Climatic controls of decomposition drive the global biogeography of forest-tree symbioses, Steidinger et al. 2019

This article describes three major types of microbial tree symbionts, why they matter, and maps their global distribution.

Microbial symbionts strongly influence the functioning of forest ecosystems. Root-associated microorganisms exploit inorganic, organic and/or atmospheric forms of nutrients that enable plant growth, determine how trees respond to increased concentrations of CO2, regulate the respiratory activity of soil microorganisms and affect plant species diversity by altering the strength of conspecific^[5] negative density dependence [Steidinger 2019: 404].

Arbuscular mycorrhizal and ectomycorrhizal fungi and nitrogen-fixing bacteria are the focus of the study.

Plants that are involved in arbuscular mycorrhizal symbiosis comprise nearly 80% of all terrestrial plant species; these plants principally rely on arbuscular mycorrhizal fungi for enhancing mineral phosphorus uptake. In contrast to arbuscular mycorrhizal fungi, ectomycorrhizal fungi evolved from multiple lineages of saprotrophic^[6] ancestors and, as a result, some ectomycorrhizal fungi are capable of directly mobilizing organic sources of soil nutrients (particularly nitrogen). Associations with ectomycorrhizal fungi—but not arbuscular mycorrhizal fungi—have previously been shown to enable trees to accelerate photosynthesis in response to increased concentrations of atmospheric CO2 when soil nitrogen is limiting, and to inhibit soil respiration by decomposer microorganisms. Because increased plant photosynthesis and decreased soil respiration both reduce atmospheric CO2 concentrations, the ectomycorrhizal symbiosis is associated with buffering the Earth’s climate against anthropogenic change.

In contrast to mycorrhizal fungi, which extract nutrients from the soil, symbiotic N-fixers (Rhizobia and Actinobacteria) convert atmospheric N2 to plant-usable forms. Symbiotic N-fixers are responsible for a large fraction of biological soil-nitrogen inputs, which can increase nitrogen availability in forests in which N-fixers are locally abundant [Steidinger 2019: 404].

The study finds that climatic controls on litter breakdown determine fungi type in a given region, where colder climates favor ectomycorrhizal fungi, which are more efficient at extracting nutrients from organic material, and warmer climates favor arbuscular mycorrhizal fungi, which efficiently extract phosphorus from the soil. Warmer climates also favor nitrogen-fixing bacterial symbionts. Based on symbiosis distribution vis-a-vis existing spatial climate gradients, the authors predict changes in forest symbiosis distribution as the climate changes overtime.

To illustrate the sensitivity of global patterns of tree symbiosis to climate change, we use the relationships that we observed for current climates to project potential changes in the symbiotic status of forests in the future. Relative to our global predictions that use the most-recent climate data, model predictions that use the projected climates for 2070 suggest that the abundance of ectomycorrhizal trees will decline by as much as 10%… Our models predict that the largest declines in ectomycorrhizal abundance will occur

along the boreal–temperate ecotone, where small increases in climatic decomposition coefficients cause abrupt transitions to arbuscular mycorrhizal forests [Steidinger 2019: 407].

The authors explain that existing transitions between arbuscular and ectomycorrhizal forests are abrupt due to positive feedbacks maintaining these systems. For instance, the chemical composition of the leaves of trees forming ectomycorrhizal symbioses resists decomposition, meaning that their leaf litter reinforces the presence of ectomycorrhiza in cooler regions. Once a small temperature threshold is breached, however, climate controls on decomposition speed up litter breakdown and favor arbuscular mycorrhizal fungi, along with their tree hosts.