1 Northern Arizona University 1School of Forestry, 3Department of Biology

1
Carbon Dioxide and Methane Fluxes in Disturbed
Southwestern Ponderosa Pine Forests
Kolb1, T.E., M. Montes-Helu1, S. Dore1, S. Hart1,
J. Kaye2, B. Sullivan1, B. Hungate3, G. Koch3
1 Northern Arizona University 1School of
Forestry, 3Department of Biology 2The
Pennsylvania State University, Department of Crop
and Soil Science
STUDY OVERVIEW This poster introduces
overall project goals and study sites under
current investigation by the Northern Arizona
University Carbon Flux Research Group. Results of
the effects of wildfire on energy balance and net
ecosystem exchange of carbon dioxide (CO2), and
on soil fluxes of CO2 and methane (CH4), are
reported in this session in posters by Dore et
al. and Sullivan et al., respectively. Our
study system is ponderosa pine (Pinus ponderosa)
forests in northern Arizona. These forests are
dominated by dense thickets of small trees that
are susceptible to high-intensity, large,
wildfires, as are many forests of the interior
western US. Fuel-reduction, or restoration,
treatments are being implemented in such forests
over large landscapes with uncertain effects on
carbon, water, and energy balances. Below is
a diagram of trajectories of net ecosystem
production (NEP) for ponderosa pine stands where
the frequent-fire regime, which was common prior
to European settlement, was interrupted by fire
suppression leading to an increase in NEP,
followed by three possible scenarios a) NEP
peaks and gradually declines in the absence of
management and wildfire b) Intense wildfire
rapidly converts the carbon sink to a source to
the atmosphere, followed by unknown rates of
recovery c) Restoration thinning is used to
avoid stand-replacing wildfire by reducing fuels,
followed by unknown effects on NEP and rates and
trajectories of recovery.
STUDY SITES

Control, unmanaged site The site (left) is
located on one section (267 ha) that has not
been thinned nor prescribed burned for many
decades. It is typical of unmanaged stands in
ponderosa pine forests of the southwestern US.
Most old- growth trees in the stand were
harvested between 1914-16. The stand currently
contains trees from several regeneration
cohorts most overstory trees are about 87
years old. In 2006, tree basal area averaged
30 m2 ha-1, leaf area index (projected) averaged
2.3 (97 in trees), which is close to the maximum
for southwestern ponderosa pine forests, and
coarse woody debris averaged 13.3 Mg ha-1.
Instrument
Restoration site This site (left)
is located 8 km from the control
site on similar basalt-derived
soil. The stand was lightly thinned
for saw timber and poles several
times between 1956 and 1991. The
latest thinning occurred in
September 2006 (after the far left photo
was taken) and removed thickets of
small-diameter trees following restoration
guidelines commonly used for fuel
reduction projects in northern
Arizona. The 2006 thinning occurred on 167 ha in
the footprint of the instrument tower that
measures fluxes of CO2, water, and energy. Slash
produced by the thinning was piled on site and
will be prescribed burned in 2007 or 2008. The
thinning reduced tree density by 67 (465 to 154
trees ha-1), tree basal area by 39 (20.5 to 12.6
m2 ha-1), and leaf area index by 41 (1.6 to
0.9). Coarse woody debris in 2006 before thinning
averaged 8.4 Mg ha-1. The site is surrounded by
the larger Woody Ridge Forest Restoration Project
administered by the Coconino National Forest. The
sections to the east of the site were thinned to
an average basal area of 11 m2 ha-1 in winter
2006 (visible in the photo, left, taken in early
2006), and the sections to the south and north
are scheduled for thinning in 2007. Thus, the
site is embedded in a larger, managed landscape
to ensure similar land use and stand structure in
the measurement footprint of the instrument
tower.
Instrument

Wildfire site This site
(left) is located on the
Coconino National Forest
approximately 32 km from
the control and restoration sites
and within the
perimeter of the
Horseshoe-Hochderffer fire complex
that burned 10,500 ha of ponderosa
pine forest in 1996. Prior to the
fire the stand had a tree
density of 343 ha-1, tree basal
area of 31 m2 ha-1, and leaf area
index of 2.4, based on measurements of
adjacent unburned stands.
The fire killed all trees within major portions
of the burned area. Natural regeneration of trees
has not occurred in the tower footprint in the 10
years since the fire. Slow regeneration is common
in southwestern ponderosa pine forests after
severe fire. Leaf area index averaged 0.6 in 2006
and is comprised entirely of herbaceous plants
and a few shrubs. All trees in the tower
footprint killed by the fire are present as
standing snags or downed logs salvage logging
did not occur after the fire. Coarse woody debris
averaged 16.0 Mg ha-1 in 2006.
Instrument
Our overall objectives are to better
understand fluxes of CO2, CH4, water, and energy
for three of these scenarios which are most
common in southwestern ponderosa pine forests 1)
unmanaged, 2) managed by restoration treatments,
3) conversion to grass/shrubland by wildfire.