Air pollution increases forest susceptibility to wildfires in southern California
1
Nancy Grulke1, Richard Minnich2, Tim Paine3, Phil Riggan1
Pacific Southwest Research Station, USDA Forest Service, Riverside, CA
2 Department of Geography, University of California, Riverside, CA
3Department of Entomology, University of California, Riverside, CA
Abstract--Many factors increase susceptibility of forests to wildfire. Among them are increases in human population,
changes in land use, fire suppression, and frequent droughts. These factors have been exacerbating forest susceptibility to
wildfires over the last century in southern California. Here we report on the significant role that air pollution has on
increasing forest susceptibility to wildfires, as described in a case study in the San Bernardino Mountains. Air pollution,
specifically ozone (O3) and wet and dry deposition of nitrogenous compounds (by-products of fossil fuel combustion), has
significantly increased with population and industrialization of the region after WWII. Ozone and elevated nitrogen
deposition cause specific changes in forest tree carbon, nitrogen, and water balance that enhance individual tree susceptibility
to drought, bark beetle attack, and combined, contribute to whole ecosystem susceptibility to wildfire. For example, elevated
O 3 and N deposition increase leaf turnover rates, leaf and branch litter, and decrease decomposability of litter.
Uncharacteristically deep litter layers develop in mixed conifer forests affected by air pollutants. Elevated O3 and N
deposition decrease the proportion of whole tree biomass in foliage and roots, the latter effect increasing tree susceptibility to
drought and beetle attack. Because both foliar and root mass is compromised, carbohydrates are stored in the bole over
winter. Elevated O3 increases drought stress by significantly reducing plant control of water loss. The resulting increase in
canopy transpiration, combined with [O3 + N deposition]-induced decreases in root mass significantly increases tree
susceptibility to drought stress, and contributes to the success of bark beetle when combined with increased bole
carbohydrates. Phenomenological and experimental evidence is presented to support the role of these factors contributing to
increasing the susceptibility of forests to wildfire in southern California.
Factors contributing to forest susceptibilty
to wildfires:
• 100 yrs of compounding factors
• Rapid change in human populations and land
use patterns
• Shift from forest utilization to recreational use
• Fire suppression and reduced management
• Forest densification
• Periodic drought
• Explosion in bark beetle populations
• AIR POLLUTION IS A SIGNIFICANT,
PREDISPOSING STRESSOR
CONTRIBUTING TO INCREASED
SUSCEPTIBILITY TO WILDFIRE
High O3 exposure and excess N deposition:
>Lower leaf mass
>Lower root mass
>Lower carbohydrate content in roots and leaves
>Carbohydrates stored in bole!
Grulke and Balduman, 1999; Grulke et al. 2001;
O3-induced
thinning crown of
ponderosa pine in
the San Bernardino
National Forest,
CA. Lower
branches are
abscissed due to
higher O3 uptake in
the shaded canopy
and overall lower
carbon balance in
shaded branches.
Rapid population increase,
Change in land use
Increased O3 & N
Periodic drought
Fire suppression,
Reduced management
Increased tree
densification
Increased tree susceptibility to
drought stress
Increased litter
litterfall, kg m-2
Direct mortality
Increased success
of bark beetle, pathogens
20
18
16
14
12
10
8
6
4
2
0
pine
Increased susceptibility
to fire
+
= WILDFIRE!
300
3
4
5
6
7
11
8
9
250
Total annual ppt, cm
TOTAL
ANNUAL PPT, CM
2
Air pollution is comprised of:
CO2, CO, particulates, NOx, and O3.
10
Nitrogen oxides (and associated nitrogenous compounds) and ozone exposure stimulates changes
in ecosystem structure and function that increases forest susceptibility to wildfire.
>Nitrogenous compounds stimulate forest stand densification, leaf turnover rates, and reduce root
mass
>Ozone exposure also increases leaf turnover rates, increases lower branch abscission, and reduces
root mass. Ozone exposure also increases lignification in leaves, reducing decomposition rates of
decomposition.
>Litter layer significantly builds in pollution-impacted sites
>Both excess nitrogen deposition and ozone exposure shift within-tree allocation of resources:
>Carbohydrates are stored in subcortical tissues in the bole, just where bark beetles feed.
Air pollution transported from Los Angeles east into the mountains.
250
200
150
100
6
8
Big Pine Flat
Forest densification
Recovering stand
200
150
100
50
0
1883
1893
1903
1913
1923
1933
1943
1953
1963
1973
1983
2003
20
0
1938
1965
1994
2003
M
60
40
20
0
1938
1965
1994
100
220
200
180
160
140
120
100
80
60
40
20
0
COVER
2004
80
M
60
40
20
0
1938
1965
1994
220
200
180
160
140
120
100
80
60
40
20
0
100
COVER
80
T re e c o ve r, %
40
COVER
80
M
60
40
20
0
1938
2004
1965
1994
2004
1st yr of
dates of
drought
Aerial
photos
% of 123 yr record [1]
4
3-4 yr
bark
pollution
average
beetle [2]
SUM06 [3]
1
2
3
54
35
60
68
60
70
51
65
84
79
70
1947
78
88
84
64
79
1958
71
73
46
1969
55
66
55
1973
61
63
79
42
47
37
48
48
39
56
59
25
1922
1927
200
“Old Fire” in 10/03
post-drought mortality, #/ha
Chronic + acute drought ’02
66
76+5
63
++
59
+++
170
61
++
140
45
++
160
45
++++
1965
Time series of imagery at
Camp Paivika composited
red, near infra-red, and thermal
bands clearly shows dead trees,
and burned areas.
(Imagery from Phil Riggan)
2004
180
160
100
2
120
100
3
5
80
9
4
60
40
20
6
7
11
10
8
0
0
ngrulke @ fs.fed.us 951-680-1556
50
100
150
83+3
95+1
110
34+5
1
y = 0.7235x - 53.207
R2 = 0.6376
140
% [4]
[logged]
1938
1999
tree cover
50
1994
Chronic drought ’99-’01
2003
A long term record of precipitation shows frequency of drought in the eastern San
Bernardino Mountains. A year with 60% of the long term average precipitation results in
severe drought stress in ponderosa pine, and 80% results in moderate drought stress.
Approximately one half the time, ponderosa pine experiences moderate (32%) or severe
(16%) drought stress. The 2002 drought was a once in 200 yr acute drought event which
was preceded by three years of chronic, severe drought (40 to 60% of long term average).
1986
Continued mortality, 9/03
1993
Oxide concentrations
Nitrogen accumulation
1897
T ree m o rta lit y, # /h a
40
20
0
0
60
220
200
180
160
140
120
100
80
60
40
20
0
100
10 Snow Valley (fire in 1970)
Tree mortality, #/ha
20
M
Tree cover, %
40
220
200
180
160
140
120
100
80
60
40
20
0
COVER
80
9 Barton Flats
Tree mortality, #/ha
M
100
Tree cover, %
160
140
120
100
80
60
Tree mortality, #/ha
Tree cover, %
8 Cam p Angeles
Tree cover, %
220
200
180
COVER
1994
Using historic aerial photographs, tree cover, and tree density and mortality rates were measured in four to eight 1-ha plots.
The same location was sampled in 1938, 1965, 1994, and 2004 (R. Minnich). A subset of sites are presented below:
11
120+ yr average
80% of average
60% of average
2002, acute drought
Late 1800’s
80
1965
10
4 Rim of the World
100
1938
campground
S
9
1 Cam p Paivika
60
Barton Flats
Angeles
Camp
7
Barton Flats
Springs
Running
5
Snow Valley
W
RW-Switzer
4
Tree m ortality, #/ha
3
back
2
Arrowhead
1
RW-switch
0
Pavika
50
Seeley Flats
pre-drought tree density, #/ha
300
Known outbreaks
Cut
Expected beetle outbreaks
How can there be a link between air pollution and forest susceptiblity to wildfires?
S a n B e r n a r d in o M o u n t a in G r a d ie n t
1
open
Barton Flats
Thinning crown leads to excessive litter
build-up. Leaves produced in a high O3
environment are highly lignified and are
slow to decompose. Fenn & Dunn 1989;
Fenn et al., 2005
Increased fire starts,
Continued fire suppression
Topology of O3 concentrations across the Transverse Range from
Bytnerowicz and Arbaugh, unpublished data (2005). Below: Stand
density shown across a pollution gradient from west to east in the
San Bernardino Mountains. Higher stand density was correlated to
higher pollutant exposure, and to higher rates of mortality in the
1999 to 2002 drought.
oak
Camp Paivika
150
200
250
pre-drought tree density, #/ha
300
At the site with the most pollutant exposure, tree mortality after the 19992002 drought and subsequent bark beetle attack was 60%, 3x higher than at
sites with less pollutant exposure. At sites with prescribed or wildfire and
lower stand density, tree mortality was 4%. However, all stressors contributed
to the recent high mortality observed in the San Bernardino Mountains, many
of which were set in motion decades ago: population growth, land use
changes, and air pollution-effected stand density and within-tree allocation of
resources (lower root mass, higher bole carbohydrates). These long term
effects predisposed the trees, and the stands, to be particularly sensitive to
drought and successful bark beetle attack.