Competition and facilitation over space and time: can N-fixing species increase forest

productivity?

Stephanie YelenikSteve PerakisDave Hibbs

What structures plant communities?

Competition: Negative interactions between individuals due to the simultaneous use of resources such as light, water, or nutrients

Facilitation: Positive interactions between individuals

The balance between competition and facilitation determines net interactions between species

Low ←resource availability → High

CompetitionFacilitation

Low ←stress→ High

Stress Gradient HypothesisThe balance between competition and facilitation will change over environmental gradients

The balance between competition and facilitation will also change over the lives of the plants

Plant Age

CompetitionFacilitation

How does the relationship between competition and facilitation

change across environmental gradients and over time?

+ = ?

Potential for facilitation:• Many Douglas-fir forests are

nitrogen limited• N accretion rates under red

alder are high(50-100 kg N/ ha yr)

NNN4+NH4

+

NO3-

Potential for competition:• Alder has higher growth rates than Douglas-fir

– Light limitation– Water limitation

Douglas-fir

red alder

Potential for change in net balance across environmental gradients:

• N – limitation of Douglas-fir forests changes across the Pacific Northwest

• Baseline soil N levels heterogeneous • Water availability can drive alder N-fixation rates

Low ←soil N availability → High

CompetitionFacilitation

Potential for change in net balance over time:• Alder quickly overtops Douglas-fir, but then its

growth rate slows down• Soil N levels under alder will increase over the life of

the stand

Plant Age

FacilitationCompetition

This is important because:• Both species are valued for timber• Positive effects on various ecosystem services

– Overall production– Carbon sequestration– Biodiversity through trophic levels– Ecosystem nutrient retention

• Would be of ecological and economic value to know where and when red alder/Douglas-fir mixtures are likely to increase yields

QUESTIONS:

Question 1: Does the NET outcome of competition and facilitation change across environmental gradients and over time in mixed stands of red alder and Douglas-fir?

Question 2: How does the provision of alder-derived N to Douglas-fir (facilitation) change across environmental gradients and over time?

Question 3: How does competition between Douglas-fir and red alder change across environmental gradients and over time?

0.5 0.5

1.0 1.0= 1

0.6 0.40.6 0.6= 1.2

Table 1: Descriptions of long-term study sites

H.J. Andrews Cascade Head

Longitude 122°10’W 124°00’W

Latitude 44°14’W 45°05’W

Elevation (m) 800 330

Soil Parent Material Andesite Basalt

Annual precipitation (cm) 230 250

Mean min. temperature (°C) -8.5 2.2

Mean max. temperature (°C) 26.9 20.9

Growing season (frost-free days) 134 180

Soil N concentration at 0 – 15cm (ppm) 1500 4200

Soil P concentration at 0 – 15cm (ppm) 1800 1600

HJ Andrews Cascade Head

RA 1.8 × taller than DF

RA 1.4 × taller than DF

RA 4.4 × taller than DF

RA 2.7 × taller than DF

Already evidence that species interactions shift across sites, and over time.

Question 1: Does the NET outcome of competition and facilitation change across environmental gradients and over time in mixed stands of red alder and Douglas-fir?

• Resample plots for tree height and dbh

Question 2: How does the provision of alder-derived N to Douglas-fir (facilitation) change across environmental gradients and over time?

• Track alder-derived N into Douglas-fir and soils with stable isotopes

• Assess how quickly red alder can increase soil N cycling rates

NH4+

NO3-

microbes

NH4+

NO3-

microbes

NNN4+

N-FIXER

litterfall rates

N mineralization rates

Question 3: How does competition between Douglas-fir and red alder change across environmental gradients and over time?

•Use tree rings and carbon and oxygen isotopes to look at competition for light, water, and nitrogen over the life of the experiment

Water use efficiency = [Carbon gained] per [Water lost] = [Photosynthesis] / [Stomatal conductance] = A/gs

WUE = A/gs ∝ δ13C

Changes in leaf-level physiology with different types of plant competition

Type of limitation

Change in A

Change in gs

Change in WUE

Nitrogenn/a

Light

Water

WUE = A/gs ∝ δ13C

gs ∝ δ18O

0 % 0 %

100% Red Alder100% Douglas-fir

Stan

d A

ge

LIMITING RESOURCE:N : nitrogenW: waterL: light

W

N

L

L

Soil N increases

Douglas-fir resource limitation

Soil

N in

crea

ses SITE:

H.J. Andrews (lower N, W; higher L) Cascade Head (higher N, W; lower L)

• Resource limitation tells us when facilitation most likely to be important

• Can relate tree growth in given years to resource limitation:When individuals show strong resource limitation, but do

NOT show changes in growth, we expect to find a strong signal of alder-derived N (facilitation).