Ectomycorrhizal abundance and community composition shifts with drought: predictions from tree rings

Mycorrhizae play a key role in ecosystem dynamics, and it is important to understand how environmental stress and climate change affect these symbionts. Several climate models predict that the intercontinental western United States will experience an increase in extreme precipitation events and warming temperatures. In 1996, northern Arizona, USA, experienced a 100-year drought that caused high local mortality of pinyon pine (Pinus edulis), a dominant tree of the southwest. We compared trunk growth, water potentials, and ectomycorrhizal dynamics for surviving trees at three high-mortality sites and adjacent low-mortality sites. Four major patterns emerged. First, surviving trees at sites that suffered high mortality exhibited reduc.ed long-term growth and increased water stress relative to adjacent sites where little or no mortality occurred. Second, surviving trees from high-mortality sites had 50% lower ectomycorrhizal colonization and showed a pronounced shift in fungal community composition relative to low-mortality sites. Third, in support of an intermediate-host plant stress hypothesis, trees that experienced intermediate levels of stress supported two-fold greater ectomycorrhizal colonization than trees at the high or low end of a stress gradient. Fourth, we observed a strong correlation between trunk growth and ectomycorrhizal colonization and validated the resulting regression model with independent data. This relationship suggests that tree rings can be used to reconstruct past and predict future ectomycorrhizal colonization. Overall, our findings suggest that predicted climate changes might be accompanied by both qualitative and quantitative changes in ectomycorrhizal dynamics that could affect ecosystems by altering nutrient cycling, carbon dynamics, and host-plant performance.