Feedbacks and transitions linking climate change, hydrologic
partitioning, and carbon cycling in continental montane forests
Paul D. Brooks, Marcy Litvak, Adrian Harpold, Jennifer McIntosh,
Joel Biederman, Noah Molotch, Peter Troch, Xavier Zapata, and many others
Focus on the role of variability in snow cover
Why focus on snow?
Snowmelt is the primary
renewable water resource in
western North America
Research has demonstrated
that snowmelt is critical for
sustaining net carbon
uptake throughout the
summer and fall
The amount and timing of
snow cover is changing in
much of the west
Figure from Schimel et al. 2002
How Will Climate Change Affect Seasonal Snow Cover?
Need to Consider Natural Spatial and Temporal Heterogeneity
Seasonal snow cover exhibits
reproducible spatial patterns
related to redistribution and to
local energy balance
August 8, 1995
July 1, 1996
Net snow water input (SWE:P)
is variable in space and time
Snow cover radically changes
surface energy balance
(albedo, longwave radiation,
and latent energy fluxes)
Development of snow cover
has a positive feedback
increasing the duration of
snow cover
July 1, 1997
From Dozier et al.
The Big Questions: How much precipitation? When is it
available? and What fraction is available to vegetation?
How does variability in
vegetation structure affect
the spatial heterogeneity
and hydrologic partitioning
of seasonal snow cover?
Carbon Flux
NEE g C m-2
How do changes in
precipitation, especially in
the amount and timing of
seasonal snow, affect
ecosystem carbon balance?
Data from Monson et al.
JSC Critical Zone Observatory/ SAHRA Ecohydrological
Observatory: Valles Caldera National Preserve, NM
Colorado SWE
New Mexico SWE
•  Southern margin of cold,
temperate snowpacks
•  Sensitive indicator for climate
change in mountain forests
How does variability in vegetation structure influence the
development and partitioning of seasonal snow cover?
40
40
40
40
35
35
SWE [cm]
30
30
35
35
30
30
25
25
25
25
20
20
20
20
15
15
15
15
12-Feb
12-Feb
12-Feb
14-Mar
14-Mar
10
10
55
16-Apr
00
10
10
55
00
44
33
22
0.5 00 0.5
0.5 11
11 0.5
22
33
44
Distance From
From Tree
Tree Bole
Bole [m]
[m]
Distance
1. Vegetation shading
increases snow depth and
delays spring melt
2. Modeling suggests that
this vegetation shading
increases snowpack water
storage approximately 25%
Shading
Interception
How does variability in vegetation structure influence the
development and partitioning of seasonal snow cover?
Mean Open
Large-scale field survey to
evaluate the effects of forest
density on SWE
Mean Forest
n
=
40
11
0
=
n
=
n
n
• Approximately 20% more net
water input than if there were no
trees
=
65
13
0
• SWE peaks in moderately dense
forests
• Tracers confirmed that the
differences are due to spatially
variable sublimation related to
vegetation controls on local energy
balance (Gustafson et al. 2010)
How does variability in vegetation structure influence the
development and partitioning of seasonal snow cover?
N
Confirms that these
patterns in net SWE
input are widespread
High 1.1
Largely match energy
balance model
predictions
Low 0.0
Because vegetation changes on short time scales changes in
vegetation structure can have a major impact on net water input
(Harpold et al. 2012; Broxton et al. 2012; Rinehart et al. 2009)
Take home points #1
•  Snow cover is highly heterogeneous; A large and variable
fraction of winter snowcover sublimates before melt season
•  Vegetation structure strongly controls spatial (and temporal)
variability in snowpack development, ablation, and net water
balance
•  Moderately dense forest canopies increase net snow water
input relative to open areas or closed canopies by
minimizing sublimation
How does spatial and temporal variability in snow cover
affect ecosystem carbon balance?
Address this question using five years continuous eddy flux
observations from our mixed conifer forest site
Snow cover
Melt
Precip Tave
SWE
2007
274
340
-4
2008
262
359
-5
2009
188
232
-2
2010
330
473
-5
2011
102
51*
-11
Oct
Dec
Feb
Apr
Broad range of climate
•  T varied by a factor of 5
•  SWE varied by a factor of 3
•  Snow cover duration by 1.6
How does spatial and temporal variability in snow cover
affect ecosystem carbon balance?
• Directly through the amount of
water released from seasonal
snow cover
SWE (mm)
• Indirectly through timing of snowpack development
Reduced infiltration
Soil frost
%
%
'%
)%
*%
'%
)%
*%
,%
&%%
"%
!2"
!
"
" !%
! %
4%%,-*3&&*%(
.4%+-
+ %( '&"
)+ (%
# $ % & Summary
Water balance: A large and variable
fraction of winter snow sublimates
before melt
Vegetation Controls: These
sublimation fluxes are minimized
in moderately dense forests
Energy Balance: Snow changes
local energy balance and has a
positive feedback on the persistence
of snow cover
Climate Sensitivity: There is more
climate sensitivity in the start date
of snow cover than in melt initiation
Carbon Linkages: Both the amount and especially the
timing of snow cover are strongly related to winter
carbon efflux and growing season carbon uptake