Tree Regeneration Strategies in Response to Burning and Thinning Restoration Treatments in a Sierran Mixed-Conifer Forest

1
Tree Regeneration Strategies in Response to
Burning and Thinning Restoration Treatments in a
Sierran Mixed-Conifer Forest
Harold S.J. Zald and Andrew N. Gray USDA Forest
Service, Pacific Northwest Research Station 3200
SW Jefferson Way, Corvallis, OR 97331
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Fire History and Suppression in Sierran
Mixed-Conifer Forests

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

Generalized impacts of 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)
Historical Old-Growth
Current High Density Stand
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Forest Regeneration Species Strategies
P. lambertiana
P. jeffreyi
A. magnifica
A. concolor C. decurrens

• 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

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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
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Study Area Teakettle Experimental Forest

• 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

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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

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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

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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

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Post-treatment Mortality and Subsequent Response
White fir

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

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Post-treatment Mortality and Subsequent Response
Incense-cedar
1742

• All treatments resulted in regeneration decreases
  
• Only BS treatments resulted in suppressed
  regeneration response

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Post-treatment Mortality and Subsequent Response
Jeffrey pine

• Jeffrey pine regeneration was not present in all
  treatment combinations
• 100 Mortality in all treatment combinations
• Subsequent response only found in BS and US plots

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Post-treatment Mortality and Subsequent Response
Sugar pine

• Thinned treatments had regeneration mortality,
  while un-thinned treatments had 1st
• year increases in regeneration
• All treatments had increased regeneration
• BS and BN treatments had highest 2nd year
  response

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Post-treatment Regeneration Micro-site Conditions

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

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Post-treatment Micro-site Conditions

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

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

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Post-treatment Germinant Survivorship

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

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Species Regeneration Strategies Conclusions

• Pre-treatment regeneration dominated by fir and
  incense-cedar
• Pre-treatment regeneration distribution and
  abundance influenced
• by light level, soil moisture, and litter cover
• If the goal was to increase pine regeneration and
  suppress fir and
• cedar, Burn/Shelterwood was the most effective
  treatment option
• Burn treatments favored pine germinant
  survivorship
• Study suggests regeneration of pines alone may
  not be sufficient
• to accomplish pine restoration

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Species Regeneration Strategies Conclusions

• Post-treatment regeneration suggest pine
  regeneration is
• occupying lighter and drier sites than white fir
  and incense-cedar
• Treatment influence on light level 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

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