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Examining the role of competition in ectomycorrhizal interactions

This study investigates the role of competition in ectomycorrhizal (ECM) interactions, including the mechanisms, competitive hierarchies, and effects on plant growth. The research is conducted in Point Reyes National Seashore, CA, focusing on Rhizopogon species. Results show asymmetry, priority effects, and corresponding plant responses. Findings highlight the importance of ECM in plant growth and suggest that competitive dominants may be the best symbionts. The implications for forestry are discussed, emphasizing the need for further research on competitive ability and timing of colonization.

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Examining the role of competition in ectomycorrhizal interactions

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  1. Examining the role of competitioninectomycorrhizal interactions Peter Kennedy NPER post-doctoral fellow - UC Berkeley pkennedy@berkeley.edu

  2. Competition: A major factor structuring natural assemblages • Typically highly asymmetric. • Competitive hierarchies are common. • Order of arrival can significantly effect outcome. Generalizations

  3. 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?

  4. Study System Point Reyes National Seashore, CA Bishop Pine (Pinus muricata) Rhizopogon spp.

  5. Mycorrhizal abundances at Pt. Reyes ECM Species Forest type 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.

  6. 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.

  7. 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

  8. R. occidentalis 0.4 RO single-species a RO two-species a 0.35 a 0.3 a a a 0.25 log (x+1) DNA yield 0.2 0.15 0.1 0.05 0 D ranch PPP 1B PPP 2B R. salebrosus a a a b a b Results: Fungi • Highly asymmetric • (i.e. mostly all or nothing) • Priority effect again observed • lab and field results = good correspondence

  9. What about the few co-colonized seedlings? a a R. salebrosus is not always the competitive inferior!

  10. Results: Plants Being mycorrhizal is very important in terms of growth, but ECM competition has little effect

  11. ECM Species Forest type Young Burned Mature Unburned (0-10 yr) (40-60 yr) Rhizopogon occidentalis Rhizopogon vulgaris Rhizopogon salebrosus Rhizopogon evadens Abundances based on Gardes and Bruns (1996), Horton et al. (1998), Taylor and Bruns (1999), Baar et al.(1999), and personal observation (T. Bruns). What about the natural pattern?

  12. Types of ECM Competition Time Exploitation Interference

  13. Competitive hierarchy R. vulgaris R. salebrosus R. occidentalis R. evadens R. evadens R. vulgaris R. occidentalis 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

  14. RV RS RO RE R. vulgaris (RV ) + + + (10/10) (7/10) (10/10) _ + + R. salebrosus (RS) (0/10) (9/10) (10/10) _ _ + R. occidentalis (RO) (3/10) (1/10) (10/10) _ _ _ R. evadens (RE) (0/10) (0/10) (0/10) Results: A Competitive Hierarchy!

  15. a ab bc c What about the plants? Seedling biomass Shoot Nitrogen a a b b The best competitors may be the best symbionts

  16. 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.

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

  18. 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?

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