Joshua Wasyliw | ALES Graduate Seminar

A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES MSc Final Exam Seminar by Joshua Wasyliw. This seminar is open to the general public to attend.

Thesis Topic: Ectomycorrhizal functional diversity parallels fine root and leaf abundance with forest stand age.

MSc with Dr. Justine Karst

Seminar Abstract:

The abundance of fine roots and leaves in forests is predicted to peak during mid-succession and then decline. If fine roots decline more rapidly than leaves, reductions in fine roots could contribute disproportionately to stand decline. Ectomycorrhizal fungi (EcM), symbionts that facilitate nutrient acquisition of fine roots, may complement or parallel these shifts in root abundance. Two competing hypotheses frame the response of EcM functional diversity to stand age: a) the ‘host-filter’ hypothesis states that EcM fungi with emanating tissues (Distance mycorrhizas) increase with stand age, and b) the ‘energy-limited’ hypothesis states that carbon available for root symbionts decreases with stand age resulting in fewer Distance mycorrhizas. In the first hypothesis, EcM functional diversity complements root abundance, while in the second it parallels root abundance.

To test these competing hypotheses, I sampled fine roots to a depth of 90 cm below the soil surface and used allometric equations to estimate changes in root and leaf area index across a chronosequence of Pinus banksiana Lamb. stands ranging from 2–76 years average tree age. In addition to estimating changes in fine root and leaf area, I examined roots microscopically to track changes in the abundance of EcM functional types.

Both fine root and leaf area increased for the first 30–36 years and then plateaued, while the ratio of leaf to fine root area remained unchanged across the age gradient. Changes to fine root area with stand age depended on soil depth, with indications that old stands could be shifting a larger proportion of roots to deeper soil. This result is important because previous studies typically focused on upper soil horizons, thus, changes in root abundance below typical sample depths may have gone undetected. The abundance of Distance mycorrhizas did not increase in old stands, contrary to the host-filter hypothesis. Instead, the mean abundance of Distance mycorrhizas paralleled changes to leaf area, a finding more in-line with the energy-limited hypothesis. Taken together, these results suggest that the soil exploration benefits of Distance mycorrhizas do not outweigh their cost in old forests, but that they too are constrained by reductions in productivity.