Harold S.J. Zald and Andrew N. GrayUSDA Forest Service, Pacific Northwest Research Station

3200 SW Jefferson Way, Corvallis, OR 97331

Tree Regeneration Strategies in Response to Burning and Thinning Restoration Treatments in a Sierran

Mixed-Conifer Forest

•Historic fire return interval 12-17 years•Fire suppression has been a dominant management strategy in Sierran mixed- conifer forests on public lands

Generalized impactsof fire suppression

Increased stand densities Increased ladder fuels

and fuel loadings

Reduced fire frequency Increased fire severity

Increased dominance of fire intolerant and shade tolerant species (firs and

incense-cedar)

Fire History and Suppression in Sierran Mixed-Conifer Forests

Historical Old-Growth Current High Density Stand

•Regeneration dynamics a major driver of future composition, structure and function

•Seeding germination and establishment is a highly sensitive life history stage

•Restoration treatments may impact future regeneration patterns

Forest Regeneration: Species Strategies

P. jeffreyiP. lambertiana A. concolor & C. decurrensA. magnifica

Study Objectives

Past: Pretreatment regeneration composition and abundance

Pretreatment regeneration with respect to environmental conditions

Pretreatment environmental conditions

Present: Treatment mortality and subsequent response

Germinant success in relation to treatments

Post-treatment regeneration with respect to environmental conditions

Future: Treatment effects on environmental conditions

Study Area: Teakettle Experimental Forest

Fresno

California Teakettle Experimental Forest

Forest BoundaryRoadsStreamsStructures

•Full factorial design contrasting two levels of burning and three levels of thinning treatments•Burn treatments: no burn (U), and understory burn (B)•Thinning treatments: no thinning (N), “CASPO” understory thinning (C), and overstory shelterwood thinning (S)•Each treatment unit is a 4 ha plot with three replicates, for a total of 18 plots

Sampling Methodology

Regeneration and Micro-site Conditions

•All trees less than 5cm DBH tallied on 402 systematically placed 3.5m radius plots•Solar radiation estimated by hemispherical photography•Volumetric soil moisture estimated using time domain reflectometry (TDR)•Vegetation and substrate cover tallied

Germination and Survivorship

•Predation exclosures (18 per treatment combination) •Seeded with dominant overstory species in the first post-treatment year (Oct 2002)•Germinants closely monitored during summer 2003 to record total germinants and 1st year mortality

Pretreatment: Regeneration Pool

•A. concolor and C. decurrens are the most common overstory trees, P. jeffreyi and P. lambertiana are major overstory components•Regeneration pool dominated by firs and incense-cedar, with a reduced pine component

Pretreatment Regeneration: Micro-site Conditions

• A. magnifica(ABMA) low DSF, moderate soil moisture, high litter cover

• C. decurrens(CADE) low DSF, high soil moisture

• A. concolor (ABCO) intermediate DSF and soil moisture

• P. lambertiana (PILA) intermediate DSF and soil moisture

• P. jeffreyi (PIJE) high DSF, low soil

moisture

Post-treatment Mortality and Subsequent Response: White fir

•Initial mortality for A. concolor highest in BS, UC, and US treatments•US, UC, BS, and UN (control) treatments had the lowest subsequent regeneration response

A. concolor

TREATMENT COMBINATION

BN BC BS UN UC US

% C

han

ge fr

om P

retr

eatm

ent D

ens

ity

-100

-50

0

50

100 1st year mortality (all size classes)2nd year reponse (size class "A")

•All treatments resulted in regeneration decreases •Immediate post-treatment seedling distribution influenced regeneration response•Only BS treatments resulted in suppressed regeneration response

Post-treatment Mortality and Subsequent Response: Incense-cedar

1742%

•Jeffrey pine regeneration was not present in all treatment combinations•Burning, thinning, and burn/thinning combinations influenced mortality•Subsequent response only found in BS and US plots Unbalanced distribution prevented response model development

Post-treatment Mortality and Subsequent Response: Jeffrey pine

P. jeffreyii

TREATMENT COMBINATION

BN BC BS UN UC US

% C

han

ge fr

om P

retr

eatm

ent D

ens

ity

-100

0

100

200

300

400

500

1st year mortality (all size classes)2nd year mortality (size all "A")

Post-treatment Mortality and Subsequent Response: Sugar pine

• Pretreatment regeneration distribution, thinning, and burn/thinning combinations affected regeneration mortality• Thinned treatments had regeneration mortality, while un-thinned treatments had 1st year increases in regeneration•1st year seedling distribution was the dominant factor influencing regeneration response

P. lambertiana

TREATMENT COMBINATION

BN BC BS UN UC US

% C

HA

NG

E F

RO

M P

RE

TR

EA

TM

EN

T D

EN

SIT

Y

-200

0

200

400

600

1st year mortality (all size classes)2nd year response (size class "A")

1500%

NOSPP

ABCOABMA

CADE

PIJEPILA

PREM

QUKE

AVG02SMAVG03SM

DSF

-0.4

-1.2

0.0 0.4 0.8

-0.8

-0.4

0.0

0.4

TK POSTREAT SDL

Axis 1

Axi

s 2

TRT0

Axi

s 2

Post-treatment Regeneration: Micro-site Conditions

•Increased separation of sugar pine and white fir based on light levels and soil moisture

Post-treatment: Micro-site Conditions

POSTTREATMENT (2002)

DIRECT SITE FACTOR

0.2 0.3 0.4 0.5 0.6 0.7

AV

ER

AG

E S

OIL

MO

IST

UR

E (

% B

Y V

OLU

ME

)

8

10

12

14

16

18UNUCUSBNBCBS

•Thinning treatments have dominant effect on light levels and soil moisture•Burning treatments have a lesser but consistent influence•Increased light levels and increased soil moisture

Post-treatment: Germination Study

•Natural regeneration surveys do not address seed source

•Firs and incense-cedar are prolific seed producers compared to pines

•High proportion of firs and incense-cedar in the overstory indicates regeneration surveys may have a seed input bias

•Pines seeds also tend to have high seed predation

•Seed sowing in predation exclosures allows for a more controlled examination of regeneration with respect to treatments

Post-treatment: Germinant Survivorship

•Burned treatments had higher pine germinant survival •BC and BS also had the lowest white fir and incense-cedar survivorship

0

2

4

6

8

10

12

14

16

BN BC BS UN UC USTREATMENT COMBINATION

1ST

YR

SU

RV

IVO

RS

HIP

ABCO ABMA CADE PIJE PILA

Species Regeneration Strategies: Conclusions

•Pre-treatment regeneration dominated by fir and incense-cedar

•Pre-treatment regeneration distribution and abundance influenced by light levels, soil moisture, and litter cover

•If the goal is to increase pine regeneration and suppress fir and cedar, Burn/Shelterwood was the most effective treatment option

•Burn treatments favored pine germinant survivorship

•High germinant study response of pines combined with low natural regeneration of pines suggests natural regeneration may not be sufficient to accomplish pine restoration

•Post-treatment regeneration suggest pines are occupying lighter and drier sites than white fir and incense-cedar

•Treatment influence on light levels and soil moisture may benefit future pine regeneration when light levels increase with little increase in soil moisture

•Management goals for specific species or structural conditions may not be compatible with restoration activities whose objectives are increased pine components

Species Regeneration Strategies: Conclusions

Primary funding provided by the Joint Fire Sciences ProgramAdditional support provided by The USDA Forest Service PSW Research Station