1. Moisture(v/v)
0
10
20
30
40
50
Mar May Jul Aug Oct Dec Mar May Jul Aug
NH4-N(mg/kgsoil)
Hollow
N-facing, Low
S-facing, Low
N-facing, High
S-facing, High
0
2
4
6
8
10
12
Mar May Jul Aug Oct Dec Mar May Jul Aug
NO3-N(mg/kgsoil)
Date
Hollow
Methods
Study Site: Four topographically contrasting sites in Lubrecht
Experimental Forest, located in SW Montana
Site characteristics: South-facing vs. North-facing at low (~1280m)
and high elevation (~1800m). Topographically contrasting subsites
within each site (side slope vs. hollow).
Metrological measurements: Air temperature; soil moisture and
temperature at 10, 30, and 50cm deep.
Snow-water equivalent (SWE) and snow chemistry: Snow cores
were collected from each site . SWE and the concentrations of NH4
and NO3 were determined.
Plant-available N: Top 10cm soil was collected monthly from 5
hollow locations and 20 slope locations per site. The soil was
extracted with 1N-KCl, and NH4 and NO3 concentrations were
determined.
Plant tissue: Buds and needles were harvested biweekly from five P.
menziesii trees in two S-facing sites, and total C and N determined.
Dominant tree species: Pseudotsuga menziesii (Douglas fir), Pinus
ponderosa (Ponderosa pine), Larix occidentals (Western larch), Abies
lasiocarpa (Subalpine fir), Picea Engelmannii (Engelmann spruce),
and Pinus contorta (Lodgepole pine).
Introduction
Seasonal snowpack plays a critical role in montane forest
ecosystems in the Western U.S. in two processes; first, the snowpack
stores much of annual precipitation, and second, it controls nitrogen
(N) dynamics by influencing N mineralization rates over winter by
insulating the soil underneath it. While projected decline of seasonal
snowpack with warmer air temperature is expected to alter water
availability and N dynamics in these forests, how the changes affect
forest productivity and whether the changes differ across complex
mountain topography are poorly understood. To predict the
response of Western forests to reduced snowpack in the future, it is
essential to understand the baseline mechanisms of how topography
controls temporal and spatial distribution of plant-available water and
N.
Objective
To characterize spatial and temporal distribution of soil water and
plant-available N across a complex mountain forest landscape.
Discussion & Ongoing efforts
• Topography (elevation, aspect, hollow vs. slope) controls temporal
windows during which the environmental conditions are favorable for
N mineralization.
• Lower SWE and early snowmelt may influence N availability in early
spring.
• The soil N availability appeared to affect N levels in buds. The similar
%N and C:N in mature needles suggests greater allocation, not
uptake, of N in the drier and warmer site.
• Micro-topography plays a larger role than elevation in controlling soil
moisture and N availability.
• Whether trees can utilize N that becomes available seasonally is
currently being investigated in the field and in a greenhouse
experiment.
Results
c
a
b b
0.0
0.5
1.0
1.5
2.0
2.5
3.0
S-facing, Low S-facing, High
Nconcentration(%)
a
c
b
b
0
20
40
60
80
100
S-facing, Low S-facing, High
C:Nratio
Fig. 1. Temperature and soil moisture content across contrasting topography.
The shaded areas indicate relatively favorable conditions for N mineralization:
soil temperature above 0°C with soil moisture content above 5%. The favorable
window was longer in the hollows and at high-elevation sites because of deeper
snowpack, which insulated the soil beneath it while providing moisture.
Fig. 2. Plant-available N in soil across sites. The levels of available N were
consistently higher at the high elevation sites and within hollow. Larger
snowpack year (2015) resulted in higher N availability in early growing
season.
Fig. 3. Percent N and C:N ratio of buds and mature P. menziesii needles. N
availability in soil (2015) was reflected to 2016 buds, which are formed in 2015. The
effect appeared to be carried on to 2016 mature needles.(bud, n=5; needles, n=3-4)
SWE: 27% (Low elev.) and 41% (high elev.) less than 2015.
First snow-free day: 3-4 weeks earlier than 2015.
0
10
20
30
40
50
Mar May Jul Aug Oct Dec Mar May Jul Aug
NH4-N(mg/kgsoil)
Slope
0
2
4
6
8
10
12
Mar May Jul Aug Oct Dec Mar May Jul Aug
NO3-N(mg/kgsoil)
Date
Slope
2015 2016 2015 2016
S-facing, Low (Hollow)
S-facing, High (Slope)S-facing, High (Hollow)
Available N in soil
Temperature(°C)Moisture(v/v)
Temperature(°C)Moisture(v/v)
Temperature(°C)Moisture(v/v)
Temperature(°C)
Elevation
(m)
Total PPT
(cm)
Max SWE*
(cm)
N inputs via
snowmelt**
(kg/ha)
Low elev. 1426 44 15 0.1
High elev. 1905 41 26 0.2
* Snow -w ater equivalent
**estimated by multiplying maximum SWEby inorganic N conconcentrations in snowpack
Table 1. Snow and N inputs at high and low elevation SNOTEL sites
in 2015 water year. N inputs via snow is a background level.
S-facing, Low (Slope)
Tissue N
2016 buds, immediately before bud break ( ) grew into 2106 mature needles ( )
Linking Topography, Snow Regimes, Nitrogen Dynamics, and Montane Forest Productivity
Yuriko Yano1, Jia Hu1, Claire Qubain1, Kelsey Jensco2
1Montana State University; 2University of Montana, Montana