Carbon Dioxide and Methane Fluxes in Disturbed Southwestern

Carbon Dioxide and Methane Fluxes in Disturbed Southwestern Ponderosa Pine Forests Kolb 1, T. E. , M. Montes-Helu 1, S. Dore 1, S. Hart 1, J. Kaye 2, B. Sullivan 1, B. Hungate 3, G. Koch 3 1 Northern Arizona University 1 School of Forestry, 3 Department of Biology 2 The Pennsylvania State University, Department of Crop and Soil Science STUDY OVERVIEW Restoration thinning Absence of management and wildfire C Sink Frequent, low- Possible post-thinning responses w/out lowintensity frequent fire Intensity fire Fire suppression Post-thinning reestablishment of frequent, low-intensity fire 0 Intense Conversion to wildfire sparse forest Conversion to C Source Net Ecosystem Production (g m 2 yr-1) 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 (CO 2), and on soil fluxes of CO 2 and methane (CH 4), 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. Fuelreduction, 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. grass/shrubland Time (yr) Our overall objectives are to better understand fluxes of CO 2, CH 4, 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. 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 Instrument pine forests of the southwestern US. Most oldgrowth trees in the stand were harvested between 1914 -16. The stand currently contains trees from Control Site several regeneration cohorts; most overstory trees are about 87 years old. In 2006, tree basal area averaged 30 m 2 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. 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 Instrument 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 Restoration Site Thinning, 2006 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 CO 2, 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 m 2 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 m 2 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 structure in the measurement footprint of the instrument tower. Wildfire site: This site (left) is located on the Coconino National Instrument Forest approximately 32 km from the control and restoration sites and within the perimeter of the Wildfire Site 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 m 2 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.