Ecosystem Responses to Mountain Pine Beetle & Management in Colorado Forests

Chuck Rhoades, Kelly Elder & Rob Hubbard USFS Rocky Mountain Research Station Fort Collins, CO

Western Water Assessment Beetle-Water Workshop Salt Lake City, UT 1 December 2011

(aka: Hydrologic Implications of Beetle-Related Management and Composition of Post-Infestation Forests)

Watershed Change

Responses Regulated by Change in Canopy interception & Snowpack accumulation Water uptake & Soil nutrient use

Complicating Factors Responses may lag, difficult to detect, prolonged Complex spatial & temporal patterns

Timing

Mag

nit

ud

e

Harvest MPB Mortality

Photo: Bill Romme

Yellowstone Area ‘60 & ‘70s About 40-70% of the overstory trees died

Surviving trees increased growth by 2-3 fold for two decades

Romme et al. 1986

Stem Volume

Increment

Previous Outbreak Forest Growth Response

Overstory Mortality in Colorado

Pine losses 80-90% of basal area

Residual live trees 15-35% of stand BA

24 pine-dominated stands Trees >10 cm DBH

Yampa RD, MBR NFBlacktail Project

Bas

al A

rea

(m2

ha-1

)

0

10

20

30

Sulphur RD, AR NFUpper Fraser Project

Bas

al A

rea

(m2

ha-1

)

0

10

20

30

LodgepoleAspen Fir

Spruce

Colorado State ForestCuster/Big Meadows

0

10

20

30

MPB-KilledLodgepole

Parks RD, MBR NFWillow Crk Project

0

10

20

30

LodgepoleAspen Fir

Spruce

Overstory 310 t/ha (126 t/acre) 71% LPP; 17% AS; 7% SF

Understory Trees

445 t/ha (180 t/Ac) 68% LPP; 12% AS; 15% SF

New Recruits

1820 t/ha (736 t/Ac) 54% LPP; 19% AS; 25% SF

*Stocking Levels 370 t/ha (150 t/Ac)

Growing Stock in MPB Forests Residual Live & New Trees

Den

sity

(ste

ms

ha-1

)

0

200

400

600

800

LodgepoleAspen Fir

Spruce

Den

sity

(ste

ms

ha-1

)

0

400

800

1200

1600

Live UnderstoryLive Overstory

New Seedlings

Live UnderstoryLive Overstory

35% of trees grew > 25% faster since the infestation

16% of trees grew faster than ever.

Unrelated to precipitation

Decline in basal area explained 10-20% of response

*Assessed 123 cores in 4 basins

Lodgepole Pine

0

1000

2000

3000

Englemann Spruce

Rin

g W

idth

(µm

)

0

1000

2000

3000

1900 1920 1940 1960 1980 20000

1000

2000

3000Post-Harvest

Release

Post-MPBRelease

Overstory Growth Response

Annual height growth of Fir & Pine has doubled since infestation beneath the dead overstory, but neither has responded in cuts.

Subalpine Fir

Understory Growth

Engelmann Spruce

0

10

20

30

40

Subalpine Fir

2007 Height Growth (cm)0 10 20 30 40

0

10

20

30

40

Lodgepole Pine

2010

Heig

ht G

rowt

h (cm

)

0

10

20

30

40

Reduction in Basal Area (%)0 20 40 60 80 100

Heig

ht In

crem

ent (

cm)

0

10

20

30

40

40% of trees added > 2X more height in ‘10 as in ’07. Proportionally, fir was most likely to double height; spruce was least likely. Loss of basal area explains 13 - 23% of height increment. Pine most sensitive to BA; spruce least sensitive.

Understory Growth

Understory Trees May Reduce Transpiration Loss

100 yrs 10 yrs

Understory tree growth & water use increase after overstory mortality Young trees use more water for a given amount of needles than old trees.

Month

2 4 6 8 10 12

Sta

nd T

rans

pira

tion

(mm

)

0

5

10

15

20

25

30

35

Pre MPBPost MPBPost MPB with Understory Release

2) Water use by understory returns 20% of savings

Pre MPB

Post MPB

How Will MPB Influence Stream Flow? Understory may Change Outcome

Transpiration savings from dead LPP

1) Reduced Annual transpiration by 60%

R. Hubbard, unpublished

Part 2 - Management Implications

CO State Forest Willow Ck, Parks RD Gore Pass, Yampa RD Fraser Expt Forest

Harvest vs. Retain?

Specific harvesting practices

1910

- 191

9

1920

- 192

9

1930

- 193

9

1940

- 194

9

1950

- 195

9

1960

- 196

9

1970

- 197

9

1980

- 198

9

1990

- 199

9

2000

- 200

9

Hec

tare

s

0

1000

2000

3000

4000

Single Tree/Group Selection

Shelterwood/Seed Tree

Clearcut

Management Response to MPB

Arapaho-Roosevelt NF, Colorado Most harvesting since 1970s Greatest extent of clear cutting

However: <50% of infested area is treatable;

of that < 30% will be cut 90% of infested area will be untreated

Post-Harvest Recruitment Are there concerns about seedling colonization after harvest of MPB stands? Since the outbreak, pine recruitment has been at least equal to previous decades > 90% of units meet minimum stocking requirements

(Collins et al. 2010a)

Pre-Outbreak Outbreak

Seed

ling

Rec

ruits

(tre

es h

a-1)

0

5000

10000

15000

(1980s) (2000s)

Compared USFS stocking surveys in pre- and post-outbreak harvest units. n = 30 stands; 3rd yr surveys ; AR NF; Sulphur RD

Species Composition of Recruits

Harvesting stimulates new pine seedlings and aspen sprouts. 5 times more pine, aspen compared to uncut stands Fir recruitment is promoted in uncut stands

*Cut stands meet minimum stocking requirements (i.e., > 150 t/acre) *8 paired sites at Fraser

(Collins et al. 2010b)

Years Since Treatment0 50 100 150 200

Bas

al A

rea

(m2 h

a-1)

0

10

20

30

40

Bas

al A

rea

(m2 h

a-1)

0

10

20

30

40

Lodgepole PineSubalpine FirEngleman SpruceQuaking Aspen

UntreatedForests

HarvestedForests

Forest Recovery - MPB-killed stands recover to

pre-MPB stand structure in a century

Uncut & Partial Cut Stands Dominated by fir Clear Cut Stands Similar to pre-MPB stands Dominated by pine

Years Since Treatment0 50 100 150 200

Bas

al A

rea

(m2 h

a-1)

0

10

20

30

40

Bas

al A

rea

(m2 h

a-1)

0

10

20

30

40

Lodgepole PineSubalpine FirEngleman SpruceQuaking Aspen

UntreatedForests

HarvestedForests

(Collins et al. 2010b)

Stand Dynamics Future Species Composition

Harvesting adds ~4X fine fuels (1 + 10 hr) ~3X total surface fuels The increase in surface fuels may result in greater flame lengths (i.e., under extreme weather conditions: 2.3 vs 1.7 m compared to 5m). 1.2 m - halt direct-attack 2.5 m - halt dozers Windthrow will increase the surface load in uncut areas ~1.4x higher than cut areas

1 Hr10 Hr

100 Hr1000 Hr

Total Woody

Post Windthrow

Fuel

Loa

ding

(Mg

ha-1

)

0

20

40

60

80 Uncut Cut

Response to Management Surface Fuels

Recovery of the forest canopy determines fire behavior Risk of crown fire is low and will differ little between treated & uncut stands until crown develops (~20 yrs). More fir in uncut stands = increases canopy BD, crown base height and flame length. Green Stands – Greater risk, intensity of crown fire: *6m total flame length *Crowing Index 55 km/hr (34 mph – moderate risk)

CanopyBulk Density

Bul

k D

ensi

ty (k

g m

-3)

0.00

0.05

0.10

0.15

0.20

0.25

UncutCut

CrowningIndex

Years0 20 40 60 80 100

Win

d S

peed

(km

hr-

1 )

0

30

60

90

120

150

TotalFlame Length

Flam

e Le

ngth

(m)

0

5

10

15

UntreatedHarvested

*

*

*

Response to Management Fire Behavior

(Collins et al. under review)

Previous MPB Outbreak White River NF 1980s

Recovery in uncut stands depends on pre-outbreak composition Lodgepole Stands

62% aspen, 27% fir, 7% lodgepole Mixed Conifer Stands

87% fir, 7% spruce, 3% aspen, 3% lodgepole

Downed Wood 70 Mg/ha in affected stands

Mostly large diameter Highly variable (0.5 to 314Mg/ha)

(Pelz, 2011 MS Thesis)

No Action Untreated Beetle-Killed Stands

Management Alternatives on MPB Acres

Water Delivery Lop and Scatter Slash Retention

Fuel Reduction Whole Tree Harvest

Forest Regeneration Mechanical Scarification Site Prep

Uncut

Lop S

catte

r

Who

le Tr

ee

Scarif

yUnc

ut

Lop S

catte

r

Who

le Tr

ee

Scarif

yUnc

ut

Lop S

catte

r

Who

le Tr

ee

Scarif

y

Gra

vim

etri

c M

oist

ure

(%)

5

10

15

20

25

30

35

40Spring Fall

p < 0.1

Summer

p < 0.05

Soil moisture was highest in slash retention treatment

Scarification

driest cut option

Response to Management Options Soil Moisture

Uncut

Lop S

catte

r

Who

le Tr

ee

Scarif

yUnc

ut

Lop S

catte

r

Who

le Tr

ee

Scarif

yUnc

ut

Lop S

catte

r

Who

le Tr

ee

Scarif

y

Ext

ract

able

N (m

g/kg

)

0

2

4

6

8

10

12Spring FallSummer

p < 0.05NH4+

NO3- p < 0.01

p < 0.05NH4

+p < 0.1NH4

+

Soil N was highest in slash retention treatment

Ammonium (NH4)

35% to 2.5X > uncut 20 - 30% > Whole Tree

Nitrate (NO3) 1.3 to 5.2 fold > uncut

Cut vs Uncut

3 to 6 fold increase *Extractable Soil N (0-15 cm mineral

soil)

Response to Management Options Soil Nitrogen Fertility

Greater in lop & scatter than other treatments (p = 0.1)

74% survival overall commonly greater on scarified plots (i.e., in 5 of 6 plots).

Uncut

Lop Scatter

Whole TreeScarify

Hei

ght G

row

th (c

m)

3.0

3.5

4.0

4.5

5.0

5.5

a 

bc 

c 

b 

Response to Management Options Seedling Height Growth

Seedling Occurrence Whole Tree 58% of plots

Scarification 50%

Lop and Scatter 33%

Uncut 42%

Harvested areas were dominated by pine seedlings and aspen sprouts (i.e., 80-100% of recruits)

Uncut stands were dominated by fir and spruce

Seedling density: 9 – 18 k seedlings/ha Adequately stocked units require 370 trees/ha

Response to Management Options Seedling Establishment

Take Home Messages

1.  Tree regeneration is abundant in beetle-infested stands 2.  Growth of residual overstory & understory trees are responding to loss of lodgepole 3.  Harvesting leads to development of different stand types - with likely implications on future fire potential and effects 4.  Slash Retention (Lop and Scatter) has positive effect on soil resources and seedling growth; Reduced colonization of new seedlings

Passive Crown Fire

Active/Conditional Crown Fire

Active/Conditional Crown Fire

Passive Crown Fire

Harvested

Years0 20 40 60 80 100

Fire

Typ

eUntreated

0 20 40 60 80 100

Fire

Typ

e

Surface Fire

Surface Fire

Percentile Weather

Years

Can

opy

Bul

k D

ensi

ty (k

g m

-3)

0.00

0.05

0.10

0.15

0.20

Can

opy

Bul

k D

ensi

ty (k

g m

-3)

0.00

0.05

0.10

0.15

0.20

CBD

97th90th80th50th