Suzanne Simard


Relevant Thesis-Based Degree Programs


Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Fish, forests, fungi: soils in the 'salmon forests' of British Columbia (2022)

In this thesis, I examine the effects of returning salmon on riparian soils in Heiltsuk traditional territory, near the community of Bella Bella on the Central Coast of British Columbia, Canada. My main objectives were to: 1) quantify how salmon affect forest soil chemistry; 2) use nitrogen stable isotopes to measure salmon contributions to fungi, soil, and litter nitrogen metabolism; 3) quantify how salmon affect soil fungal communities; 4) quantify how salmon affect soil bacterial communities; and 5) test the role of fungi in plant ammonium uptake by disrupting fungal metabolism. To achieve objectives 1 through 4, I used two observational approaches: the first examined watersheds along a natural gradient of salmon density; and the second used sites where waterfalls block salmon migration, allowing for within-watershed comparisons above and below these barriers. To achieve objective 5, I employed a nitrogen stable-isotope addition experiment. I found that salmon affected soil chemistry, with impacts on concentrations of nitrogenous compounds, exchangeable cations, phosphorus, and metals, as well as differences in pH. I found that δ15N was greater in sporocarps when salmon were present, and interpreted this as evidence fungi acquire salmon nutrients. Using next-generation environmental sequencing, I found salmon inputs impact fungal relative abundance, soil fungal β- and α- diversity, but did not affect phylogenetic dispersion. In general, symbiotrophic fungi were affected by salmon inputs more than saprotrophs. Soil bacterial communities had fewer correlations with salmon inputs. Finally, I found that disruption of fungal metabolism slowed the uptake of ammonium by riparian shrubs. These results show that salmon are important in structuring riparian soil chemistry and microbial ecology, and support the hypothesis that fungi play an important role in salmon nutrient uptake by plants.

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Investigation of ecosystem legacy-based mine reclamation techniques: effects of soil transfers, forest connectivity and plant-soil feedbacks on soil food web recovery and plant establishment (2021)

Primary successional post-mining landscapes present physical, chemical and biological barriers to terrestrial ecosystem recovery. I investigated if legacies of the pre-disturbance ecosystem can be applied to help overcome these barriers, leveraging the processes that guide succession following natural disturbances. In a three-year field experiment at the Mount Polley Mine, Canada and in a greenhouse experiment using tailings, overburden and forest soil from the mine, I tested if (1) adding small volumes of forest soil to the seedling rooting zone during planting and (2) spatial connectivity with undisturbed forest would improve soil food web recovery and plant establishment. Soil additions improved seedling growth and/or nutrition in the greenhouse and field, corresponding with greater root colonization by microbial mutualists. In the field, soil additions increased soil faunal diversity and caused bacterial community shifts consistent with advancing succession, with greater benefits in lower quality soils. Sterilized greenhouse controls revealed that a later successional conifer benefited primarily from the biological inoculum, while an early successional shrub benefited more from the improved soil physiochemical conditions. Greater proximity to undisturbed forest increased plant diversity and shifted soil microbial communities, showing that retained patches of intact ecosystems can counteract dispersal limitations. Short-distance belowground connectivity with undisturbed forest improved plant diversity and mycorrhizal fungal community establishment but decreased seedling growth compared to areas “disconnected” with trenches. Field-testing of soil additions and spatial connectivity in conjunction with site preparation showed that application of ecosystem legacies can have additive benefits with reclamation methods targeting soil physical improvements. In a second greenhouse experiment, I tested if plants establishing in mine reclamation materials regenerate soil microbial legacies that facilitate successional advancement. I found negative feedbacks due to fungal pathogen accumulation promoted turnover of an early successional shrub, while formation of distinct mycorrhizal fungal communities selectively promoted establishment and stability of compatible later successional tree species. This reinforces that reclamation plant species selection can influence long-term plant community assembly. Ecosystem legacy-based mine reclamation methods could be used operationally to simultaneously enhance dispersal and establishment of native plants and soil organisms, which, over the long-term, is anticipated to have positive feedbacks on ecosystem recovery.

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Influence of kin, density, soil inoculum potential and interspecific competition on interior Douglas-fir (Pseudotsuga menziesii var. glauca) performance and adaptive traits (2020)

Positive interactions between plants, in addition to competition, can help shape a plant community and ecosystem. Positive interactions can come in the shape of intraspecific interactions such as kin selection, or interspecific interactions, such a mycorrhizal symbiosis. Previous studies have provided evidence for both of these processes occurring in interior Douglas-fir (Pseudotsuga menziesii var. glauca) with the possibility that they are linked. This thesis focuses on potential influences on the processes of kin recognition and selection including density, soil (mycorrhizal) inoculum potential and species-specific community composition. In three studies regarding kin relationships of year-old seedlings, evidence supporting kin recognition via differences in morphological traits and kin selection via differences in performance between kin and stranger seedlings was provided. We found that increasing the plant density created environments where kin seedlings behaved in a more similar manner to strangers when seedlings were grown in pots with limited resources. Decreasing the soil inoculum potential, while decreasing overall performance, increased the kin response. When grown in the field, seedlings required a greater density for a kin/stranger differential response to be detected. By changing community composition at a consistent density, we observed cooperative behaviours in kin seedlings grown with only other kin and unique responses when kin and strangers were grown together in a group suggesting integration of multiple cues. Our results could have important forest management implications, particularly surrounding the concepts of legacy trees and natural regeneration of the locally adapted seed they produce, maintaining access to mycorrhizal associations and networks and the potential for family substructuring. Kin relationship considerations may be particularly important in harsh climates or at the leading edge of the range of Douglas-fir, which is expected to move northward and upward as the climate shifts.

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The effects of stumping and tree species composition on the soil microbial community in the interior cedar-hemlock zone, British Columbia (2020)

Stump removal (stumping) is an effective forest management practice used to reduce the mortality of trees affected by fungal pathogen-mediated root diseases such as Armillaria root rot, but its impact on soil microbial community structure has not been ascertained. This study investigated the long-term impact of stumping and tree species composition on the abundance, diversity and taxonomic composition of soil fungal and bacterial communities in a 48-year-old trial at Skimikin, British Columbia. We used DNA metabarcoding targeting the fungal internal transcribed spacer (ITS) marker and the bacterial 16S rRNA gene to decipher the microbiomes. A total of 108 samples were collected from the FH (fermented and humus layers), 0-10 cm (A horizon) and 10-20 cm (B horizon) layers in 36 plots, 18 stumped and 18 unstumped, that were planted with pure stands and admixtures of Douglas-fir, western redcedar and paper birch. Fungal α-diversity in the A horizon increased with stumping regardless of tree species composition and had a tendency to increase in the FH and B horizons. In the FH horizon, the relative abundance of the saprotrophic fungal community declined while that of ectomycorrhizal (ECM) fungal community increased with stumping. Bacterial α-diversity in the B horizon declined with stumping, irrespective of tree species, and also tended to decrease in the A horizon. The B horizon of stumped plots was significantly enriched with potential plant growth-promoting bacteria (PGPR), such as rhizobia. Similarly, Pseudomonadales, known for their antagonistic role against pathogens, increased significantly in all three soil horizons with stumping and was especially observed in association with birch and its admixtures. The culture-based assessment focused on 16S rDNA substantiated the dominance of potential PGPRs in the stumped plots. Furthermore, molecular characterization of Armillaria using translation elongation factor-1 alpha (tef-1) and ITS revealed the occurrence of A. gallica, reported for the first time at this site. Overall, we conclude that stumping along with plantation of resistant tree species with susceptible ones, led to a healthy fungal community structure and promotion of a beneficial bacterial microbiome, thus proves as a potent practice for the suppression of Armillaria root rot and promotion of forest health.

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Going underground: patterns of fine-root and mycorrhizal fungal trait variation across a biogeographic gradient in western Canada (2019)

Understanding fine-root adjustments to the environment and identifying factors that shape mycorrhizal fungal communities is a prerequisite for predicting the response and feedbacks of plants to global changes. As a consequence, trait-based plant ecology, which has mostly focused on above-ground traits, is increasingly placing the emphasis below-ground.To improve our functional understanding of fine roots, we first quantified root morphological, chemical and architectural trait variation in interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) forests across a biogeographic gradient in Western Canada. We found substantial within-population root trait variation, which may enable acclimation of trees to future environmental conditions. Yet, we also identified moderate but consistent trait-environment linkages across populations of Douglas-fir. We provided evidence for decoupled variation in fine-root morphological and chemical traits. Our results highlight the existence of multiple axes of within-species fine-root adjustments that were consistent with a potential increase in fine-root acquisitive capacity with environmental limitations.Next, to better integrate mycorrhizal symbiosis into trait-based plant ecology, we combined trait measurements of fine roots and ectomycorrhizal fungi with next-generation sequencing. We found temperature, precipitation and soil C:N ratio affected ectomycorrhizal community similarities and exploration type abundance but had no effect on fungal richness and diversity. We did not provide evidence for a functional connection between root traits and fungal exploration types within Douglas-fir populations. Our study clarifies ectomycorrhizal taxonomic and functional responses to environmental factors but warrants further research to broaden root trait frameworks and evaluate the role of mycorrhizal fungi in mediating ecosystem responses to environmental changes. This line of inquiry will be particularly important to better manage existing forests and to ensure that well-adapted forest tree populations are regenerated in the future.

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Kin-selected signal transfer through mycorrhizal networks in Douglas-fir (2017)

Mycorrhizal networks create pathways for movement of resources and information molecules belowground. A mycorrhizal network is formed when two or more plants are linked by the same mycorrhizal fungus. Experiments have demonstrated movement of carbon and nitrogen between Douglas-fir and neighboring plants in response to source-sink dynamics, seasonality, and differences in age of linked plants. Furthermore, the network appears to act a conduit for information chemicals, where defense chemicals are transferred in response to herbivory or pathogen attack. Because of recent evidence implying the capacity for Douglas-fir to recognize kin, as well as differential colonization of Douglas-fir by ectomycorrhizas based on tree relatedness, this thesis aimed to determine whether Douglas-fir would preferentially transfer carbon and/or nitrogen through mycorrhizal networks to kin over strangers in response to herbivory treatment. Using seedlings with and without access to a mycorrhizal network (restricted or permitted via mesh pore size), stable isotope probing was used to track carbon and nitrogen in the system. One seedling of a pair was designated as the 'donor' and defoliated immediately prior to photosynthesizing with 99%-¹³C-CO₂ as well as pulse-labelling with 99%-¹⁵N ammonium nitrate. Both a greenhouse and field experiments were performed to corroborate results. Transfer was determined by measuring δ¹³C and δ¹³N in tissues (needle, stem, root) of kin and stranger seedlings. Data was analyzed using linear mixed effects models. Significantly more carbon was transferred to kin than strangers, and through the mycorrhizal network than when the mycorrhizal network was blocked. Furthermore, herbivory (in the form of western spruce budworm defoliation as well as manual defoliation) induced transfer of carbon to kin over strangers. Douglas-fir families differed in their tendency to transfer carbon and nitrogen to kin. Molecules potentially involved in defense signaling were identified using liquid chromatography coupled with mass spectroscopy. Ectomycorrhizal fungi that can form mycorrhizal networks were found on all seedlings. We conclude that preferential carbon transfer through mycorrhizal networks occurs between kin in Douglas-fir and is amplified by herbivory stress. Herbivory is not necessary for transfer, as some transfer also occurred in the no-herbivory treatment.

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Mycorrhizal fungi: unlocking their ecology and role in the establishment and performance of different conifer species in nutrient-poor coastal forests (2016)

This thesis explored the fungal communities of arbuscular mycorrhizal-dominated Cedar-Hemlock (CH) and ectomycorrhizal-dominated Hemlock-Amabilis fir (HA) forests on northern Vancouver Island, British Columbia, Canada and examined the role of mycorrhizal inoculum potential for conifer seedling productivity. Objectives of this research project were to: (1) examine the mycorrhizal fungal communities and infer the inoculum potential of CH and HA forests, (2) determine whether understory plants in CH and HA forest clearcuts share compatible mycorrhizal fungi with either western redcedar (Thuja plicata) or western hemlock (Tsuga heterophylla), (3) test whether differences in mycorrhizal inoculum potential between forest types influence attributes of seedling performance during reforestation and (4) test effectiveness of providing appropriate mycorrhizal inoculum at the time of planting on conifer seedling performance. Molecular and phylogenetic techniques were utilized to compare mycorrhizal fungal diversity between forest types and to identify mycorrhizal fungal associates of the plant species occurring in clearcuts. In a field trial utilizing seedling bioassays, the role of mycorrhization of western redcedar and western hemlock on seedling growth was evaluated; reciprocal forest floor transfers from uncut forests were incorporated into the project design as inoculation treatments. Though diversity was similar, ectomycorrhizal and saprophytic fungal community composition significantly differed between CH and HA forests; arbuscular mycorrhizae were widespread in CH forests, but rare in HA forests. There was high similarity of arbuscular mycorrhizal fungi to those found in western redcedar among the dominant plant species colonizing CH clearcuts, including the ericoid plant Gaultheria shallon and in Blechnum spicant growing sparsely in HA clearcuts. No alternative ectomycorrhizal host species were detected. Mycorrhization greatly influenced productivity of western redcedar seedlings; without mycorrhizal inoculum, redcedar did not achieve its full growth potential in HA clearcuts. Mycorrhization of western hemlock seedlings did not differ between forest clearcut type or treatment group; however, an inhibitory effect of forest floor collected under mature western redcedar trees on the growth of western hemlock seedlings was unexpectedly detected. These results have implications for sustainable forest management practices, including retention of legacy trees and plants with timber harvesting and inoculation of seedlings with mycorrhizal fungi at the time of planting.

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Vascular Plant Response to Slashburning and Clearcutting in Central British Columbia: A 20 Year Study of Plant Functional Type Resilience (2014)

How resilience is understood and measured has become increasingly challenging for ecologists, particularly as terrestrial ecosystems are undergoing radical change as climate changes. This body of work proposes a specific approach to studying resilience and applied it to Interior Cedar-Hemlock (ICH), Sub-Boreal Spruce (SBS) and Engelmann Spruce-Subalpine Fir (ESSF) forests extending across central British Columbia, Canada. Repeated measurements (% cover and height) of vascular plants were collected between 1981 and 2008 (1, 2, 3, 5, 10 and 20 years after clearcutting and slashburning) in permanent research installations. Individual species sensitive to the forestry treatment (recorded exclusively pre-burn) included Rhododendron albiflorum, Menziesia ferruginea and Prosartes hookeri in the ICH; Rubus pedatus in the SBS; and Orthilia secunda, Listera cordata and Moneses uniflora in the ESSF. Post-burn shifts in species dominance consisted of substantial loss of Abies lasiocarpa, Oplopanax horridus and Listera cordata, and increases in Alnus spp., Salix spp., Epilobium spp. and Calamagrostis spp., indicating possible transition from conifer forest to mixed forest or open meadow ecosystems at several study sites.To overcome the difficulty of evaluating ecosystem resilience from measurements of 183 individual species recorded in experimental plots, I created plant functional types (PFTs) based on 15 common plant traits. PFTs were determined by grouping together plants that behave in similar ways or produce similar outcomes despite having different physical characteristics or evolutionary paths. PFT models of abundance and richness along gradients of soil nitrogen and fire severity over time indicated linear and non-linear response trends, and lasting and temporary effects. Structural equation modeling (SEM) was used to measure the relative importance of factors driving the responses observed. The SEM indicated that mean annual precipitation (MAP) negatively influenced fire severity; mean annual temperature (MAT) positively influenced fire severity and soil nutrients; and MAP and MAT directly and/or indirectly influenced most PFTs. My research suggests that clearcutting and slashburning do not alone alter the diversity or function of mesic ESSF, SBS and ICH forests; however, past and future anthropogenic disturbances combined with non-historical climate and interrelated edaphic factors may place long-term stability of these ecosystems at risk.

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Juvenile growth of subalpine fir (Abies Iasiocarpa) in the Montane Spruce ecological zone of British Columbia Canada (2013)

Subalpine fir (Abies lasiocarpa (Hook.) Nutt.) is a widely distributed western North American conifer that can grow under a wide range of light environments, initial densities and site qualities. It can be a major component of stands found within the Thompson Dry Mild variant of the Montane Spruce ecological zone (MSdm2) in the southern interior of British Columbia, Canada. In chapter 2 of this dissertation, I examined the effects of light, moisture, nutrients and neighbor density on juvenile subalpine fir growth. This led me to conclude that: 1) light availability had the largest influence on juvenile tree growth; 2) Delta-13C was the second most important growth predictor; 3) tree size also improved growth predictions; 4) soil moisture was a weak growth predictor; 5) foliar N levels did not improve growth predictions; and 6) density, as expressed as stems/ha, improved growth predictions negligibly. The results from chapter 2 helped to determine the important predictor variables (light and tree size) that were used in investigating the importance of spatially explicit competition on the development of juvenile trees (chapter 3). The chosen spatial model utilized tree size and the crowding effect of neighbors to predict juvenile radial growth. This model was then incorporated into SORTIE-ND as a new juvenile growth behavior, “Juvenile NCI Growth”, and used to test whether juvenile or mature trees have a greater competitive influence on juvenile subalpine fir growth under three basal area classes. Here, I found that juvenile radial growth was faster under the canopy of mature trees than in the neighborhood of similar sized juveniles at the two lowest density classes, 7 and 20 m²/ha. This indicated that symmetric competition processes dominated. I also found that at the highest density class, there were no differences in juvenile radial growth between the two neighbor strata. Chapter 4 was designed to test the influence of site series on growth predictions using SORTIE-ND. I found that site series did have an influence on the growth and development of the stand, as would be expected, which suggests that incorporating site quality into SORTIE-ND would improve growth and yield predictions.

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The complex socio-spatial architecture of Rhizopogon spp. Mycorrhizal networks in xeric and mesic old-growth interior Doublas-fir forest plots (2012)

Mycorrhizal networks (MNs) can influence tree establishment and resource competition but little is known regarding their underlying architecture in situ. This study examined the socio-spatial architecture of MNs between Rhizopogon spp. genets and interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) trees in an old-growth forest. MN features were contrasted between plots with xeric versus mesic soil moisture regimes as a proxy for changes in site water stress anticipated with climate change. My objectives were to: (1) describe the fine-scale spatial patterns and autecological traits of R. vesiculosus and R. vinicolor mycelia systems and compare these between xeric and mesic plots; (2) describe the spatial patterns and architecture of Rhizopogon spp. MNs at the forest stand scale; (3) contrast MN architectures between phytocentric and mycocentric perspectives and between xeric and mesic plots, and identify critical determinants of MN architectures. Rhizopogon vesiculosus mycelia occurred deeper, were more spatially prolific, and colonized more tree roots than R. vinicolor mycelia. Both species were associated with moist microsites within plots, and had more prolific mycelia in mesic compared to xeric plots. The occurrence of R. vesiculosus shifted in the presence of R. vinicolor towards deeper soil horizons, suggesting competition and foraging strategy are important for niche partitioning between these species. At the forest stand scale, Rhizopogon spp. genets spanned tens of metres and colonized up to 19 trees, but R. vesiculosus genets were larger and linked more trees than R. vinicolor genets. Multiple tree cohorts were linked, with saplings and mature trees sharing the same fungal genets. Across all plots, the physical size of individual trees or fungal genets was positively related to their MN connectivity. This together with size asymmetries among different genets and trees resulted in the self-organization of complex, hierarchical scale-free MN architectures. The MNs appear robust to random perturbations but susceptible to the loss of large trees or fungal genets. No MN structural differences were found between phytocentric and mycocentric models or between xeric versus mesic plots. The pervasive mycelia and extensive MNs formed by these Rhizopogon spp. could influence interior Douglas-fir stand dynamics and resistance to water stress.

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Natural Regeneration Potential of Douglas-Fir Following Wildfire and Clearcut Harvesting (2011)

The McLure fire of August, 2003, affected over 26,000 hectares in the interior of British Columbia. The study objective was to determine the impact of wildfire and clearcutting severity on Douglas-fir regeneration potential in the Interior Douglas-fir biogeoclimatic zone. The study design consisted of five treatments that compared a range of disturbance severities: high severity burn, low severity burn, clearcut, screefed clearcut, and undisturbed forest. At each of four replicate sites per treatment, 1000 seeds were sown in late spring of 2004. Natural regeneration potential was assessed by measuring seedling performance and mycorrhizal diversity over a three-year period. Overall, the treatments with the greatest disturbance severity had the greatest natural regeneration potential due to increased resource availability. Seedling survival was considerably higher in the burn and clearcut treatments than the undisturbed forest. Seedlings in the high severity burn had significantly greater shoot height, biomass, and foliar N and P content than those in the clearcut treatments. Seedlings regenerating in the burn treatments had the lowest ectomycorrhizal colonization in the first growing season but all seedlings in all treatments were colonized by the start of the second growing season. Increased disturbance severity, either by wildfire or clearcutting, led to a uniform ectomycorrhizal community dominated by Wilcoxina sp. In contrast, the undisturbed forest was dominated by a more diverse ectomycorrhizal community. The simplification of the ectomycorrhizal community did not negatively affect seedling growth or survival. The highest biomass accumulation and foliar nitrogen content occurred in the high severity burn and were associated with the lowest levels of mycorrhizal colonization and diversity. This supports the hypothesis that plants reduce their carbon investment in mycorrhizal fungi when growing under favorable conditions. Overall, the results indicate that, given a seed source, the natural regeneration potential of Douglas-fir is high after both wildfire and clearcut harvesting.

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The Role of Ectomycorrhizal Networks in Plant-to-Plant Facilitation Across Climatic Moisture Gradient (2011)

Common ectomycorrhizal (EM) networks are expected to facilitate conifer regeneration under abiotic stress, such as drought exacerbated by climate change. This study examined effects of climate, CO₂ concentration ([CO₂]), and EM networks on Douglas-fir seedling establishment. My objectives were (1) to determine the effects of regional climate (represented by a drought index) on EM network facilitation of Douglas-fir seedling establishment; (2) to separate genotypic effects from climatic effects; (3) to compare the importance of EM networks to 3-year-old outplanted nursery seedlings versus 1st year seedlings germinated in the field; (4) to parse the competitive from facilitative effects of residual Douglas-fir trees on small seedlings; and (5) to determine the interaction between soil water and [CO₂], in their effects on EM network-facilitated seedling establishment and C-transfer between different sized Douglas-fir seedlings. Survival was maximized when seedlings were able to form an EM network in the absence of root competition, both in growth chambers and in the field for the medium moisture provenance. When drought conditions were greatest, growth of these same seedlings increased when they could form an EM network with nearby trees in the absence of root competition, but it was reduced when they were unable to form a network. Overall, survival was greatest for these seedlings relative to those from the wet or dry provenances, but decreased with summer heat:moisture index more rapidly. I found no evidence of C transfer between seedlings through growth chamber ¹³CO₂ labeling, but D₂O labeling and natural abundance H₂¹⁸O measurements are suggestive of increasing water transfer from donor to receiver seedlings as receiver water deficiency increased.

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Carbon, Plant and Microbial Dynamics in Low-Arctic Tundra (2010)

Anthropogenic climate change threatens the stability of Arctic C stores. Soil microbes are central to the C balance of ecosystems as decomposers of soil organic matter and as determinants of plant diversity. In four experiments in the tundra, I address critical gaps in our understanding of the role of soil microbial communities in the response of an Arctic ecosystem to climate change. My objectives were 1) to asses the role of mycorrhizal networks (MN) in plant-plant interactions; 2) to determine the effects of warming and fertilization on the ectomycorrhizal (ECM) community of Betula nana; 3) to determine the effect of warming on soil fungi and bacteria over time; 4) to assess the role of the mycorrhizal symbiosis in C-allocation to rhizosphere organisms. I show that MNs exist in tundra and facilitate transfer of C among Betula nana individuals, but not among the other plants examined. C-transfer among Betula nana pairs through MNs represented 5.5 ± 2.2% of photosynthesis, total belowground transfer of C was 10.7 ± 2.1%. My results suggest that C-transfer through MNs may alter plant interactions, increasing competition by Betula nana, and that this will be enhanced with warming. I show that warming leads to a significant increase of fungi with proteolytic capacity, particularly Cortinarius spp., and a reduction of fungi with high affinities for labile N, especially Russula spp. My findings suggest that warming will alter the ECM community and nutrient cycling, which may facilitate Betula nana in tundra. I show that warming leads to a 28% and 22% reduction in the richness of soil fungi and bacteria in tundra, respectively, as well as corresponding declines in diversity. My data agree with reductions in plant community richness with warming at this site, and suggest that warming will reduce total community diversity in tundra. I show that Gram-negative bacteria and a species-specific community of mycorrhizal fungi are the primary consumers of rhizodeposit C among tundra shrubs. Together, these results strongly suggest that soil microbes play a critical role in plant community dynamics and C-cycling in Arctic tundra, and that this role will become increasingly important as climate warms.

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Effects of Nurse Tree Species on Growth Environment and Physiology of Underplanted Toona ciliata Roem in Subtropical Argentinian Plantations (2009)

No abstract available.

Interacting effects of soil nitrogen supply and light availability on understory sapling growth and foliar attributes (2008)

Light availability in forest understories is a well recognized constraint on sapling growth, but limitations in soil nitrogen (N) availability, and the link to foliar photosynthetic capacity, typically receive less consideration in describing stand dynamics. My primary hypothesis is that light and soil N availability have species-specific effects on photosynthetic activity and growth, and that together these resources will better define understory development in complex forests. To test these relationships, I examined 1) soil N indices and the tradeoffs between soil fertility and light attenuation in old-growth forest understories; 2) the effects of light and N constraints on understory sapling foliar N concentration (N%), N per unit area (Na), and natural abundance of ¹³C; 3) the effects of light and soil N supply on species growth and photosynthetic activity in a factorial field experiment; and 4) the mechanisms responsible for the stagnation of understory saplings. Soil N indices incorporating dissolved inorganic N and organic N were useful in characterizing differences in N supply among contrasting sites. Understory light availability declined with increasing soil N supply, while understory Abies lasiocarpa had strong correlations between foliar N% and soil N availability, despite shading effects. In partial-cut forests, understory Tsuga heterophylla and Picea glauca x sitchensis had consistent foliar N% across gradients of light availability; in contrast, foliar N% of Betula papyrifera and Thuja plicata declined with increasing shade, which would distort assessments of soil fertility and perhaps contribute to increased mortality of these species in deep shade. Strong correlations between foliar Na and ¹³C or growth increment suggest foliar N per unit area is the simplest integration of light availability and N nutrition on leaf photosynthetic activity. Ontogenic interactions that occur among foliar attributes and tree size in forest understories, especially for saplings
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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Of mice and moose: small and large mammal responses to a gradient of forest harvesting intensities across interior British Columbia (2021)

British Columbia’s interior forests have been heavily logged, burnt and subject to beetle outbreaks for decades. The compounding effects of these disturbances on wildlife and their habitat must be considered. Partial retention forest harvesting may be a method that could mitigate some of the negative effects of clearcut harvesting on wildlife. However, tests of the effects of partial harvests on ecosystem patterns and processes in different contexts are needed. From December 2018 and June 2020, we conducted live trapping for small mammals and camera trapping for medium-to-large-bodied mammals to estimate species diversity, population density, habitat use, and behaviours across different forest harvesting practices across a 900 km gradient in John Prince Research Forest, Alex Fraser Research Forest, and Jaffray (east Kootenays), BC. We detected 7 small mammal species, with diversity highest in the control (mean Shannon Index = 1.01, SE = 0.14) and partial retention treatments (means = 0.99, 0.98; SE = 0.17, 0.17) and significantly lower in the seed tree treatment (mean = 0.63, standard error = 0.17, p-value = 0.02). Population densities of North American deer mouse (Peromyscus maniculatus) and Southern red-backed vole (Myodes gapperi) estimated with spatially explicit capture-recapture models highlight the importance of partial harvest practices that maintain sufficient cover to support a higher diversity of small mammals and higher densities of forest specialists. Our medium- to large-bodied mammal diversity analysis suggests that the regional environmental context had a stronger effect on mammal communities than local-scale differences in harvesting practices. Vegetation productivity measured with normalized difference vegetation index was a more important predictor of habitat use for ungulates than harvest treatment, potentially due to the importance of forage availability. Across both small and large mammals, responses to forest harvesting were variable; several species used partial harvests more than clearcuts. Forest practices should consider broader implementation of partial harvests to provide suitable habitats for a broader range of species. More experimental approaches to forest operations are needed across larger spatial scales, such as adaptive management of forest practices with rigorous wildlife monitoring to ensure ecological objectives are met.

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Western hemlock regeneration on coarse woody debris is facilitated by linkage into a mycorrhizal network in an old-growth forest (2018)

Western hemlock (Tsuga heterophylla (Raf.) Sarg.) is the climax tree species in most of the low elevation Pacific northwest coastal forests. Regeneration of western hemlock under a closed canopy it is tightly associated with coarse woody debris (CWD). Nurse-logs contain physical, chemical and biological features that make them the most suitable seedbed for hemlock in old-growth forests. However, the preference of western hemlock for CWD is still not completely understood. Few studies have investigated belowground dynamics related to western hemlock growing on decayed wood, specifically ectomycorrhizal (EM) associations and the potential access to a mycorrhizal network (MN). My research addressed two objectives: 1) To compare establishment and survival of western hemlock germinants among different microsites (CWD with different wood breakdown levels and forest floor) in the forest understory; and 2) To investigate whether carbon is transferred through mycorrhizal networks from mature trees to seedlings established on CWD. To accomplish this, I pulse-labeled ¹³C-glucose solution into the phloem of mature trees. I found that western hemlock successful recruitment was associated with the presence of medium to advanced decay classes of CWD. I also found the first evidence that there is carbon transfer from mature trees to regenerating seedlings established on CWD. I conclude that western hemlock regeneration on nurse-logs is facilitated by mycorrhizal networks of canopy trees in an old-growth forest of coastal British Columbia.

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Forest canopy gap size affects regeneration potential of interior Douglas-fir (2016)

There is growing concern about the long term productivity of forests in British Columbia due to changing climatic conditions. Interior Douglas-fir, an economically and culturally valuable conifer species, has recently had inconsistent regeneration success in the dry climatic regions of its distribution due to high summer soil surface temperatures, drought and growing season frost. Seeds of interior Douglas-fir germinate after mixed severity disturbances, but their survival appears to depend on the size of disturbance gaps, environmental resources and conditions, and colonization by mycorrhizal fungal symbionts. In two separate experiments that differed in climate (very dry, hot and dry, cool Interior Douglas-fir (IDF) subzones), and disturbance agent (natural and harvested), I sowed interior Douglas-fir seed into different sized forest canopy gaps. In both experiments, I tested the effects of canopy gap size and access to mycorrhizal networks on seedling performance (establishment, growth, water use efficiency, foliar nutrition, mycorrhizal colonization) and environmental resources and conditions (light, temperature, soil moisture). In the first experiment, regeneration failed in all canopy gap sizes and network treatments due to the harsh climatic conditions. There, neither protection in small gaps nor access to mycorrhizal networks were sufficient to create favourable regeneration conditions. In the second experiment, where the climate was cooler and wetter, seedling survival reached 74% in harvested gaps that were 80 – 300 m² in area, corresponding with greater soil moisture availability. Gaps of 20 – 80 m² were too small to initiate gap-phase regeneration, however, as indicated by low seed emergence and slow height growth rates. Gaps >300 m² resulted in high emergence but low survival (26 %) due to low soil moisture availability. Access to mycorrhizal networks had minor effects on mycorrhizal colonization and water use efficiency. My study shows that regeneration potential of interior Douglas-fir is severely limited by the very dry, hot climate in the low elevation IDF forests, but can be increased in wetter, cooler climates with forest harvesting practices that create moderately sized canopy gaps.

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Networks of communication: defense-related signal transfer between tree seedlings via mycorrhizal networks and an educational mycorrhizal-focused video game (2016)

The majority of terrestrial plants associate with fungi in symbiotic resource-exchange relationships called mycorrhizae. Mycorrhizal networks (MNs) arise when the same fungus is connected to multiple plants, allowing for interplant resource transfer and impacting ecosystem functions. Recent work suggests MNs also transfer defense-related information from pathogen-, herbivore-, or mechanically-damaged plants to unharmed neighbors. I investigated the defense pathways involved in defense-related signal transfer in ectomycorrhizal systems. Paired Douglas-fir seedlings were grown with varying levels of belowground connectivity (soil water only; soil water and MNs; soil water, MNs, and roots), and a defense response was stimulated in donor seedlings by methyl jasmonate. After 24 and 48 hrs, I measured expression of two regulatory genes on the jasmonate and ethylene pathways. Receiver response was unrelated to hormone treatment of donors in either gene, but the jasmonate response of donor and receiver pairs was correlated across treatments. Positive expression of both genes across donors and receivers and pervasive presence of spider mites suggested signal transfer may either have not occurred or been masked by already ongoing defensive responses. Results indicate the complexity of these systems, and further work is needed to better characterize defense signal transfer via ectomycorrhizal networks. Because of the importance of these mycorrhizal systems to ecosystem functioning, it is crucial that resource managers and scientists have a good understanding of mycorrhizal ecology. However, lower student interest in plants and fungi combined with difficulties visualizing belowground processes present challenges for teaching and learning mycorrhizal concepts. To address this, I co-created the digital plant-centric action-based game Shroomroot for use in lower level postsecondary settings. I conducted a pre-test/post-test evaluation of Shroomroot in a 2nd year postsecondary Introduction to Soil Science course. Students’ knowledge of mycorrhizal ecology increased after playing Shroomroot, and engagement with mycorrhizal content tended to increase after gameplay. These exploratory results suggest positive potential for action-based plant-oriented digital games in the higher education classrooms. Both studies focus on improving our understanding of mycorrhizae and mycorrhizal networks, ecologically and pedagogically. Greater understanding of mycorrhizae has the potential to improve our multi-faceted relationships with the ecosystems upon which we depend.

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Interspecific interactions in mixed stands of paper birch (Betula papyrifera) and interior Douglas-fir (Pseudotsuga mensiezii, var. glauca) (2015)

Growing mixed conifer-broadleaf forests instead of monoculture coniferous forests could reduce problems with seedling regeneration, disease and volume loss, all of which are expected to increase with warmer climates and more frequent droughts. Understanding mixed forest dynamics, as well as their quickly evolving mycorrhizal symbionts, could reveal key management strategies for adapting to climate change. This study is a long-term analysis of two field experiments established in 1992 in the southern interior of British Columbia, Canada, where I sought to gain insight into the outcomes and mechanisms of interspecific interactions in mixtures of broadleaves and conifers. The broader experiment examined interactions within mixed stands of Douglas-fir and paper birch in an extensive response surface design, while a second experiment isolated rooting areas of individual trees within two density pairings embedded in the larger experiment. The treatments were replicated across three geographically distinct sites within the same BEC subzone (ICHmw). Twenty-one years after the experiments were established, I found strong evidence of reduced Armillaria root disease and increased foliar nutrition in interior Douglas-fir with increasing density of paper birch neighbours, but no negative effect of paper birch competition on interior Douglas-fir growth. This last result may be due in part to the comparatively weak status of the planted paper birch, which never overcame early poor performance. The mechanisms by which the struggling paper birch interacted with interior Douglas-fir were revealed in the trenching experiment, where ability to form mycorrhizal networks resulted in cumulative benefits to paper birch over time, with significantly less growth loss in untrenched than trenched treatments. This benefit was consistent across densities and regardless of climatic stress, pointing to a pattern of constant benefits of belowground interactions for subordinate tree species. This finding points towards belowground interactions as a medium for balancing species inequalities and, by extension, of maintaining ecosystem diversity and stability. Taken as a whole, these results illustrate the possible benefits of maintaining broadleaves in commercially valuable conifer plantations, both in terms of direct health benefits to conifers, and in the broader sense of providing negative feedback mechanisms to species loss and ecosystem instability.

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Biogeoclimatic Ecosystem Classification of Subalpine and Alpine Communities in the Cariboo Mountains (2014)

Terrestrial ecosystems in British Columbia are cataloged using the Biogeoclimatic Ecosystem Classification (BEC). My research is a continuation of the BEC program, specifically aimed at classifying high-elevation ecosystems (alpine and subalpine) of the western Cariboo Mountains of British Columbia. I’ve included a literature review describing the factors that contribute to the formation of high-elevation plant communities. My results include: five new site series for the Interior Mountain Heather wet cold alpine subzone; b) seven new site series for the Engelmann Spruce Subalpine Fir wet cold parkland subzone; c) two plant orders (one not previously described); d) 5 alliances (2 not previously described), and 19 plant associations (17 not previously described). These results contribute to the description of Site Associations, Site Orders and Site Alliances for the provincial Alpine classification. To develop the classification, I explored the interaction of vegetation with topographic and edaphic variables, and followed phytosociology methods to develop the nomenclature for the plant communities

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Mycorrhizal facilitation of kin recognition in interior Douglas-fir (Pseudotsuga menziesii var. glauca) (2013)

Insight into influences on successful seedling establishment could be essential to future regeneration of British Columbia’s interior Douglas-fir (Pseudotsuga menziesii var. glauca) forests, particularly as climate changes. Areas of harsh climatic conditions have low regenerative capacity and require management decisions leading to enhanced seedling establishment. Variable retention harvesting and natural regeneration from residual trees, for example, may become increasingly important for their locally adaptive traits as climate changes. Kin recognition, mycorrhizal networks, or the combination of the two may be important mechanisms for enhanced seedling establishment in these regions. We examined the effects of relationship (kin vs. non-kin) and mycorrhizal networks on regeneration from seed in greenhouse and field settings. In the greenhouse, kin recognition was evident in differing foliar microelement (Fe, Mo, Al and Cu) and growth variables (total leaf area, volume and stem length) according to relationships between seedlings. Kin recognition was also weakly evident in the field, where it was expressed as differential survivorship among kin versus non-kin seedlings. Kin selection was evident in the greenhouse, where microelement content of kin was greater than non-kin. Greater mycorrhizal colonization of kin compared to non-kin as well as greater donor total leaf area, volume and stem length also suggest kin selection, although not consistently in all experiments. In the field, survivorship was greater among non-kin; however, detection of kin recognition may have been masked by the large effects of site and seed origin on germination and survival. Mycorrhizal networks and carbon transfer occurred within all greenhouse seedling pairs, and enhanced mycorrhization of kin suggests network colonization was involved in kin selection, but our data does not strongly support our hypothesis that kin recognition was facilitated by mycorrhizal networks. While the mechanism of kin recognition is still not well understood, we provided evidence of kin recognition in interior Douglas-fir seedlings, particularly those that originate from harsh climates, and observed subtle indicators of kin selection or reduction of competition due to a close genetic relationship. Accounting for these phenomena in forest management could be helpful to successful regeneration of interior Douglas-fir forests as stresses associated with climate change increase.

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