Different effects of thinning and burning on

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Different effects of thinning and burning on
Sierran mixed-conifer ecosystems
Malcolm North, USFS, Sierra Nevada Research
Center, Davis, CA
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(No Transcript)
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Teakettle Location, Design, Plot Layout and
Timeline
18 plots, 4 ha each, 3 reps of each of the 6
treatments

• Burned November 2001, off season, for
  containment air quality

Thinning Level No Burn Understory Burn
None Control Burn Only
25 cm lt thin lt76 cm Thin from below/No Burn Thin from below/Burn
25 cm lt thin leave 22 large t/ha Overstory thin/No Burn Overstory thin/Burn
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Thinning Effects (using all trees in a 4 ha plot)
Overstory (shelterwood) Thin gt 25 cm leave 22
large tree/ha
Understory 25 cmlt thin lt 76 cm
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Teakettles Focus Ecosystem Structure,
Composition and Function
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Collaborators
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Teakettle Results Focus on 4 Vegetation
Questions Relevant to Developing Thinning and
Burning Prescriptions

1. What effect do shrubs have on forest succession
   and water dynamics?
2. Why is mixed conifer so strongly clustered? Are
   trees grouped by age cohorts?
3. Do significant reductions in canopy cover reduce
   regeneration survival and understory herb cover?
4. Did the fire and thinning treatments produce a
   desired diameter distribution?

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1. What effect do shrubs have on forest
succession and water dynamics?

• Saplings and shrubs compete for shallow water in
  early summer
• Trees rely on deep water through summer drought

• Low tree seedling survival in shrubs and
  without root exclosure

From A. Plaemboeck

• Shrubs become islands that can lock a site
  against seedlings and herbs

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2. Why is mixed conifer so strongly clustered?
Are trees grouped by age cohorts?
Bonnicksen and Stone (1982) suggested pattern is
due to grouped cohort age structure
clustered
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• Are trees grouped by age cohorts?
• No ages were not spatially autocorrelated
  (variogram not shown)

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Why is mixed conifer so strongly clustered?

• One influence may be that tree groups reflect
  the geomorphic template (depth to bedrock)

Note correlation between the grouping of tree
basal area, mostly driven by clusters of large
trees and the depth to bedrock
Depth to bedrock for the a 4 ha area, where
deeper soils are in red. (Determined by seismic
survey)
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How Might This Explain Current Mortality
Patterns?

• Mortality episodic associated with La Nina
  droughts
• Most mortality in areas of high density
• Significantly higher than expected mortality for
  large trees
• Are small/intermediate trees depleting deep soil
  water pockets?

Significantly different than expected
(Chi-square)
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3. Do significant reductions in canopy cover
reduce regeneration survival and understory herb
cover?
Surface temperature by Julian date (y axis) and
hour (x axis) for 3 and 76 canopy cover
Persistent gaps in mixed-conifer without
regeneration and having few herbs
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Post-treatment Germinant Survivorship

• Burned treatments had higher pine germinant
  survival
• Burn and thin treatments (BC and BS) also had the
  lowest white fir and incense-cedar survivorship
• Herb cover and richness (not shown)
  significantly greater in all burned vs. thinned
  treatments, regardless of canopy cover reduction

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Increased Micro-site Heterogeneity
Data and slide from Harold Zald and Andy Gray,
PNW Research

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

Post-Treatment
Pre-Treatment
Light
Light
Soil H2O
Soil H2O
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4. Did the fire and thinning treatments produce
a desired diameter distribution?
Current diameter distribution
Before fire suppression (1865) recruitment and
mortality was pulsed by fire and El Nino events
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• Did the fire and thinning treatments produce a
  desired diameter distribution?
• No Not enough small and too many intermediate
  size trees taken

VTM is gt2400 vegetation type mapping plots
collected in the early 30s
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What did we learn at Teakettle?

• Thinning alone stalls herb development and does
  not favor shade-intolerant regeneration
• Fires important for reducing shrubs which are
  strong competitors with trees and herbs
• Group selection doesnt appear to be needed
• Thinning treatments may need to target small
  trees in high density areas, particularly around
  large, old trees
• If prescribed fire is applied off season,
  thinning must be aggressive about cutting small
  trees
• Observation Thinning prescriptions should stay
  focused on trees as fuels rather diameters or
  board feet