Ectomycorrhizal associations of northern red oak (Quercus rubra) seedlings along an environmental gradient

Inconsistencies in the relationship between abundance of ectomycorrhizae (ECM) and characteristics of their host plants abound in the published literature. To determine whether such inconsistencies are related to environmental conditions, we conducted a study of ECM of Quercus rubva L. seedlings along an environmental gradient in oak and pine forests in northern Lower Michigan. The gradient was created by retaining canopy cover at 0%, 25% (50% in first year), and 75%, as well as uncut controls. ECM development, seedling growth, and resource levels were analysed against the gradient of percent canopy cover. ECM development (percent ECM and total number of ECM tallied per seedling), seedling development (total biomass, stem diameter, shoot and root mass), and resources (photosynthetically active radiation (PAR), volumetric soil moisture (moisture), and nitrogen mineralization (N)) showed a quadratic, negative exponential, and negative linear relationship, respectively, with percent canopy cover. The best development of ECM was observed at 38% canopy cover, where N was 31-43 kg.ha(-1).season(-1), moisture was 12-18%, and photosynthetically active radiation was 45-56% of open sky conditions. Four general conceptual models of ECM relationships were hypothesized. "Fungal parasitism" defines the low end of the percent canopy cover gradient and "plant parasitism" defines the high end. "True symbiosis" exists somewhere between the extremes. These models provide a conceptual framework for understanding ECM relationships in the context of ecosystem dynamics and a practical guide for oak regeneration management in forestry.