Evaluating the Resiliency of Ponderosa Pine Forests to Bark Beetle Infestations A
Decade Following Fuel Reduction Treatments (WC-EM-F-11-02)
Christopher J. Fettig and Stephen R. McKelvey
Invasives and Threats Team, Pacific Southwest Research Station, USDA Forest Service, Davis, California
Figure 1. Mechanical thinning,
California, U.S., 2005.
Figure 2. Prescribed burning,
California, U.S., 2005.
Both thinning and the application of prescribed fire may influence
the amount and distribution of bark beetle-caused tree mortality
at various spatial and temporal scales. For example, the health
and vigor of residual trees; the size, distribution and abundance
of preferred hosts; and the physical environment within forest
stands may be impacted (Fettig et al. 2007. For. Ecol. Manage.
238: 24-53). Furthermore, tree volatiles released during harvest
operations, including monoterpenes, are known to influence the
physiology and behavior of bark beetles and colonization rates of
trees by bark beetles in ponderosa pine forests (Fettig et al.
2006. For. Ecol. Manage. 230: 55-68).
We are monitoring levels of tree mortality a decade after the
application of fuel reduction and forest restoration treatments,
and determining the resiliency of these structures to bark beetle
infestations during a period of time largely dominated by drought,
elevated growing season temperatures, and elevated bark beetle
populations. Studies are conducted at two sites in northern
California: (1) Blacks Mountain Experimental Forest (BMEF;
40°40’ N, 121°10’ W; 1700-2100 m elevation) located on the
Lassen National Forest, and (2) Goosenest Adaptive
Management Area (GAMA; 41°30’ N, 121°52’ W; 1500-1780 m
elevation) located on the Klamath National Forest. Both studies
were originally established to evaluate the ecological
consequences of such treatments on forest health and
productivity.
Portions of BMEF were divided into three blocks based on tree
composition: (1) high pine, (2) pine and fir, and (3) high fir.
Four plots were established within each block ranging in size
from ca. 77 to 144 ha, and randomly assigned one of two
structural types, high diversity (HiD) or low diversity (LoD).
Each plot was subsequently divided in half with one half
randomly assigned prescribed fire (B), resulting in four
treatments. Fettig et al. (2008, Can. J. For. Res. 38: 924-935)
describe the climatic conditions, fuel moistures and other
parameters that occurred during prescribed burns. Oliver
(2000, PSW-GTR-179) and Zhang et al. (2008, Can. J. For.
Res. 38: 909-918) provide comprehensive accounts of
treatments and forest conditions. A 100% cruise (census) was
initially conducted on each plot to locate dead and dying trees
two years after the application of prescribed fire. Methods are
reported in Fettig et al. (2008).
Fettig and McKelvey (2010, Fire Ecol. 6: 26-42) described
responses of bark beetles to these treatments and resulting
structures five years after the application of prescribed fire at
BMEF. A total of 6,877 pine and fir trees (4.0% of all trees)
were killed by bark beetles (all bark beetle species combined),
the majority of which (41.5% of trees killed by bark beetles;
2,857 trees) occurred on HiD + B. Significantly higher levels of
tree mortality were attributed to bark beetles on LoD + B (5.3%)
and HiD + B (4.8%) than LoD (1.1%) (Fig. 3).
Portions of GAMA were divided into twelve 10-ha plots randomly
assigned one of four treatments with three replicates per
treatment. Treatments included: (1) untreated control (C), (2) burn
- prescribed burning in the fall (B), (3) thin - thinning from below
and selection harvest with leave trees including all stems >76.2 cm
dbh (T), and (4) thinning from below and selection harvest
followed by prescribed fire (T + B). Our work is also associated
with the Fire and Fire Surrogate study, a national network of 12
long-term study sites. Ritchie (2005, PSW-GTR-192) provides a
comprehensive account of treatments and forest conditions.
Fettig et al. (2010, For. Sci. 56: 60-73) described methods and
responses of bark beetles to these treatments and resulting
structures two and four years after the application of prescribed
fire. A total of 1,822 pine and fir trees (5.1% of all trees) were
killed by bark beetles. Significantly higher levels of bark beetlecaused tree mortality occurred on B (9.2%) than C (3.2%), T
(<1%) or T + B (3.3%) cumulatively during the four year period
(Fig. 4). Ten year assessments of tree mortality will be completed
in September 2012.
12
A. Sample period 1 (2-yr)
a
10
Control
Burn
Thin
Thin + burn
ab
8
10
5-yr
HiD + B
HiD
LoD + B
LoD
a
8
ab
ab
a
a
a
6
a
a
a
a
a
ab
a a
4
a
b
b
2
a
ab
a
a
ab
ab
b
b
b
Figure 3. Mean
percentage of trees
killed by diameter
class (mid-point of
10-cm diameter
classes shown except
for largest) five years
after the application
of prescribed fire.
Means (+ SEM)
followed by the same
letter within groups
are not significantly
different (P > 0.05).
b
0
24.1
34.3
44.5
54.7
>59.7
Mean percentage (+ SEM) of trees killed by bark beetles
Large and severe wildfires in the western U.S., and particularly in
California, have aroused public concern and manifested the need
for well-designed treatments to reduce their extent and severity.
Creating more fire resilient stands generally requires treatment of
surface and ladder fuels; reductions in crown density; and
maintenance of large diameter trees (Agee and Skinner 2005.
For. Ecol. Manage. 211: 83-96). A combination of thinning (Fig.
1) and prescribed fire (Fig. 2) has been shown to be highly
effective for reducing the severity of wildfires in interior ponderosa
pine forests (Ritchie et al. 2007. For. Ecol. Manage. 247: 200208).
Goosenest Adaptive Management Area
Ecological Research Project
Blacks Mountain Ecological Research Project
Mean percentage (+ SEM) of trees killed by bark beetles
Introduction
a
6
a
a
4
b
2
ab
b
b
b
b
b
b
b
b
b
0
b
b
b
B. Cumulative
a
15
ab
12
a
9
a
a
6
ab
ab
ab
3
ab
ab
b
b
34.3
44.5
b
b
a
a
b
All
DBH class (cm)
a
a
a
Figure 4. Mean
percentage of trees
killed by bark beetles
by diameter class
(mid-point of 10-cm
diameter classes
shown except for
largest) for
evaluations
completed two and
four years
(cumulative) after
prescribed burns
were implemented.
Means (+ SEM)
followed by the same
letter within groups
are not significantly
different (P > 0.05).
b
0
24.1
>49.5
All trees
DBH class (cm)
The amount of bark beetle-caused tree mortality increased
substantially on unburned split plots compared to burned split
plots following our initial survey (i.e., during three to five years
after the application of prescribed fire). Ten year assessments
of tree mortality will be completed in May 2012 following
surveys of plots 40, 48 and 49.
We thank L. Fischer (FHP) for her support of this work. To date, R. Borys,
C. Dabney, M. Patterson, and C. Stoll (PSW) have provided technical
assistance, and J. Baldwin (PSW) has assisted with statistical analyses. We
acknowledge the numerous contributions of our colleagues at the Redding
Laboratory (PSW) and Lassen and Klamath National Forests to the
maintenance of infrastructure at BMEF and GAMA. This research was
supported, in part, by a Forest Health Monitoring-Evaluation Monitoring
grant (WC-EM-F-11-02) and the Pacific Southwest Research Station.
Poster presented at the 2012 FHM Workgroup Meeting, Tucson, AZ , 16-19
April 2012.