Linking stream nitrate to forest response and recovery after severe bark beetle infestation
Chuck Rhoades, Rob Hubbard, Kelly Elder, Derek Pierson; USFS Rocky Mountain Research Station (AGU POSTER H11D-118)
BIOGEOCHEMICAL RESPONSE AND LINKAGES
Increased Tree Growth =
Increased Soil Nutrient Demand
Reduced Nutrient
Uptake
Leaf Area Index
Mixed Age > Old Growth
Soil Resource Demand
Mixed Age >> Old Growth
Overstory Mortality
Old Growth > Mixed Age
Increased Soil
Resources
Stream
Export
Response
Proportional to Mortality
20
20
10
10
0
1990
1955
Spruce
Fir
Managed
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.
Stream NO3-N (g L-1)
Unmanaged
Lodgepole
100
Lexen
40
20
80
60
40
20
0
2000
100
Fool
MPB Infestation
80
60
40
20
0
Lexen
p = 0.028
20
p = 0.003
p = 0.095
10
30
10
2010
120
100
MPB Infestation
Deadhorse
80
60
40
20
0
30
10
1000
0
1980
1990
Deadhorse
20
10
2000
2010
25
Falling
Limb
Base
Flow
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.
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. 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).
Lodgepole Pine
30
20
10
0
40
Subalpine Fir
30
20
10
0
0
10
20
30
40
2007 Height Growth (cm)
is less in Mixed stands;
>small, young, non-host trees
20
15
Young trees use more soil resources
10
~ 5X more water / unit leaf area
than old trees (Hubbard unpublished)
5
Rising
Limb
2010
40
MPB effect on basal area and LAI
Fir
Spruce
Pine
0
1990
2000
2010 Height Growth (cm)
Radial Growth (m)
20
30
20
Engelmann Spruce
2000
Peak
SnowWater (cm)
Fool
0
2000
30
At Fraser there is widespread evidence of
stimulated post-outbreak plant growth
and nutrient demand. For example, we
have found:
• Radial growth higher for 30% of trees
• Abundant new seedlings under dead pine
• Understory tree growth is 2X faster
• Foliar N increased following attack
0
2010
120
30
Lodgepole Pine
Engelmann Spruce
1970
60
0
2010
MPB Infestation
1990
Stream NO3-N (g L-1)
Fir
Mixed Age
Spruce
p < 0.001
Pre-MPB
MPB
120
1990
Lodgepole
2000
80
p < 0.001
Average Leaf Area (m2 plot-1)
30
30
50
East St Louis
40
60
50
40
30
-1000
20
10
Old Growth
Deadhorse
100
100
Mixed Age
Mean Basal Area (m2 ha-1)
Lexen
150
Stream NO3-N (g L )
10
MPB Infestation
East St Louis
Stream NO3-N (g L )
10
200
Median Radial Increment
0
250
Stream NO3-N (g L-1)
20
Stream NO3-N (g L-1)
20
30
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).
-1
Live
MPB-Killed
30
Fool Creek
Old Growth
Mean Basal Area (m2 ha-1)
East St. Louis
FOREST GROWTH
3000
Streamwater Nitrate: At Fraser, stream nitrate concentrations were statistically higher in
-1
40
COMPENSATORY
after bark beetle attack. This was documented under Colorado lodgepole pine (Clow et al.
2011) 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.
Stream NO3-N (g L-1)
40
Soil Nitrogen: Lower water and nutrient uptake by dying trees is expected to increase soil N
Stream NO3-N (g 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 in
2002. By 2007, bark beetles had killed 50 to 80% of the overstory pine
in Fraser’s research watersheds.
Seasonally Dependent
Old Growth > Mixed Age
0
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Related Citations
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Biogeochemistry of Beetle Kill: Explaining a weak nitrate response.
Rhoades et al. 2013; PNAS
Changes in transpiration and foliage growth in lodgepole pine trees following mountain
pine beetle attack and mechanical girdling. Hubbard et al. 2013; FEM
Responses of soil and water chemistry to mountain pine beetle induced tree mortality in
Grand County, Colorado, USA. Clow et al. 2011; Applied Geochem
Tree regeneration and future stand development after bark beetle infestation and
harvesting in Colorado lodgepole pine stands. Collins et al. 2011; FEM