Moving belowground with drought: Mycorrhizal association controls C allocation responses to water stress by temperate forest trees

Background/Question/Methods

The potential for global change to enhance drought frequency and severity threatens the ability of temperate forests to continue sequestering carbon (C). While aboveground responses to water stress are relatively well understood, the degree to which belowground responses sustain tree growth under water stress remains uncertain. Differences between trees in their mycorrhizal symbionts and belowground investment may be the key to reducing this uncertainty. Ectomycorrhizal (ECM) trees allocate more C to rhizosphere microbes to gain nutrients and tend to be drought tolerant. Opposingly, arbuscular mycorrhizal (AM) trees rely less on belowground C allocation to acquire nutrients and tend to be drought sensitive. As such, we hypothesized that ECM trees would have a greater ability to upregulate belowground C allocation in response to drought than AM trees, which enables ECM trees to capture water and nutrients necessary to sustain growth. To test this hypothesis, we used throughfall exclusion to induce water stress in stands dominated by ECM and AM trees in West Virginia over three growing seasons spanning 2017-2019. We measured aboveground net primary production, belowground root growth, and mycorrhizal colonization rates in both control and drought plots for each mycorrhizal type.

Results/Conclusions

In contrast to our hypotheses, both ECM and AM trees altered their belowground C allocation in response to water stress and the magnitude of these responses changed over time. ECM trees were found to continually grow aboveground with no difference belowground in response to drought. By contrast, AM trees rerouted C allocation from aboveground to increased belowground root growth under water stress. Moreover, AM tree roots grew deeper into the soil during the growing season in the third year of drought, potentially driven by water foraging lower in the soil profile. Mycorrhizal colonization rates for AM and ECM trees increased in the second year. However, in the third year, AM fungal colonization declined while ECM colonization remained elevated, suggesting that a legacy of water stress reduced the ability of AM trees to maintain their symbionts. Our results suggest drought significantly altered C allocation in temperate forest trees. ECM trees sustained aboveground growth and symbionts; whereas AM trees partitioned C to increased root growth. These results refine our understanding of whole tree responses to water stress and suggests that mycorrhizal association may be an important driver of forest ecosystem responses to water stress.