Monday, August 6, 2007: 2:50 PM
Willow Glen II, San Jose Marriott
Coast live oak (Quercus agrifolia) forms tripartite symbiotic associations with arbuscular (AM) and ectomycorrhizal (EM) fungi. We selected two neighbouring oak stands with differential access to groundwater (mesic valley vs. xeric hill) to investigate inter-annual changes in tree mycorrhizal status. We found higher EM and lower AM colonization at the valley than at the hill site in two consecutive years with contrasting rainfall. Oak mycorrhizal status was very sensitive to inter-annual rainfall variability, but also remarkably resilient against extreme drought. Differences in % EM root colonization, EM diversity, and density of viable hyphae between a wet and a dry year were much greater at the hill stand than at the valley stand. Oaks at the hill stand showed EM colonization levels under 1% during drought, whereas oaks in the valley stand maintained significantly higher EM root colonization (19%). EM root infection and soil hyphal density and viability increased sharply at both stands during the ensuing wet year. Across sites and years, percent EM root colonization, EM diversity and soil hyphal density and viability were strongly positively correlated with soil moisture potential, but % AM root colonization was not. The mycorrhizal status of oaks exhibited high plasticity at the xeric hill site, where roots in upper soil layers shifted from almost exclusively AM during severe drought to predominantly EM during the ensuing wet year. By contrast, the mycorrhizal status of oaks in the mesic valley stand was less strongly coupled to current meteorological conditions, as they remained predominantly EM during both years. Taproot access to groundwater attenuated the adverse effects of extreme drought on tree EM status. Canopy shading and hydraulic lift by oaks in the valley stand prevented excessive desiccation of upper soil layers and contributed to maintain the integrity and viability of EM roots and rhizosphere hyphae during drought.
