Differences in depth to groundwater modulates the mycorrhizal responses of oak trees to interannual rainfall variability

Background/Question/Methods

Soil moisture availability is a major driver of mycorrhizal root colonization.  Studies have shown that ectomycorrhizal (EMF) colonization of plant roots can be negatively affected by drought, whereas arbuscular mycorrhizal fungi (AMF) may be capable of enduring more negative soil water potentials.  What is less well known, however, is how plants that host both EMF and AMF might respond to changes in soil moisture. Of particular importance are the responses of mycorrhizal roots within the nutrient-rich topsoil layers, in which temporal and spatial changes in moisture content are often most dramatic.  In this study, we used Quercus agrifolia (coast live oak) individuals differing in topographic position to evaluate the effects of groundwater table depth, i.e., shallow (4-5m) in mesic valley bottom locations versus deep in xeric hill sites, on mycorrhizal status (root colonization, hyphal abundance, community structure).  We also examined the inter-annual variations in root colonization by AMF and EMF to elucidate whether mycorrhizal status in Q. agrifolia is temporally stable or sensitive to changes in rainfall and soil moisture. 

Results/Conclusions

Both interannual rainfall variability and depth to soil moisture were major factors influencing the mycorrhizal status of coast live oaks. Across sites and years, EMF root colonization and soil hyphal density and viability were strongly positively correlated with soil moisture potential, but AMF root colonization was not.  Most interestingly, the mycorrhizal status of oak trees was particularly responsive to changes in soil moisture at the hill sites, where roots in upper rhizosphere soil shifted from almost exclusively AMF during severe drought to predominantly EMF during the ensuing wet year. In contrast, the mycorrhizal status of oaks in the valley sites was less strongly coupled to current meteorological conditions, as roots in upper soil layers remained predominantly EMF during both a dry and a wet year. Canopy shading and hydraulic lift by oaks in valley sites likely contributed to maintaining the integrity and viability of EMF roots and extraradical hyphae in the upper rhizosphere soil.  These findings suggest a degree of temporal instability in oak mycorrhizas in hill soils (deep water table) that is imposed by sensitivity to rainfall events, and that coast live oak woodlands may become increasingly reliant on AMF under future climate change scenarios.