A Decade of Watershed Responses to
Mountain Pine Beetle in Colorado Subalpine Forests
Chuck Rhoades, Rob Hubbard, Kelly Elder, Derek Pierson; USFS Rocky Mtn Research Station
Expected & Observed Responses
Soil
Nutrients
0.25
40
East St. Louis
Fool Creek
Mean Basal Area
2
-1
(m ha )
expected to increase soil N after bark beetle attack. This was
documented under Colorado lodgepole pine (Clow et al. 2011; above)
and in every study where it has been examined (Rhoades in prep.).
The increase in soil N occurs prior to significant needle drop (red
needle phase) or other forest structure or microclimatic change.
Live
MPB-Killed
30
20
30
20
10
2
-1
(m ha )
Mean Basal Area
30
30
20
20
10
10
Clearcut
Sweden (20)
Strip cut
Eastern US (21)
Partial clearcut
Eastern US (22)
Ice Storm
Eastern US (23)
0.20
Beetle Kill
Colorado (24)
MPB Outbreak
0
200
0.15
400
600
Increase in nitrate-N, %
0.10
A number of studies
in Colorado found
that stream nitrate
responses to bark
beetles were minor
compared to
harvesting and other
disturbances (Rhoades
et al. 2013).
At FEF there is widespread evidence of
stimulated post-outbreak plant growth and
nutrient demand. For example, we have found:
0.05
0.00
1990
Deadhorse
Clearcut
Colorado (14)
East St Louis Creek (ESLC)
10
Lexen
Clearcut plus Herbicide, Eastern US (19)
2000
Abundant new seedlings
under dead pine.
Understory tree growth 2X
faster annually.
Radial growth higher for
30% of trees.
Foliar N increased following
attack.
•
2010
Streamwater Nitrate:
At Fraser, stream nitrate concentrations
were statistically higher in the East St Louis Ck basin the decade after
bark beetles killed 75% of lodgepole pine basal area compared to
the pre-outbreak period. Nitrate increased in another unmanaged
basin (Lexen), but not 2 managed basins (Fool, Deadhorse).
•
•
•
40
20
10
0
3000
Mountain pine beetles
prefer larger diameter
lodgepole pine, so areas
populated by smaller
diameter trees are more
resistant to attack at the
onset of an outbreak.
Fraser basins that were
treated in the 1950’s &
1970’s have less pine
basal area and have lost
< ¼ of their total basal
area to beetles.
Fool Creek: Half the forested area was cut in the mid
1950s to quantify changes in snowpack and water yield.
400
300
200
100
0
Jan
Base
Flow
b
Fe
r
Ma
r
Ap
y
Ma
Rising
Limb
J un
J ul
g
Au
Falling
Limb
S ep
t
Oc
v
No
Base
Flow
Magnitude of Change: Stream
nitrate concentrations remained low
(< 0.1 mg NO3-N L-1); beetle-related
change in streamwater N was < 2% of
annual deposition.
0.15
0.10
2000
1000
40
Lodgepole Pine
30
20
10
0
40
Subalpine Fir
0.05
30
0.00
c
De
Post-outbreak differences were greatest
on the rising limb of the stream
hydrograph and during baseflow. On
average, monthly nitrate export was
30% higher after
beetles entered
unmanaged
basins;
the
largest
differences were in May and June.
(ESLC)
(m)
ESLC
Rising Limb
(Apr - Jun)
Radial Growth
Managed
0.20
500
Lodgepole Pine
Engelmann Spruce
0.20
0.15
0
20
60
50
40
30
-1000
20
10
10
Peak
SnowWater (cm)
1955
Fir
Stream NO3-N (mg L-1)
Unmanaged
Lodgepole Spruce
Stream NO3-N (mg L-1)
Fir
Falling Limb
(Jul - Sep)
0.10
0
0
1970
1980
1990
2000
2010
10
20
30
40
2007 Height Growth (cm)
0.05
Related Citations:
0.00
Nitrogen dynamics in conifer forests affected by bark beetles.
(in prep). Rhoades 2013
0.20
Stream NO3-N (mg L-1)
Lodgepole Spruce
Stream Discharge (L sec-1)
Median Radial Increment
600
Engelmann Spruce
30
2010 Height Growth (cm)
40
Compensatory Responses
Soil Nitrogen: Lower water and nutrient uptake by dying trees is
Stream NO3-N (mg L-1)
Forested watersheds of western North
America are experiencing rapid and
extensive canopy mortality caused by a
variety of insect species. The mountain pine
bark beetle (Dendroctonus ponderosae)
began to attack lodgepole pine (Pinus
contorta) at the USFS Fraser Experimental
Forest (FEF) in 2002. By 2007, bark beetles
had killed 50 to 80% of the overstory pine
in Fraser’s research watersheds.
Nutrient
Leaching
0.15
Base Flow
(Oct - Mar)
0.10
0.05
0.00
Pre Beetle
Beetle Outbreak
Biogeochemistry of Beetle Kill: Explaining a weak nitrate
response. Proc Nat Acad Sci. Rhoades et al. 2013
Changes in transpiration and foliage growth in lodgepole
pine trees following mountain pine beetle attack and
mechanical girdling. For Ecol Mgmt. Hubbard et al. 2013
Responses of soil and water chemistry to mountain pine
beetle induced tree mortality in Grand County,
Colorado, USA. Appl Geochem. Clow et al. 2011
Tree regeneration and future stand development after bark
beetle infestation and harvesting in Colorado
lodgepole pine stands. For Ecol Mgmt. Collins et al. 2011