An exploration of aboveground and belowground interactions that shape forest dynamics

  • Author / Creator
    Cooper, Joseph
  • 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 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. 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 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, 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 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 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 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 displayed increasing dissimilarity between soil and root communities with tree age. These results suggest distinct but overlapping fungal communities 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 influence of aboveground and belowground interactions in shaping forest health, as well as how these interactions are variable across space and time.

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.