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 life span drives soil organic C input and, thus, nitrogen (N) availability. The two main 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 life span are not clear. 2. We explored differences in fine root life span between four AM and four ECM tree species using mini-rhizotrons in a factorial drought experiment in large mesocosms. 3. Median root life span of young AM and ECM trees differed fundamentally in its response to soil moisture and season of root birth; ECM root life span was reduced from 176 to 81 days in dry soil compared to moist soil, independent of season. By contrast, AM root life span was less responsive to drought, but decreased from 185 to 127 days when comparing roots produced early in the growing season versus mid-season. In both mycorrhizal types, root life span 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 life span across tree species, they also indicate fundamental differences in the environmental response of root life span in young AM and ECM trees. This knowledge helps to improve global predictions of root life span.