Streamwater Nitrogen During Mountain Pine Beetle Infestation of Subalpine Watersheds
at the US Forest Service, Fraser Experimental Forest
Chuck Rhoades, Kelly Elder, Rob Hubbard & Mark Dixon; U.S. Forest
Forest Service, Rocky Mountain Research Station, Fort Collins, CO
Unmanaged
175
50
Max
(m)
Overstory
Composition
Pine
ha
(%)
Spruce
Fir
20
20
10
10
Total
m2 ha -1 and (%)
290.9
30.9
32.4
17.1
(56.4) (29.7)
8.0
(13.9)
57.5
Lexen
129.9
3267.8
3527.6
15.9
12.3
30.5 17.5
(47.0) (26.9)
16.9
(26.1)
65.0
307.3
3208.9
3503.3
1.6
0.5
18.7
15.4
13.1
(39.7) (32.6) (27.7)
47.2
Deadhorse
319.5
3187.8
3526.3
14.1
4.4
17.0
17.1
(35.8) (36.1)
47.4
* Treeline averages 3482 m elevation (range: 3242 - 3654) at the Fraser Experimental Forest.
13.3
(28.1)
Mean Basal Area
Fool
Deadhorse
30
30
20
20
10
10
Lodgepole
Spruce
Fir
Unmanaged
Lodgepole
-2
-1
L s km
-1
μS cm
2
10
Spruce
Fir
Managed
4
6
8
10
12
Dissolved Inorganic N
80
Total D IN
60
40
N itrate
0
Lexen
0
Deadhorse
60
2
4
30
20
20
10
10
6
8
10
12
2
4
6
8
10
12
Month
Nitrate Export
4
8
10
12
Pre MPB (1982 - 2002)
Post MPB (2003 - 2008)
60
40
20
0
2
-1
0
2
80
East St. Louis
-1
-1
40
30
(g N ha mo )
40
6
M onth
50
0
30
Lexen
4
6
8
10
12
Pre MPB (1982 - 2002)
Post MPB (2003 - 2008)
20
On average monthly nitrate export was 30%
10
higher after beetles entered unmanaged basins;
Bark beetles preferentially the largest differences were in May and June.
0
2
4
6
8
10
12
attack
larger
diameter
200
Month
lodgepole pine, so areas
East St Louis
150
populated
by
smaller
100
diameter trees regenerating
Fraser watersheds responded rapidly to
50
after harvesting or wildfire
beetle attack. Few physical changes have
0
may be resistant to change
altered forest structure at this point, so
during the initial outbreak
1990
2000
2010
higher nitrate concentration and export
200
phase. Fraser basins managed
are the likely result of decreased nutrient
East St Louis
150
May & June
in the 50’s and 70’s have
demand following pine mortality.
100
proportionally less pine and
50
have lost < ¼ of total basal
Streamwater nitrate remains extremely
0
area to beetles.
low; change in N export is equivalent to
only about 2% of annual N deposition
1990
2000
2010
Year
(~2 kg N ha-1 yr-1).
-1
3847.6
-1
3360.9
2
941.3
(m ha )
Lexen
East St Louis
90
60
20
50
50
-1
2
-1
(m ha )
Fool Creek
30
12
20
NO3- N (μg L )
Mean
ha
Alpine
Area
Live
MPB-Killed
30
10
Conductivity
100
30
75
0
NO3- N (μg L )
Elevation
East St. Louis
Mean Basal Area
Watershed
Area
2008
40
8
100
Nitrate Export
1955
6
70
During six years following the onset of bark beetle
activity spring and fall nitrate was higher than preoutbreak concentrations in unmanaged basins. In
these basins, beetle attack killed >75% of the pine
and 40% of the total overstory basal area.
40
4
80
Pre-MPB (1982-2002)
Post-MPB (2003-2008)
40
100
Month
FEF is 100km west of Denver on the western edge of Colorado’s
Front Range. The 9,300 ha research forest ranges from 2,684 m to
3,903m elevation and contains subalpine and alpine ecosystems.
Lodgepole pine dominates the lower elevations and south aspects.
2
110
-1
125
25
Half the forested area was clear cut in 1954-55 to study
changes in snow accumulation and water yield.
2527 m
0
120
60
Fool Creek:
3955 m
10
Fool
60
-1
The biogeochemical implications of the current MPB outbreak
result from the aggregate of short and longer term change in factors
that regulate nutrient retention and release. For example, water
and nutrient demand by pines cease within weeks after beetles
attack and introduce blue stain fungi into the host tree’s vascular
system and thus may rapidly alter nutrient leaching and
streamwater export. In contrast, changes in canopy interception
and understory growth following overstory mortality and
windthrow will continue for years to decades.
East St. Louis
30
120
Managed
150
40
20
Dissolved N (μg N L )
2006
Nutrient
Leaching
NO3-N (μg N L )
The hydrologic, climatic, biogeochemical and vegetation
records available at the Fraser Experimental Forest (FEF)
provide a unique opportunity to quantify the impacts of this
widespread, but poorly understood disturbance in basins
relative to a multi-decade pre-disturbance period. Here we
compare streamwater chemistry and nutrient export in the six
years since the bark beetle outbreak began with the twenty year
pre-attack record for Fraser’s four main research watersheds.
50
-1
Soil
Nutrients
D ischarge
60
Seasonal hydrochemistry of Fraser streams mirrors
spring snowmelt; concentrations are diluted during the
period of peak stream runoff for most analytes.
Nitrate represents 50-90% of the dissolved inorganic
nitrogen in Fraser streams. Nitrate concentrations are
highest during late winter and spring and low
throughout summer and fall.
-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 in 2002. By 2007, bark
beetles had killed 50 to > 80% of the overstory pine in Fraser
research watersheds.
70
Biogeochemical Patterns
(g N ha mo )
Expected Watershed Responses
NO3-N (μg N L )
Rapidly Changing Forest Watersheds
Implications