Bill ParkerResearch ScientistOntario Forest Research Institute

Can we improve them?

COMPETITION RULESCOMPETITION RULES……

• de Crescentis (1305): advocates thinning to improve residual tree growth.

• Malthus (1798): plant competition as it affects human society through food supply

• Darwin (1859): competition as a selective factor in “The Origin of Species”

• Clements (1905): first comprehensive experimental examination of competition in “Research Methods in Ecology”

• Clements et al. (1929): “Plant competition: An analysis of community functions”

A Long History of Writings on Plant Competition

From: Lee and Kirby (1963) Amazing Fantasy 15: 1-6.

Outline of presentation

1. Principles of interspecific competition (“Some general rules”)

2. Competition indices and their capacity to predict current and

future growth (“Some more general rules”)

3. Using 45% sunlight as a threshold for management of white pine

4. Myths surrounding vegetation management and white pine

Effects of competing vegetation on crop trees

• Reduced availability of growing space and resources

• Altered physical environment (site and seedling energy

balance, temperature, humidity, wind)

• Facilitate / diminish herbivory, mycorrhizal colonization

• Vectors for pathogens (blister rust, Coleosporium asterum)

• Allelopathy (chemical growth inhibition)

• Mechanical damage

• Effects may be positive or negative

PART 1: General principles of plant competition

Interspecific competition has two primary components:

1. the effect of plant species on resource supply2. the response of species to resource supply

Environment

Resource

Plant B

EFFECT EFFECT

RESPONSERESPONSE

Plant A

PLANT ABUNDANCE

RES

OU

RC

E AV

AILA

BIL

ITY

(%) EFFECT

RESOURCE AVAILABILITY

PLAN

T R

ESPO

NSE

RESPONSE

General form of common competition effect and response relationships

Example: Light as a resource

EFFECT RESPONSE

PPFD (umol m-2 s-1)

0 500 1000 1500 2000

Net

pho

tosy

nthe

sis

(um

ol C

O2

m-2

s-1

)

-2

0

2

4

6

8

10

12

14

Fir

Hazel

Fern

Overstory

Cover of overtopping vegetation (%)

0 50 100 150 200 250

Mea

n da

ily s

unlig

ht (%

)

0

20

40

60

80

Relationship of 5th year seedling height and diameter with foliar N content for white pine growing in a clearcut (McConnell Lakes).

Example: Nitrogen as a resource

Leaf N (%)

1.2 1.4 1.6 1.8 2.0 2.2 2.4

Seed

ling

diam

eter

(mm

)

10121416182022242628

cncvr vs cnn

Leaf N (%)

1.2 1.4 1.6 1.8 2.0 2.2 2.4

Seed

ling

heig

ht (c

m)

40

60

80

100

120

140

160

180

Relationship of needle conductance to soil moisture content as affected by competition control for white pine growing in a clearcut(McConnell Lakes)

Example: Water as a resource

Soil moisture content

0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

Nee

dle

cond

ucta

nce

(mm

ol m

-2 s

-1)

0

50

100

150

200

250

300

350NoneBothHerbaceousWoody

“Competition for resources increases with site productivity”

Hypothetical relationship of light and soil resource availability with site productivity

PRODUCTIVITY

LIG

HT

AVA

ILAB

ILIT

Y

NO VEGETATION

WITH VEGETATION

PRODUCTIVITY

SOIL

RES

OU

RC

E A

VAIL

AB

ILIT

Y NO VEGETATION

WITH VEGETATION

“Competitive capacity and effects on resource supply to crop trees differ among plant growth forms”

Competition for light, water, nutrients, and space

•Overtopping crowns•Large root systems

Overstory trees

Competition for light, sometimes water, nutrients

•Overtopping crownsTall shrubs, saplings, midstory trees

Competition for light, sometimes water, nutrients

•Dense cover•High shoot growth rate

Low shrubs

Competition for light, water, and nutrients

•Dense cover•High shoot growth rate

Herbs

Competition for water, nutrients, and soil space

•Dense root systems, short shoots•High uptake and growth rates

Graminoids

Main effectsMain traitsGrowth form

“Competitive effects of different plant functional groups differ with density and time”

Quantification of competition effects on biomass production

For a given resource (R):

Biomass = R supply * proportion R captured * efficiency of R use

Resource Amount captured

- water - transpiration

- light - light interception (LAI)

- nutrients - foliar nitrogen content

Efficiency = (C fixed / R used)

e.g., Water use efficiency = (Net photosynthesis / transpiration)

Two primary strategies for competition-induced resource limitation:

1. Maximize uptake and growth (“preemptive strike”)

2. Conservation (“reduce, reuse, recycle”)

“Tree growth is determined by the interaction of light, water, and nutrient supply, but light availability is generally the dominant factor.”

“Water and nutrient have greatest influence under high light.”

Seedling and sapling mortality under heavy shade

• “Probability of mortality is higher for pioneer species and smaller

individuals of a given species”

• “Trees predisposed to mortality under heavy shade undergo radial

growth decline over several years”

• “Shade tolerance decreases as size increases”

- Aw, By, Or, Mh, Ms,

Bf, Ce, Sw, Po, Bw

Radial growth rate

Prob

abili

ty o

f mor

talit

y (%

)High

LowLow High

Annual mortality rates of white pine is higher in smaller seedlings (Wharncliffe)

Time (yr)

1 2 4 5 6 8 10 11

Ann

ual m

orta

lity

(%)

0

2

4

6

8

Ht > meanHt < mean, Ht > 25 percentileHt < 25 percentile, Ht > 10 percentileHt < 10 percentile

Control plots

Time (yr)

1 2 4 5 6 8 10 11

Ann

ual m

orta

lity

(%)

0

2

4

6

8Release plots

11313.750.43Dead in 2004

33337.351.38Living in 2004

(mm yr-1)

NRelative diameter growth rate

(10 yrs)

Annual diameter increment

(10 yr)

White pine that suffer shade-induced mortality show long term radial growth reductions (1994-2003)

Relevant operational variables to define competition and response/effect to changes in resource availability

Space occupied

Spatial arrangement

NutrientsMortalityProximity

Water Vigour (increment, RGR)Size

LightGrowthAbundance

ResourceResponseCompetition

PART 2: Quantifying competition and its effects on growth using competition indices

Measures of Competition

Abundance: percent cover (C), biomass, leaf area, density

Size: height (H), crown area

Proximity: distance from crop tree (P), crown overlap

* Based on horizontal point sampling

“Crop tree growth decreases as abundance, size, and proximity of competing vegetation increase”

Competition index

Rel

ativ

e gr

owth

rate

High

LowLow High

A. Vertical point sampling to quantify cover of overtopping vegetation (accounts for proximity and height of this competition)

B. Horizontal point sampling to quantify cover of encroaching and ground vegetation within circular plot w/ radius determined by longest lateral branch

An alternative approach to quantification of competing vegetation

Using regeneration survey data to develop competition rules

Competition indices (CI) developed using combinations of these measures can be highly predictive for given species, site, silvicultural system, etc.

• CI = Sum of (C * Hcomp )/H crop) for all species within 1.3 m radius of crop tree (spruce plantations, sc BC). (Comeau et al. 1993)

CI’s are often strongly correlated with light availability and provide good measures of shade or light interception by overtopping competition

• LI = Sum of (C * (Hcomp / P)) all competing species (Sw plantations, nc BC) (DeLong 1991)

Use of competition indices to identify thresholds for maximizinggrowth, minimizing mortality, etc.

Competition index

Rel

ativ

e gr

owth

rate

High

Low

Low High

Threshold for growth, e.g. 30% reduction in height growth

PART 3: Using light threshold targets for managing white pine inclearcuts and shelterwoods

Logan (1965 – 1973)

Percent sunlight

0 20 40 60 80 100

Seed

ling

heig

ht (c

m)

0

50

100

150

200

250

Pj

By

MhPwPr

Percent sunlight

0 20 40 60 80 100

Seed

ling

heig

ht (%

of m

axim

um)

0

20

40

60

80

100

PjPwMhByPr

Target light (45-50%) achieved through manipulation of percent cover of overtopping competition (clearcut – McConnell Lakes)

Cover at mean seedling height (%)

0 20 40 60 80 100 120

Mea

n da

ily s

unlig

ht (%

)

0

20

40

60

80

100

120

2002200320042005

Y = 92.3 - 0.71 X, r2 = 0.72

Shelterwood used to achieve light target in understory

Seasonal variation in understory light levels in clearcut and shelterwood in absence of competing vegetation (McConnell Lakes)

D ay

140 160 180 200 220 240 260 280 300

Mea

n da

ily P

FD (m

ol m

-2)

0

10

20

30

40

50

60

70

C learcutShelterw ood

M ay June July August Septem ber O ctober

Percent cover or light interception by shelterwood canopy must be accounted for in meeting light threshold for maximum height.

Cover at mean seedling height (%)

0 20 40 60 80 100

Mea

n da

ily s

unlig

ht (%

)

0

10

20

30

40

50

60

70

80

200320042005

Y = 50.0 - 0.41 X, r2 = 0.62

Modify shelterwood density and understory competition control to achieve light target

Heavier cutting to reduce level of overtopping vegetation control in understory?

Basal area (m2 ha-1)

0 10 20 30 40 50 60

Perc

ent s

unlig

ht

0

20

40

60

80

100 Bba vs Bpfd Cba vs Cpfd FRba vs FRpfd MCba vs MCpfd Nbba vs NBpfd Pnba vs PNpfd Ogba vs Ogpfd Rbba vs RBpfd Thba vs THpfd

Crown closure (%)

0 20 40 60 80 100

Perc

ent s

unlig

ht0

20

40

60

80

100

Using light target as a basis for competition rules for growing white pine using shelterwoods.

Understory light, understory cover, and white pine growth (Wharncliffe)

Cover of overtopping vegetation (%)

0 50 100 150 200 250

Mea

n da

ily s

unlig

ht (%

)

0

20

40

60

80

Mechanical release that decreased percent cover of overtopping vegetation increased understory light and improved growth, vigor and size of white pine regeneration, but……..

Is current light level predictive of future growth?

Mean daily sunlight (%)

0 20 40 60 80

Rel

ativ

e di

amet

er g

row

th ra

te

0

2

4

6

8

10

ControlRelease

Mean daily sunlight (%)

0 20 40 60 805-ye

ar a

vera

ge h

eigh

t inc

rem

ent (

cm y

r-1 )

0

10

20

30

40

50

Yes! Higher light in 2001 correlated with higher relative growth over the next 3 years

Mortality

H: 0.0%

M: 13.3%

L: 58.2%

(H: light > 45%, M: light 25 to 45%, L: light < 25%)

H M L

Rel

ativ

e di

amet

er g

row

th (%

)

05

1015202530354045

H M L

Rel

ativ

e he

ight

gro

wth

(%)

0510152025303540455055

Understory light level

H M LAve

rage

hei

ght i

ncre

men

t (cm

yr-1

)

0

5

10

15

20

25

30

35

Understory light level

H M L Ave

rage

dia

met

er in

crem

ent (

mm

yr-1

)

0

1

2

3

4

a

b

c

a

b

c

a

b

c

a

b

c

Are relative height differences between crop tree and competition predictive of future growth? Yes!

Mortality

FTG: 11.9%

O: 14.3%

T: 25.5%

S: 68.9%

Pw ht / Comp ht: (FTG: > 1.25, O: 1 to 1.25, T: 0.5 to 1.0, S: < 0.5)

FTG O T S

Rel

ativ

e he

ight

gro

wth

(%)

0

20

40

60

80

100

120

FTG O T S

Rel

ativ

e di

amet

er g

row

th (%

)

0

20

40

60

80

100

H eight c lass

FTG O T S

Ave

rage

hei

ght i

ncre

men

t (cm

yr-1

)

0

5

10

15

20

25

30

H eight c lass

FTG O T S Ave

rage

dia

met

er in

crem

ent (

mm

yr-1

)

0 .0

0 .5

1.0

1.5

2.0

2.5

3.0

3.5

a

a

b

c

a

b

c

d

aa

b

c

a

b

c

d

Myth: Control of herbaceous and ground vegetation competition is lessimportant to crop tree growth than is control of overtopping vegetation

PART 4: The cat in the microwave and other urban legends – common myths concerning forest vegetation management

“Herbaceous vegetation is a strong competitor for soil moisture early in stand development”

Seasonal variation in soil moisture content in a white pine plantation as affected by competition control

Day

120 140 160 180 200 220 240 260 280 300

Soil

moi

stur

e co

nten

t (%

)

468

101214161820222426

BothNoneHerb.Woody

Year 1

May June July August Sept. Oct.

Effect of competition control treatments of foliar nutrient content of white pine in a clearcut

CN - no controlH - herbaceous onlyW - woody onlyB - woody and herbaceous

“Herbaceous vegetation is a strong competitor for soil nutrients”

CN H W B

Leaf

N (%

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Competition control treatment

CN H W BLe

af C

a (g

kg-1

)0.0

0.5

1.0

1.5

2.0

2.5

3.0CN H W B

Leaf

P (g

kg-1

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Competiton control treatment

CN H W B

Leaf

K (g

kg-1

)

02468

1012141618

a

bab ab

a

b

bb

a

b b b

a

bb b

Myth: Hardwood competition benefits white pine by acting as a nurse crop

“The best shelter in wet soil is poplar or aspen, and in dry soil, Rhus, for the growth of oak. One need not fear that the sumac, aspen orpoplar can injure the oak or birch. After the latter have passed thefirst few years in the shade and shelter of the others, they quickly stretch up and suppress all the surrounding plants.” (Buffon 1742)

Woody vegetation provides shade but becomes a strong competitor for soil moisture.

Soil

moi

stur

e co

nten

t (%

)

468

1 01 21 41 61 82 02 22 42 6

D a y

1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0 2 4 0 2 6 0 2 8 0 3 0 0

Soil

moi

stur

e co

nten

t (%

)

468

1 01 21 41 61 82 02 22 42 6

B o thN o n eH e rb a c e o u sW o o d y

2 0 0 1

2 0 0 3

M a y J u n e J u ly A u g u s t S e p t. O c t.

Providing overhead shade and weevil control with a woody nurse crop in clearcuts will require herbaceous vegetation control to avoid growth losses.

Clearcut (year 5) (McConnell Lakes):

Treatment Height (cm) Volume (cm3)

No control 68 (60%) 32 (19%)

H for 2 yr 105 (93%) 125 (73%)

H for 4 yr 113 171

* ~ 9,500 seedlings ha-1 and 94% stocking 5 years before release treatment (Wharncliffe)

Myth: White pine is a mid-tolerant species and will eventually push through understory vegetation and recruit into canopy.

Mortality

Release: 26.1%

Control: 73.0%

Year after release

0 1 2 3 4 5 6 7 8 9 10 11

Tota

l hei

ght (

cm)

0

20

40

60

80

100

120

140

160

Control y = 17.63 + 5.66x - 0.163x2, r2 = 0.99

y = 13.98 + 10.35x + 0.138x2, r2 = 0.99Release

Change in status of white pine natural regeneration in a shelterwoodfollowing a single mechanical release at age 5 years (Wharncliffe)

1729.69.964.026.111

1449.49.780.010.310

11812.412.780.07.48

8921.535.357.47.46

(b) Release

720.30.0326.773.011

600.30.356.842.910

550.31.285.912.98

482.18.581.79.76

(cm)(%)(%)(%)(%)(a) Control

Mean heightFTG*OpenOvertoppedDead Years since release

* FTG refers to trees > 100 cm in height and not overtopped by competition within 1 m radius

Myth: Vegetation management treatments can be delayed without significant growth losses.

Shelterwood (year 4) (McConnell Lakes)

Treatment Stem volume (cm3)

No control 18 (15%)

H, W @ yr 2 48 (41%)

CSP, W @ yr 2 93 (80%)

H, W over 4 yr 117

Summary

• Light is primary limiting resource in GLSL forests

• Competitive strength varies with plant type and time

• Shade-induced mortality higher for smaller seedlings

• Competition indices provide a useful tool to identify thresholds

and need for future treatments

• Target light levels to drive height growth of white pine

regeneration can be achieved by controlling percent cover of

overtopping vegetation

• Incorporate new knowledge into operational practice

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