Links between root morphology and root foraging in arbuscular mycorrhizal trees

Background/Question/Methods

Co-occurring arbuscular mycorrhizal (AM) trees in central Pennsylvania forests vary widely in root morphology. Species like Magnolia acuminata and Liriodendron tulipifera have considerably coarser 1^st-order roots (stream-based ordering system) than those of species like Ulmus americana and Acer rubrum. In this study we examined how variation in root diameter and specific root length of 1^st- and 2^nd-order roots among six co-occurring tree species was linked with root tissue density, root branching intensity, root proliferation in nutrient-rich patches and mycorrhizal colonization.  We hypothesized that trees with finer absorptive roots would more readily proliferate in nutrient-rich patches but would be more slowly colonized by AM fungi than trees with coarser absorptive roots.  The study area was an approximately 70-yr-old forest and trees were canopy dominants or co-dominants.  Roots of different species were isolated by tracing the woody root back to the trunk.  Roots were pruned and allowed to regrow in mesh bags either unfertilized or amended with slow-release fertilizer at a rate intended to increase nitrate and ammonium levels about four-fold over unamended levels. 

Results/Conclusions

Species of small root diameter that constructed their roots for high length per unit dry weight (SRL) had higher branching intensity (number of 1^st-order roots/cm of 2^nd order root, R² = 0.75), higher root proliferation rate in unfertilized soil (R² = 0.74) and lower mycorrhizal colonization (R² = 0.63).  Fertilization increased root proliferation rate but decreased mycorrhizal colonization more in species with high SRL than species of low SRL.  This study underscores the different ways co-occurring trees of differing root morphology may forage for limited nutrients.  We demonstrated with naturally established forest trees that coarse-rooted, low SRL species appear more obligate in regards to mycorrhizal colonization regardless of nutrient conditions and fine-rooted, high SRL species are more facultative with mycorrhizal fungi and more readily proliferate roots in nutrient-rich patches.