Mycorrhizal-mediated nutrient foraging strategies of roots in temperate trees

Background/Question/Methods

Mycorrhizal fungi can strongly influence nutrient acquisition by plants. The extent that mycorrhizas benefit nutrient acquisition among tree species is likely related to their root morphology.Compared to tree species with fine roots, species with coarse absorptive roots are predicted to generate more mycorrhizal hyphae but less root length when foraging in nutrient-rich patches. Specifically, we hypothesized that across temperate tree species, mycorrhizal hyphae proliferation is positively correlated with diameter of absorptive roots, but root proliferation is negatively correlated with diameter when foraging in nutrient-rich patches. Thus, nutrient foraging by roots and mycorrhizal hyphae are complementary. To test this hypothesis, more than 1000 ingrowth containers were installed in June and harvested in November 2013 in a common garden located in central Pennsylvania. In half of the soilcores,we added leaf litter (organic treatment) or not (control treatment). Proliferation of roots and mycorrhizal hyphae of six arbuscular mycorrhizal (AM) tree species and seven ectomycorrhizal (EM) tree species were quantified by the ratio of organic to control patches in length of roots.  We also examined the influence of the organic treatment on root architecture (branching ratio).

Results/Conclusions

We found that proliferation of roots was strongly influenced by both root morphology and mycorrhizal status across the thirteen tree species. Across tree species, organic matter addition more commonly stimulated root proliferation in AM trees than in EM tree species. We also found root proliferation ratio (root length ratio in organic vs. control treatment) was negatively correlated with the average root diameter in EM tree species (r² = 67%; P = 0.024).  In AM tree species, the influence of root proliferation was apparently influenced both by root diameter and tree growth rate. Branching ratio (number of 2^nd order roots to number of 3^rd order roots) of EM tree species was also increased when proliferating in organic patches. These findings provide greater insight into variation in mycorrhizal-mediated nutrient foraging strategies of tree species with morphologically divergent absorptive roots.