The effect of drought and season on root life span in temperate arbuscular mycorrhizal and ectomycorrhizal tree species

1. Fine roots play a key role in carbon (C) and nutrient cycling, since fine root lifespan drives soil organic C input and, thus, N availability. The two principal fungal symbioses found in temperate forest trees, ectomycorrhizae (ECM) and arbuscular mycorrhizae (AM), induce different root morphological changes upon infection, but the consequences for root lifespan are not clear.
2. We explored differences in fine root lifespan between four AM and four ECM tree species using mini-rhizotrons in a factorial drought experiment in large mesocosms.
3. Median root lifespan of young AM and ECM trees differed fundamentally in its response to soil moisture and season of root birth: ECM root lifespan was reduced from 176 to 81 d in dry soil compared to moist soil, independent of season. By contrast, AM root lifespan was less responsive to drought, but decreased from 185 to 127 d when comparing roots produced early in the growing season vs. mid-season. In both mycorrhizal types, root lifespan was positively related to root diameter and negatively to the portion of lower-order roots.
4. Synthesis. While our results indicate morphological and architectural traits that predict root lifespan across tree species, they also indicate principal differences in the environmental response of root lifespan in young AM and ECM trees. This knowledge helps to improve global predictions of root lifespan.