Transcript Tree Regeneration in Response to Prescribed Fire, Thinning, and

Tree Regeneration in Response to Prescribed Fire, Thinning, and Microsite Conditions
in a Sierran Mixed Conifer Forest
Harold Zald (541-750-7299, [email protected], PNW Research Station, 3200 SW Jefferson Way, Corvallis, OR 97333), Andrew Gray (PNW Research Station, Corvallis, OR), and Malcolm North (Sierra Nevada Research Center, Davis, CA)
Introduction
At each grid point in the intensive and replicate plots, seedlings and saplings (trees with diameter at breast height (DBH) < 5cm,
and height > 5cm) were tallied within 3.5 m radius (38 m2) plots centered on each grid point (N=402). Each plot was divided into 4
quadrants. The total number of individuals by size class was recorded for each tree species in each quadrant. The size classes were:
Seedlings (5-50 cm in height).
Saplings (greater than 50 cm in height, and less than 5 cm DBH).
Methods: Seed Sowing Manipulative Experiment
A seed-sowing experiment was used to determine the influence of treatments combinations and dominant vegetation types on
tree germination and survival. Within each of the six treatment combinations, 9 grid points (54 total grid points) were chosen
representing the three dominant vegetation types (closed tree canopy, Ceanothus cordulatus, and bare ground). At each grid point two
0.49 m2 exclosures were constructed from 19 gauge hardware cloth with a 1.25 cm mesh size. In the fall of 2002 seeds from the five
dominant overstory species were sown in numbers to equalize the number of viable seed per species. Total germinants and germinant
mortality were recorded multiple times during the growing season (June-September) in 2003 and 2004.
Methods: Microsite Environmental Variables
The study conducted at the Teakettle Experimental Forest. Data collected
during the summers of 2000, 2002, 2003, and 2004. Teakettle is located 80 km east of
Fresno, California in the north drainage of the Kings River (Sierra National Forest,
Kings River Ranger District). Teakettle consisted of 1300 ha of old-growth mixedconifer and red fir (Abies magnifica) forest.
• Elevation range: 1980-2590 m
• Annual precipitation: 110 cm, primarily snow, from November to April
• Mean, maximum and minimum July temperatures are 17C, 30C, and 3C.
Teakettle grades from a mix of white fir (Abies concolor), sugar pine (Pinus
lambertiana), incense-cedar (Calocedrus decurrens), and Jeffrey pine (Pinus jeffreyi) at
the lower elevations to red fir, lodgepole pine (Pinus contorta) and western white pine
(Pinus monticola) at higher elevations. Soils are generally Xerumbrepts and
Xeropsamments typical of the southwestern slopes of the Sierra Nevada.
Proportional Abundance (percent)
Results: Pretreatment Seedling and Sapling Composition and Abundance
Methods: Study Design
• 2 by 3 factorial design of treatments, thinning occurred during 2000 and 2001, burning
occurred during the fall of 2001
• For each treatment: one intensive plot containing 49 grid points, and 2 replicate plots
containing 9 grid points each.
•4 ha plots for each treatment combination, 18 total plots, 72 total ha.
60
seedlings
saplings
50
Firs and incense-cedar dominate both the seedling and sapling
components of the regeneration pool.
40
•Extremely low Jeffrey pine component in the regeneration pool,
with scattered sugar pine.
30
20
50
0
P. jeffreyi P. lambertiana
Results: Changes in Sapling and Seedling Abundance in Relation to Treatments
60
600
Frequency of Quadrants
Occupied (percent)
US3
US2
30
20
10
UC2
UN2
BN2
BS2
BS1
BN1
Teakettle Experimental Forest Plot Map
Each plot identified by two letter and one digit code indicating the treatment for each block
Thinning Treatments
N-no thinning C-understory thin S-overstory thin
b
c
BN
BC
100
BS
UN
2000
2002
2003
2003
Calocedrus decurrens
40
30
20
10
d
UC
US
BN
BC
30
25
20
10
20
A. concolor
A. magnifica
C. decurrens
P. jeffreyi
P. lambertiana
15
10
5
0
0
UN
UC US BN
BC
Treatment Combination
BS
BARE CLOSED_CNPY CECO
Vegetation Type
•Germinant survivorship varies by species, treatment combination, and vegetation type
•Jeffrey pine and sugar pine overall higher survivorship than the firs or incense-cedar
•Both pine species had higher germinant survivorship in burned and shelterwood treatments
•All species had low survivorship on bare sites
BS
2000
2002
2003
2004
Calocedrus decurrens
500
400
300
200
100
0
UC
US
14
BN
BC
BS
Pinus jeffreyii
12
10
UN
18
2000
2002
2003
2004
8
6
4
2
0
16
UC
US
BN
BC
BS
2000
2002
2003
2004
Pinus lambertiana
14
12
10
8
6
4
2
0
UN
UC
US
BN
BC
Treatment Combination
Selected treatment images: a.burned/shelterwood, b. unburned/shelterwood,
c. unburned/understory thin, and d. burned/understory thin.
200
600
UN
• For each treatment combination, plots are numbered 1-3 starting from the south.
• Brown plots have 49 internal grid points (25 m ground distance spacing with a 25 m buffer) for
intensive spatial sampling. Blue plots have 9 grid points (50 m spacing with a 50 m buffer).
a
300
0
Frequency of Quadrants
Occupied (percent)
Burn Treatments
U-unburned B-burned
US
Sapling Density (Stems/ha.)
BN3
Frequency of Quadrants
Occupied (Percent)
Garage
UN1
UC
50
Cabin
BC1
400
•Frequency of white fir seedlings
increased across all treatment
combinations.
•White fir sapling densities declined
dramatically in shelterwood thinnings,
moderately in understory thinnings,
and increased slightly due to growth
of seedlings in the unthinned
treatments.
40
30
A. concolor
A. magnifica
C. decurrens
P. jeffreyi
P. lambertiana
0
UN
UC1
Fresno
500
0
UC3
US1
40
2000
2002
2003
2004
Abies concolor
Frequency of Quadrants
Occupied (percent)
UN3
Sacramento
50
Sapling Density (Stems/ha.)
2000
2002
2003
2004
Abies concolor
BC2
Results: Effects of Treatment Combinations and Vegetation Types on Germinant Survivorship
10
A. concolor A. magnifica C. decurrens
BS3
•Thinning heavily effected
Treatment Combination
Variable
UN
UC
US
BN
BC
BS
solar radiation levels
•Thinning also influenced
ISF
0.29(.01)d
0.38(.01)c
0.50(.01)b
0.28(.01)d 0.42(.01)c 0.55(.01)a
overall and species specific
TSF
0.35(.02)c
0.44(.02)b
0.57(.02)a
0.35(.02)c 0.49(.02)b 0.64(.02)a
levels of shrub cover
DSF
0.35(.02)c
0.45(.02)b
0.58(.02)a
0.36(.02)c 0.50(.02)b 0.65(.02)a
•Moisture levels appear to be
a
abc
c
ab
abc
bc
CECO
8.51(2.53) 5.37(1.71)
0.81(.45)
7.46(2.26) 2.35(.93) 1.24(.61)
lower in burned versus
a
abc
c
ab
bc
c
SHRUB
12.49(2.92) 8.66(2.40)
1.49(.68) 10.24(2.58) 3.57(1.11) 1.26(.60)
unburned treatments,
ab
ab
a
b
ab
b
H2O_03
8.26(.83)
8.870(.42)
10.93(1.00)
7.61(.60) 9.61(.54) 7.86(.40)
burned/shelterwood
ab
ab
a
b
ab
b
5.73(.76)
5.49(.27)
7.35(.87)
4.84(.48)
5.51(.28)
4.54(.24)
H2O_04
treatment combinations tended
to have low soil moisture relative to other treatment combinations
•Results suggest that solar radiation can be successful altered by silvicultural treatments, but soil moisture
is highly variable within treatment combinations, with no strong patterns present
2004 Survivorship
(percent)
The solar radiation above each grid point was estimated using digital hemispherical images taken during September 2003.
Estimated radiation variables were indirect site factor (ISF), direct site factor (DSF), and total site factor (TSF), or the indirect, direct,
and total radiation compared to an open site at the same latitude. Volumetric soil water content in the top 15 cm of soil was measured
multiple times during the growing seasons of 2002, 2003, and 2004 using calibrated time domain reflectometry (TDR, model 1502C,
Tektronix Inc., Beaverton, OR). Percent cover of shrubs, coarse woody debris by size class, mineral soil, litter cover, litter depth, and
rock were estimated within a 10 m2 micro plot centered on each grid point.
Methods: Study Area Location and Description
BC3
•Nonmetric Multidimensional Scaling (NMS) ordination
suggests species occupy different microsites that are
based on a handful of environmental gradients.
•The joint plot overlay indicates the light levels
(TSF and DSF), soil moisture (H2O_03 and H2O_04),
and shrub cover (SHRUB and CECO) are the most
important environmental factors in determining
regeneration composition and abundance.
•Many of the species are arranged in the ordination on a
gradient from high soil moisture, low light levels, and low
shrub cover to low moisture, high light levels, and higher
shrub cover.
•White fir and incense-cedar (ABCO and CADE) occupied
moist, relatively shaded sites. Sugar pine (PILA) occupied
slightly drier and more open sites, while Jeffrey pine and
bitter cherry (PIJE and PREM) occupied progressively
drier and more open sites.
•Sites with no tree seedling (NOSPP) were drier, more
open, and had higher shrub cover than sites with
seedlings.
2004 Survivorship
(percent)
The Healthy Forests Initiative and the Sierra Nevada Ecosystem Project have
proposed using prescribed fire and thinning to restore pre-settlement composition and
structure and reduce the risk of catastrophic wildfire in mixed conifer forests of the
Sierra Nevada Range, California. Rrelatively little is known about how alternative
management approaches will impact mixed conifer forest ecosystems. The Teakettle
Ecosystem Experiment was initiated to examine the multiple ecological effects of
prescribed fire and thinning treatments in a mixed conifer forest of the Southern
Sierra Nevada Range.
Sapling mortality and seedling regeneration are important processes that can
drive future stand structure, overstory composition, and susceptibility to wildfire (via
fuel loading and ladder fuels). Highly clustered patterns of trees that are not based on
tree age cohorts or shade tolerance gradients suggest that within stand microsite
conditions will influence forest regeneration, in addition to stand level treatments.
The primary objectives of this study were to:
• Describe the effects of prescribed fire and thinning treatments on seeding and
sapling mortality and regeneration.
• Examine the role of microsite environmental gradients in determining species
composition in the regeneration pool.
• Observe how environmental variable important to seedling composition vary in
response to treatments at the stand scale.
• Determine the role of treatments and patchy vegetation types in tree germinant
survivorship.
Results: Seedling Abundance and Treatment Combinations
in Relation to Microsite Environment Conditions
Methods: Seedling and Sapling Data Collection
BS
UN
UC
US
BN
BC
Treatment Combination
BS
•Frequency of incense-cedar
seedlings increased across most
treatment combinations.
•The shelterwood treatments had
essentially unchanged seedling
frequency after three years compared
to pretreatment levels (2001).
•Incense-cedar sapling densities
declined dramatically in
burned/shelterwood treatments,
and returned to approximately
pretreatment levels in all other
treatment combinations.
•Frequency of Jeffrey pine and sugar
pine seedlings increased across most
treatment combinations.
•The highest increases for
Jeffrey pine were in shelterwood
thinning and burned/understory
thinned treatment combinations.
•The highest increases for sugar
pine were in shelterwood thinned
and burned/unthinned treatment
combinations.
•Low densities prohibited analysis
of sapling data for both pine
species
Conclusions
•Immediate short term reductions in ladder fuels (fir and incense-cedar saplings) best accomplished by the
burned/shelterwood treatment combination.
•The combination of burning and shelterwood silvicultural practices may promote the abundance of
pine seedlings, but the two species appear to respond very differently to a range of burning and thinning
activites.
•Shelterwood treatments resulted in the lowest abundance of incense-cedar, but white fir appears to
regenerate well in all treatment combinations. Multiple entries over time maybe required in these forests if
reductions in shade tolerant species is a management objective.
•Analysis of environmental conditions suggests light levels, soil moisture, and shrub cover are most
important in determining regeneration composition and abundance.
•Most species fit along a gradient from high soil moisture and low light, to dry and open sites.
•Thinning treatments resulted in well defined changes in solar radiation, but soil moisture did not display well
defined patterns based on treatment combinations.
•Considerable variability exists in soil moisture and shrub cover conditions between treatments, suggesting
that two important environmental factors driving species composition and abundance cannot be
manipulated by treatments with any degree of certainty.
•Manipulative seed sowing experiment indicates that burning and thinning should result in higher germinant
survivorship for both pine species.
•The wide discrepancy between germinant survivorship and observed seedling responses in treatments
leads us to speculate that recruitment limitation (i.e. seed predation, low seed crops, etc.) may be partly
responsible for the low overall regeneration of pines in response to treatment combinations.
Primary funding provided by the Joint Fire Sciences Program
Additional support provided by The USDA Forest Service PNW Research Station,
and the USDA Forest Service PSW Research Station