Joseph Cooper | ALES Graduate Seminar

Date(s) - 08/12/2020
11:00 am - 12:00 pm

Event details:  A graduate exam seminar is a presentation of the student’s final research project for their degree.

This is an ALES PhD Final Exam Seminar by Joseph Cooper.  This seminar is open to the general public to attend via Zoom Link:

Thesis Topic: An exploration of aboveground and belowground interactions that shape forest dynamics

Seminar Abstract: Growth is limited when trees are restricted in resources or they experience conditions outside of their physiological adaptation, such as extreme cold. However, no tree exists in isolation. A wide variety of aboveground and belowground species interactions modulate the degree to which trees respond to resource limitation. Specifically, aboveground competitive or facilitative interactions may alter how trees grow, survive, and reproduce. Likewise, belowground interactions with fungi, ectomycorrhizal or otherwise, can alter access to resources and have a variety of beneficial or harmful impacts on tree growth. The overarching focus of my thesis is investigating the interactions that influence how trees experience growth-limitation and how these interactions may change with time. First, to investigate the role of aboveground interactions on forest demography and spatial patterns indicative of competition and facilitation, I compiled existing data and added to a census of a forest plot at the edge of the southern boreal forest of Alberta, Canada. I found a precipitous decline in tree recruitment and a rising mortality rate that had resulted in the loss of 70% of the trees within the last 30 years. However, density-dependent processes of facilitation and competition resulted in a near-equilibrium of the spatial distribution of trees across the stand. Density-dependent competition compounded with repeated drought, defoliator, and bark-beetle disturbances influenced spatial patterns as well as stand demography. My results suggest that the combination of density-dependent interactions and resource limitations have shifted the forest onto a novel successional trajectory, which likely will transition to an open-canopy shrubland. Second, to investigate how belowground interactions between trees, mediated by ectomycorrhizal networks, influence the growth of Pseudotsuga menziesii var. glauca, I used dendroecology to assess tree growth responses to network topology. I found that trees with more connections to other trees through the ectomycorrhizal fungal species, Rhizopogon vinicolor, had greater growth and lower interannual variation in growth than trees with fewer connections. Trees that were colonized by more unique genets of the ectomycorrhizal fungus, Rhizopogon vesiculosus, had greater growth than trees with fewer connections. My results suggest that ectomycorrhizal networks can transport biologically meaningful resources among adult trees and that interspecific differences exist between fungal species in their association with tree growth. These belowground interactions may have important implications for the magnitude and variance of mature forest growth. Third, because fungal communities driving belowground interactions can vary in composition, it is important to understand the processes that influence their assembly. Variations in the proportion of stochasticity or determinism could result in strikingly different fungal communities with cascading implications for tree growth. Stochastic community assembly emerges from functional equivalence between taxa while determinism arises from environmental filters acting on species. To investigate if stochastic and deterministic processes change for fungal communities on roots, soils, and with tree age, I surveyed four forests of Pseudotsuga menziesii across the fringes of its range in western North America. Across 1800 km, these forests vary in precipitation limitation, nutrient availability, and growing season length. I found that the proportion of assembly processes differed between sites but that deterministic pressures selecting for homogenizing communities were largely dominant. Community assembly processes were independent of tree age at all sites. My results suggest that fungal communities assemble with site-specific processes and that changes in environmental factors, such as local climate, may have unequal impacts on fungal community composition across the range of Pseudotsuga menziesii. Finally, to further explore factors affecting the community assembly of belowground fungal communities, I investigated how the identity and age of the at-risk species, Pinus flexilis and Pinus longaeva, influenced the composition of belowground fungal communities. The root-associated fungal communities were distinct for Pinus flexilis and Pinus longaeva, while soil-associated fungal communities were similar. Ectomycorrhizal fungi had the greatest number of sequences and fungal species were largely shared between the two pines. Tree age was associated with the composition of root-associated fungal communities on Pinus flexilis and were correlated with increasing dissimilarity between soil and root communities with tree age. The results suggest distinct but overlapping fungal communities were dominated by shared ectomycorrhizal fungi. The potential influence of tree age on root-associated fungal communities suggest that partner selection changes over multiple centuries as trees mature. Cumulatively, my work highlights the potential influence of aboveground and belowground interactions in shaping forest health as well as how these interactions are variable across space, and with time.