select P. fremontii populations and environments, 2) Determine whether root C exudation differs between EM and AM dominated trees and how this process interacts with the environment, and 3) Position C exudation onto two well-established gradients of root traits, namely the collaboration and conservation gradients. To address this latter objective, we measured morphological traits to understand relationships among root chemical and structural characteristics and mycorrhizal associations.

Results/Conclusions

We found that Populus fremontii associated with both AM and EM fungi, but the extent of AM versus EM colonization differed by garden and source provenance. Overall exudation rates were low (below 0.24 ug C cm-1 day-1 across the two gardens) and were not strongly associated with colonization levels of AM or EM fungi. We detected subtle patterns in this species’ root traits as a function of C economics, and that these patterns changed between gardens. Exudation’s position on the conservation gradient was inconsistent between the gardens. These results suggest that cottonwoods may deploy multiple strategies for acquiring resources depending on the environment. Taken together, three main conclusions emerge from our study: 1) Populus fremontii loses relatively small amounts of C from roots and mycorrhizal type has a weak influence on exudation, 2) the link between exudation and the conservation gradient in root traits varies with abiotic conditions demonstrating this species has flexibility in relying on different root traits to obtain resources, and 3) host identity may have a larger effect than mycorrhizal type on root C exudation.