Mo

istu

re (

v /v)

0

10

20

30

40

50

Mar May Jul Aug Oct Dec Mar May Jul Aug

NH

4-N

(m

g/k

g so

il)

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

NO

3-N

(m

g/k

g so

il)

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

N c

on

cen

trat

ion

(%

)

a

c

b b

0

20

40

60

80

100

S-facing, Low S-facing, High

C:N

rat

io

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

NH

4-N

(m

g/k

g so

il)

Slope

0

2

4

6

8

10

12

Mar May Jul Aug Oct Dec Mar May Jul Aug

NO

3-N

(m

g/k

g so

il)

Date

Slope

2015 2016 2015 2016

S-facing, Low (Hollow)

S-facing, High (Slope) S-facing, High (Hollow)

Available N in soil

Tem

per

atu

re (

°C)

Mo

istu

re (

v /v)

Tem

per

atu

re (

°C)

Mo

istu

re (

v /v)

Tem

per

atu

re (

°C)

Mo

istu

re (

v /v)

Tem

per

atu

re (

°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 SWE by 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