Root morphology and root foraging in temperate 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. We examined how variation in root diameter, branching intensity and specific root length of 1^st- and 2^nd-order roots  was linked with root proliferation, root longevity and mycorrhizal colonization in nutrient-rich patches.  We hypothesized that trees with finer absorptive roots would more readily proliferate in nutrient-rich patches and would more readily extend the lifespan of those roots but would be more slowly colonized by AM fungi than trees with coarser absorptive roots. For the proliferation/mycorrhizal study, we examined roots of trees that were canopy dominants or co-dominants in an approximately 70-yr-old forest.  For the lifespan study, we used a 15-yr-old common garden.  In the forest, 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.  In the common garden, the fates of roots were tracked with minirhizotrons in single-species plots.

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 tended to increased root proliferation rate more in species with small diameter roots. Mycorrhizal colonization was generally unaffected by fertilization in species with coarse roots but strongly diminished in species with coarse roots.  Lifespan was extended in fine-root species but unaffected in coarse-root species.  This study underscores the different ways co-occurring trees of differing root morphology may forage for limited nutrients.  Fine-root species are clearly more plastic in nutrient foraging, as indicated by greater proliferation, greater shifts in lifespan, and greater reductions in mycorrhizal colonization.  We hypothesize that coarse-root species may exploit resource-rich patches not by root proliferation, but perhaps by mycorrhizal hyphal exploitation.