Examining the role of

competition

in

ectomycorrhizal interactions Peter KennedyNPER post-doctoral fellow - UC

Berkeleypkennedy@berkeley.edu

Competition:

• Typically highly asymmetric.

• Competitive hierarchies are common.

• Order of arrival can significantly effect outcome.

A major factor structuring natural assemblages

Generalizations

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?

Study SystemPoint Reyes National Seashore, CA

Bishop Pine (Pinus muricata)

Rhizopogon spp.

Mycorrhizal abundances at Pt. Reyes

Forest type

Young Mature

Rhizopogon occidentalis

Rhizopogon vulgaris

Rhizopogon salebrosus

Rhizopogon evadens

(0-10 yr) (40-60 yr)

ECM Species

Rhizopogon is a dominant colonizer of seedlings in post-fire and primary

successional settings.

Lab Study Results

•Timing of colonization differed considerably between species

•Strong asymmetry and priority effects wereobserved

•Inoculation curves very similar between species

Kennedy and Bruns (2005), New Phyt. 166: 631-638.

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

Results: Fungi

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

D ranch PPP 1B PPP 2B

log (x+1) DNA yield

RS single-speciesRS two-species

R.

a

a

a

b

b

a

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

D ranch PPP 1B PPP 2B

log (x+1) DNA yield

RO single-speciesRO two-species

aa

a

a a

a

R. occidentalis

R. salebrosus

•Highly asymmetric(i.e. mostly all or nothing)

•Priority effect again observed

•lab and field results = good correspondence

What about the few co-colonized seedlings?

R. salebrosus is not always the

competitive inferior!

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

RO RS

Species

DNA Yield

a

a

Results: PlantsTable 3. Performance of Pinus muricata seedlings in the coastal scrub/grassland at Point Reyes National Seashore, CA, USA.Treatments are non-inoculated (NM), single-species Rhizopogon occidentalis (RO), single-species R. salebrosus (RS), andtwo-species (RO/RS). Values are means with one standard error in parentheses. Different letters indicate significantdifferences at P < 0.05.

Parameter Treatment

NM RO RS RO/RS

Survival 0.72(0.14) a 0.86(0.04) a 0.86(0.10) a 0.84(0.11) a(alive/total)

Total weight 0.14(0.03) a 0.254(0.03) b 0.212(0.02) b 0.206(0.02) ab(g)

Leaf Nitrogen 0.88(0.18) a 1.87(0.09) b 1.87(0.09) b 1.90(0.08) b(%)

Being mycorrhizal is very important in terms of growth, but ECM competition has little effect

What about the natural pattern?

Forest type

Young Burned Mature Unburned

Rhizopogon occidentalis

Rhizopogon vulgaris

Rhizopogon salebrosus

Rhizopogon evadens

(0-10 yr) (40-60 yr)

ECM Species

Abundances based on Gardes and Bruns (1996), Horton et al. (1998), Taylor and Bruns (1999), Baar et al.(1999),

and personal observation (T. Bruns).

Types of ECM Competition

Exploitation Interference

Time

Competitive hierarchy experiment

R. salebrosus

Competitive intransivity

•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

R. occidentalis

R. evadens

R. vulgaris

R. salebrosus

Competitive hierarchy

R. occidentalis

R. evadens

R. vulgaris

Results: A Competitive Hierarchy!

R. occidentalis(RO)

R. vulgaris(RV )

R. salebrosus(RS)

R. evadens(RE)

RV RS RO RE

+ + +

(0/10)

(3/10)

_

_

_

_ _

_

+ +

+

(10/10) (7/10) (10/10)

(9/10) (10/10)

(10/10)(1/10)

(0/10) (0/10)(0/10)

What about the plants?

Seedling biomass Shoot Nitrogen

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

RV RS RO RE

Species

Total Seedling Biomass (g)

aab

bc

c

0.00

0.20

0.40

0.60

0.80

1.00

1.20

RV RS RO RE

Species

Total Shoot Nitrogen

aa

b b

The best competitors may be the best symbionts

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.

Acknowledgements

• Sarah Bergemann, Sara Hortal, Tom Bruns

• Bruns lab members, UC Berkeley

• Point Reyes National Seashore

• National Parks Ecological Research Fellowship

• National Science Foundation

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?