Examining the role of competition in ectomycorrhizal interactions

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Examining the role of competitioninectomycorrhiz
al interactions

• Peter Kennedy
• NPER post-doctoral fellow - UC Berkeley
• pkennedy_at_berkeley.edu

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Competition
A major factor structuring natural assemblages
Generalizations

• Typically highly asymmetric.
• Competitive hierarchies are common.
• Order of arrival can significantly effect outcome.

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Research questions

• 1. How is ECM competition structured in the
  field?
• What are main mechanisms by which it occurs?
• 2. Are there competitive hierarchies among ECM
  fungi?
• If so, are competitive dominants also better
  symbionts?

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Study System
Point Reyes National Seashore, CA
Bishop Pine (Pinus muricata)
Rhizopogon spp.
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Mycorrhizal abundances at Pt. Reyes
Forest type
ECM Species
Young
Mature
(0-10 yr)
(40-60 yr)
Rhizopogon occidentalis Rhizopogon
vulgaris Rhizopogon salebrosus Rhizopogon
evadens
Rhizopogon is a dominant colonizer of seedlings
in post-fire and primary successional settings.
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Lab Study Results

• Timing of colonization differed considerably
  between species
• Strong asymmetry and priority effects were
• observed
• Inoculation curves very similar between species

Kennedy and Bruns (2005), New Phyt. 166 631-638.
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Field competition experiment

• Two species R. salebrosus (RS) and R.
  occidentalis (RO)
• Treatments no inoculum, single species, two
  species
• 20 replicates/treatment at three sites
• Harvested seedlings after 5 and 10 months
• Analyzed ECM root tips with real-time PCR

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Results Fungi

• Highly asymmetric
• (i.e. mostly all or nothing)
• Priority effect again observed
• lab and field results good correspondence

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What about the few co-colonized seedlings?
a
a
R. salebrosus is not always the competitive
inferior!
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Results Plants
Being mycorrhizal is very important in terms of
growth, but ECM competition has little effect
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What about the natural pattern?
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Types of ECM Competition
Time
Exploitation
Interference
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Competitive hierarchy experiment

• Four species Rhizopogon vulgaris (RV), R.
  salebrosus (RS), R. evadens (RE), R. occidentalis
  (RO)
• All pair-wise and one three-way combination
  (RO/RV/RS)
• Spores of competitors added at the same time (106
  spores/species)
• 10 replicates/treatment grown for 8 months
  (growth chamber)
• Harvested all seedlings and root tips analyzed
  with real-time PCR

Competitive intransivity
R. salebrosus
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Results A Competitive Hierarchy!
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What about the plants?
Seedling biomass
Shoot Nitrogen
a
a
b
b
The best competitors may be the best symbionts
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Applications for Forestry

• Mixed species ECM inoculations may not be
  effective - competitive interactions are strong
  and highly asymmetical.
• Differences in timing of spore germination may
  affect seedling ECM composition - priority
  effects observed in both lab and field studies.
• More studies are necessary to determine
  competitive ability - for Rhizopogon, competitive
  dominants appear to be the best symbionts.

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Acknowledgements

• Sarah Bergemann, Sara Hortal, Tom Bruns
• Bruns lab members, UC Berkeley
• Point Reyes National Seashore
• National Parks Ecological Research Fellowship
• National Science Foundation

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Ongoing experiments

• Do spore- vs. mycelial-based ECM competition have
  different outcomes?
• Can priority effects be reversed by altering the
  timing of colonization?
• Does ECM competition occur mainly through direct
  or indirect interactions?
• Does spore density and soil heating affect the
  outcome of ECM competition?