Tree Regeneration Responses to Prescribed Fire and Thinning in a Sierran Mixed Conifer Forest

Harold S.J. Zald1, Andrew N. Gray2, Malcolm North3, and Ruth A. Kern4

1. Dept. Forest Science, Oregon State University, Corvallis, OR2. U.S. Forest Service PNW Research Station, Corvallis, OR3. U.S. Forest Service PSW Research Station, Sierra Nevada Research Center, Davis, CA4. Biology Department, Cal State University - Fresno, Fresno, CA

.Fire (and the lack of fire) influence

composition, structure, and function in most Western forests.

Fire and Fire Suppression

Generalized impactsof fire suppression

Increased stem densities

Increased ladder fuels and fuel loadings

Increased dominance Of shade tolerant species

Reduced fire frequency

Increased fire severity North et al. 2005 Forest Science

Managing for Fire and Restoration Objectives

General Objectives for Sierran Mixed Conifer Forests:• Reduce probability of high severity, stand-replacing wildfire• Restore pre-settlement composition, structure, and function• Increase proportion of fire tolerant, shade intolerant pine species

Implicit assumption: all objectives can be met simultaneously in space

Primary Land Management Tools to Reach These Objectives• Silvicultural Manipulations (Thinning, Replanting)• Prescribed fire

Seed Production Seedling EstablishmentGerminationSeed Dispersal

ClimatePredation

Resource availability

Primary DispersalSecondary Dispersal

Predation

ClimateMicrositeSpecies

ClimateMicrosite

Resource availabilityPredation

Competition

Regeneration

•Influences future fire susceptibility (via horizontal/vertical fuel development)

•Influences future forest composition and structure (via specific establishment and growth)

•Often only viewed from the end product (the established seedling)

•However the What (species composition and abundance) alone does not tell us much about the Why (processes)

Study Objectives

Investigate understory tree mortality and subsequent regeneration in response to burning and thinning treatments.

Specifically, we examined fire and thinning effects on:• Seed quantity• Sown seedling germination and early survival in controlled microsites• Understory tree mortality and natural regeneration• Relationships between microsite conditions and post-treatment regeneration

Study Area: Teakettle Experimental Forest

•Located in Sierra National Forest (80km east of Fresno, CA)•1900-2600 m asl•125 cm annual precipitation (almost all snow)•Well drained loamy-sand to sandy-loam soils derived from granite

•Old-growth Sierran mixed conifer forest•Density and basal area dominated by white fir and incense-cedar•Largest trees Sugar and Jeffrey pine•Red fir, Bitter cherry, and California black oak minor components

•3 major vegetation patches (closed canopy, shrub, bare ground)

Factorial design contrasting two levels of burning, three levels of thinning

Experimental Design

Thinning treatments (summers 2000&2001):•No thin (NT)•“CASPO” understory thin (UT) – 25-76 cm DBH harvest, ≥ 40% canopy retention •Overstory thinning (OT) - > 25 cm DBH harvest with ~22 large trees per ha. retained

Burn treatments (fall 2001):•No burn (NB)•Understory burn (B) – Ground fire, ≤ 2 m flame heights

1 2 3 4 5 6 7

8 9 10 11 12 13 14

15 16 17 18 19 20 21

22 23 24 25 26 27 28

29 30 31 32 33 34 35

36 37 38 39 40 41 42

43 44 45 46 47 48 49

1 2 3

4 5 6

7 8 9

Each treatment unit is a 4 ha plot with three replicates, for a total of 18 plots

Within each treatment combination:1 replicate with 49 grid point sampling2 replicates with 9 grid point sampling402 total grid points

Sampling Methodology

Stand Structure (Plot-level)•All trees ≥ 5cm DBH measured and mapped

Germination and Survivorship•Predation exclosures (9 pairs per burn*thin combination) •Seeded with 5 conifer species in first post-treatment year (Oct 2002)•Germinants monitored during summers 2003-2005 to record total germinants and survival

Regeneration and Micro-site Conditions (Microplot)•All trees less than 5cm DBH tallied on 3.5m radius plots•Vegetation/substrate cover tallied•Solar radiation above each grid point•Volumetric soil moisture (top 15cm) estimated using time domain reflectometry over growing seasons

Seed Rain•25 0.25m2 seed traps per plot•Collected 2001-2003, counted by species

Sampling Regeneration: Frequency vs. Density

Density (stems/unit area)•Stocking guidelines•Forage estimates

Is this appropriate?•Does not incorporate evenness•Sensitive to outliers•Bias species-environmental relationships

Frequency the response variable•Defined as the odds ratio of

quadrants occupied within a plot

54 12

06

Density=72

1 1

01

Frequency=0.75

Results: Overstory & Understory Responses to Treatments

•Overstory: dominated by White fir (and incense-cedar) pre and post- treatment

•Only white fir and incense-cedar

•Slight ingrowth in controls

•Burning alone does nothing

•Thinning reduced fir, but did not consistently reduce incense-cedar •Thinning and burning resulted in greatest reduction of fir and incense- cedar

Pre1st3rd2nd

Results: Seed Rain Responses to Treatments

•Order of magnitude difference between fir and incense-cedar versus pines

•Fir seed rain declined with increased thinning intensity

•Incense-cedar and Jeffery pine seed rain did not change with treatments

•Jeffrey pine seed rain higher in burned and thinned plots

Results: Germinant Survival Responses to Treatments

•Low survival of pine germinants in controls and unburned/understory thin

•High survival of pine germinants in burn/thinned

•Low survival of fir germinants in thins

•Low survival of incense- cedar germinants in overstory thin

Results: Seedlings and Microsite Conditions

•NMS Ordination of post-treatment seedling frequency in micro-plot space

•Species close together occupy similar micro-plot

•Joint plot vectors indicate direction/strength of microsite variables in relation to ordination structure

•Primary microsite variables: moisture, light, shrub cover

•Seedling species occurrence on soil moisture/light level/shrub abundance gradient

•Sugar pine seedlings occupied moisture/light/shrub cover similar to White fir

High light, High shrub, Low H2O

Low light, Low shrub, High H2O

Results: Seedlings Responses to Treatments

•Burn/no thin: increased fir and sugar pine regeneration

•Burn/understory thin: massive fir and incense-cedar recruitment

•Overstory thin: highest Jeffrey pine regeneration

•If high pine and low fir and incense-cedar is the objective, overstory thin

•Understory thin counter- productive to many current management objectives

No Thin (NT) Understory Thin (UT) Overstory Thin (OT)

Bur

ned

(B)

Unb

urne

d (N

B)

Results: Post-treatment Environment

↑Light↑H2O

↑Litter&Slash↓Shrub

↑Light↑H2O

↑Litter&Slash↓Shrub

LightH2O

Litter&SlashShrub

↑Light↓H2O

↓Litter&Slash↓↑Shrub↑Burn

↑Light↑H2O

↓Litter&Slash↓Shrub↑Burn

Conclusions

•All treatments resulted in fir and incense-cedar still dominating sites

•Only burning and thinning reduced fir and incense-cedar sapling frequencies

•Seed rain order of magnitude greater for fir and incense-cedar versus pines •Burning alone or burning and thinning increased pine germinant survival while reducing fir and incense-cedar survival

•Species-specific post-treatment regeneration occurred along a high light- low h20 to low light-h20 gradient.

•Burning combined with overstory thinning only treatment combination with increased pine regeneration frequency while reducing fir and incense-cedar •Understory thinning results in regeneration consistent with overstory

Management Implications

•Current management not meeting multiple objectives

•Seed source for fir and incense-cedar source of inertia

•Seed source for pines may limit recruitment (need for planting)

•Current planting methods are counter-productive

•Study may be overly optimistic for younger stands (old-growth=large pine)

•Overstory thinning and burning may conflict with other objectives (wildlife habitat)

Acknowledgements

Teakettle Experimental Forest Field Crews

California State University Agricultural Research Initiative

USDA/USDI Joint Fire Sciences Program

Sierra Nevada Research Center

USDA FS PNW Research Station

Questions?