Mycorrhizal Root Foraging in Temperate Trees

Root foraging for nutrient “hot spots” is a key strategy by which some plants maximize nutrient gain from their carbon investment in roots and mycorrhizal hyphae. Foraging strategies may depend on costs of root construction, with thick roots generally costing more per unit length than thin roots. Moreover, we have found root thickness to be the most important predictor of root lifespan, with thick roots living considerably longer than thin roots. Thus the nutrient foraging strategy of a particular tree species may be strongly influenced by this “payback time”. Investment in mycorrhizal hyphae, which are considerably thinner than roots, may represent an alternative strategy for cost-effective nutrient foraging, especially for thick-root species. Type of mycorrhiza may matter, as ectomycorrhizal (EM) fungi are more associated with longer hyphae and ability to mineralize organic matter than arbuscular mycorrhizal (AM) fungi. Among AM trees, we found that thin-root tree species proliferated their roots in soil patches enriched with mineral nutrients to a greater extent than species with thick roots. In addition, thick-root species were consistently colonized more heavily with mycorrhizal fungi than thin root species, although nutrient addition tended to diminish colonization. In a common garden in central Pennsylvania of both AM and EM tree species, we found that nutrient patches enriched with organic materials resulted in greater root and mycorrhizal fungal proliferation compared to those enriched with inorganic nutrients and that thick-root species proliferated more with their mycorrhizal fungi whereas thin-root species proliferated more with their roots. Foraging precision, or the extent that roots or mycorrhizal hyphae grew in the enriched patch relative to the unenriched patch, was related to both root thickness and type of mycorrhiza. In both AM and EM trees, thick-root species were not selective foragers of either their roots or hyphae. In thin-root species, there was strong selectivity in foraging with AM trees showing high precision in root foraging and EM trees showing high precision in mycorrhizal hyphal foraging. Collectively, these results reveal some of the diverse nutrient foraging strategies of temperate trees.