Variable thinning using historical stand structure data to create fire-resilient forests and enhance

ecosystem services in a changing climateEric Knapp – USFS Pacific Southwest Research Station Roger Bales – Univ. of California MercedMalcolm North – USFS Pacific Southwest Research Station Matt Busse – USFS Pacific Southwest Research Station Scott Stephens – Univ. of California Berkeley

Background• Mixed conifer forest, Central Sierra Nevada, CA• Median historic fire return interval = 6 years, but no fire since 1889• Result: increased tree density, loss of structural heterogeneity, high

fuel loads, risk of uncharacteristically severe wildfire

1929 2008

Objectives

• Evaluate whether forest treatments designed to restore structural heterogeneity benefits a greater number of ecosystem services.

• Snow capture and melt-out date• Habitat for key old-forest associated wildlife species• Understory biodiversity• Natural regeneration of desired tree species• …while also being resilient to wildfire

• Is restoration of structure and stand resilience best accomplished with thinning, prescribed fire, or a combination?

Variable Density Thinning study - layout

Stanislaus-Tuolumne Experimental Forest

Thinning treatmentWith prescribed

fireHigh

VariabilityLow

variabilityUntreated

control

Without prescribed fire

High Variability

Low variability

Untreated control

Logging: Summer 2011 Prescribed burns: Fall 2013

Snow accumulation & melt data collection(Roger Bales, UC Merced)

• Winters of 2013-2016

California drought

20002002

20042006

20082010

20122014

2016

44.88

28.9835.2

42.42

28.91

55.9560.82

25.6626.97

43.0540.95

62.78

26.0330.37

25.0228.41

46.23

Ave. since 1922

CA Dept. of Water Resources

Year1940 1950 1960 1970 1980 1990 2000 2010 2020

Snow w

ater content (inches)

0

10

20

30

40

50

60

70Bell Meadow – snow survey

Snow Depth, DailyMet 2 Array, WY2014-16

Date

Oct-13 Apr-14 Oct-14 Apr-15 Oct-15 Apr-16 Oct-16

Sno

w D

epth

(mm

)

0

200

400

600

800

1000

D70 D72 F65F70F72

Snow Depth, DailyMet 1 Array, WY2014-16

Date

Oct-13 Apr-14 Oct-14 Apr-15 Oct-15 Apr-16 Oct-16

Sno

w D

epth

(mm

)

0

200

400

600

800

1000

V8V9X9X11

Elev. 1735 m

Elev. 1845 m

Snowfall October 2013 – October 2016

Snow survey – spring 2013At peak accumulation (March 7 survey) the high variability treatment had accumulated, on average, 9 and 3 cm more snow than the control and even thinning treatments, respectively.

By the March 21 survey approximately 86% of the sites had already melted out. The variable treatment retained the most snow followed by the control, then the even units.

M. Pickard thesis – completed Dec. 2015Paper draft in progress

Vegetation and fuels data collection• Summer 2012 – post logging, pre prescribed burning

• Summer 2014 – first full post-treatment assessment

• Summer 2016 – final field data collection

Results – tree density

Variable thin with prescribed fire

Diameter class (cm)10-20 20-30 30-45 45-60 60-75 75-90 >90

Trees ac-1

0

100

200

300

400PrePost thinning and burning1929 Reference

Control with prescribed fire

Diameter class (cm)10-20 20-30 30-45 45-60 60-75 75-90 >90

Trees ac-1

0

100

200

300

400PrePost-burning1929 Reference

Prescribed fire: 7% of trees, 2% of basal area killedThin: 77% of trees, 44% of basal area removed

Results – structural heterogeneityTree density

Sample area (m2)150 225 450

CV

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 Control High variability thinLow variability thin1929 reference condition

Results: shrub species richness(3 yrs post-thinning, 1 yr post burn)

©2013 Debra L. Cook

Ceanothus parvifolius

Shrub richness

Treatment

Species per 25m

2

0

1

2

3

4

5

C HV LV C HV LV No Burn Burn

Summary of early findings• Variable thinning produced within-stand heterogeneity closest to

historical reference condition• Prescribed fire alone did not kill enough trees to substantially

influence heterogeneity• Combined thinning + prescribed burning produced strongest

understory vegetation response• Benefits of variable thinning to physical and ecological response

variables are subtle at this early stage• Slight (non-significant) increase in snow accumulation and snow retention

Proximity to Rim Fire plus drought/ drought-induced tree mortality has focused attention on the research project• Rim Fire (2013) was the largest in modern history in the Sierra Nevada and

came within 5 miles of the study area• 66+ million trees die from bark beetles – ongoing

Success best measured by socio-political impactYosemite Stanislaus Solutions - Collaborative of diverse stakeholders from timber industry to environmental groupsJune 26 and July 24, 2015

YSS/TuCARE (Tuolumne County Alliance for Resources and the Environment)

Oct. 6, 2016

• Strong need for action• Forest management very polarized• Variable density thinning – agreement among diverse groups of stakeholders

Future opportunities• Snow accumulation, melt, soil moisture in high snow-fall winter• Do treatments improve resilience to drought-induced bark beetle

mortality?