Transcript Post-treatment Mortality and Subsequent Response

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
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 mixedconifer 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
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
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
California
Teakettle Experimental Forest
Fresno
Forest Boundary
Roads
Streams
Structures
•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
PROPORTIONAL ABUNDANCE (%)
70
Overstory basal area
Overstory density
Understory density
Sapling density
60
50
40
30
20
10
0
A. concolor
A. magnifica
C. decurrens
P. jeffreyi
P. lambertiana
•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
% Change from Pretreatment Density
Post-treatment Mortality and Subsequent Response: White fir
100
A. concolor
1st year mortality (all size classes)
2nd year reponse (size class "A")
50
0
-50
-100
BN
BC
BS
UN
UC
US
TREATMENT COMBINATION
•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
% CHANGE FROM PRETREATMENT DENSITY
Post-treatment Mortality and Subsequent Response: Incense-cedar
C. decurrens
1st year mortality (all size classes)
2nd year response (size class "A")
1742%
500
0
BN
BC
BS
UN
UC
US
TREATMENT COMBINATION
•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: Jeffrey pine
% Change from Pretreatment Density
500
1st year mortality (all size classes)
2nd year mortality (size all "A")
P. jeffreyii
400
300
200
100
0
-100
BN
BC
BS
UN
UC
US
TREATMENT COMBINATION
•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
% CHANGE FROM PRETREATMENT DENSITY
Post-treatment Mortality and Subsequent Response: Sugar pine
600
1st year mortality (all size classes)
2nd year response (size class "A")
P. lambertiana
1500%
400
200
0
-200
BN
BC
BS
UN
UC
US
TREATMENT COMBINATION
• 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
Post-treatment Regeneration: Micro-site Conditions
TK POSTREAT SDL
0.4
TRT
0
NOSPP
DSF
PREM
0.0
Axis 22
Axis
PIJE
AVG02SM
PILA
AVG03SM
QUKE
-0.4
ABCO
ABMA
-0.8
CADE
-1.2
-0.4
0.0
0.4
Axis 1
0.8
•Increased separation of sugar pine and white fir based on light levels
and soil moisture
Post-treatment: Micro-site Conditions
AVERAGE SOIL MOISTURE (% BY VOLUME)
POSTTREATMENT (2002)
18
UN
UC
US
BN
BC
BS
16
14
12
10
8
0.2
0.3
0.4
0.5
0.6
0.7
DIRECT SITE FACTOR
•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
16
ABCO
ABMA
CADE
PIJE
PILA
1ST YR SURVIVORSHIP
14
12
10
8
6
4
2
0
BN
BC
BS
UN
UC
TREATMENT COMBINATION
US
•Burned treatments had higher pine germinant survival
•BC and BS also had the lowest white fir and incense-cedar survivorship
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
Species Regeneration Strategies: Conclusions
•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
Primary funding provided by the Joint Fire Sciences Program
Additional support provided by The USDA Forest Service PSW Research Station