Authors

  • Dan Ek, FIA Supervisory Forester, Forest Inventory and Analysis Program, Colorado State Forest Service
  • Dakota Dolan, Forest Inventory and Analysis Program, Colorado State Forest Service

Editors

  • Sara Goeking, PhD, FIA Deputy Program Manager, USDA Forest Service Rocky Mountain Research Station
  • Wilfred Previant, PhD, Asst. Professor, Warner College of Natural Resources, Colorado State University
  • Amanda West Fordham, PhD, Associate Director of Science and Data, Colorado State Forest Service
  • Ethan Bucholz, PhD, Academic Liaison and Experiential Learning Specialist, Colorado State Forest Service

Introduction

The Forest Inventory and Analysis (FIA) program of the USDA Forest Service (USFS) has operated since 1930. In Colorado and Wyoming, the USFS collaborates with the Colorado State Forest Service to conduct and continuously update a comprehensive inventory of forest conditions in the two states. The FIA program annually surveys 10 percent of thousands of permanent plots in each state. Each plot represents 6,000 acres of land across all ownerships. The inventory includes information about trees, seedlings, saplings, shrubs, forbs, grasses, down woody material and soils. Additional information including lynx habitat, lichens and root diseases are sometimes collected.

Scientists from the USFS Rocky Mountain Research Station analyze FIA plot data. They produce reports every five years on the health of Colorado’s forests, along with a wide range of other scientific studies. These data are available to the public online for state-level forest analyses.

FIA data can be analyzed using tools such as EVALIDator 2.0 and DATIM, found on the USFS FIA website. These tools are public domain, used by scientists, land managers, corporations and individuals. For this analysis, EVALIDator was used to analyze carbon stores in forested land in Colorado from 2010 to 2019.

Carbon Pools in Forests

Foresters generally recognize five main carbon pools within forests. The EVALIDator tool uses species-specific allometric relationships to calculate carbon stocks across these pools from FIA plot measurement data. The summaries and graphs within this factsheet represent the cumulative total of these five pools (Burrill et al., 2018).

  • Live above-ground biomass consists of all living biomass above the soil including stems, branches and bark but excludes foliage.
  • Live below-ground biomass includes all living biomass of coarse living roots ≥ 0.1 inches in diameter.
  • Dead wood includes all non-living woody biomass either standing, lying on the ground (excluding litter) or in the soil.
  • Forest floor litter includes the organic material on the floor of the forest, including fine woody debris, humus and fine roots in the organic forest floor layer above the mineral soil.
  • Soil organic carbon includes all fine organic material in soil to a depth of 39 inches but excludes the roots of the belowground pools.

Results

Percentage of Total Forest Carbon by Pool

Forests store carbon differently based on species composition, disturbance and climate. Temperate and boreal forests store most of their carbon in dead biomass (soil organic carbon, forest floor litter, dead wood), whereas tropical forests store it mostly in living biomass (live above/below-ground biomass) (Pan et al., 2011). In our temperate Coloradan forests, the largest individual pool is live above-ground biomass, followed closely by soil organic carbon and forest floor litter pools (Fig. 1).

pie chart that show percentage of total forest carbon by Pool in Colorado: 6.2% live below-ground, 9.5% Dead wood, 26.5% Forest floor litter, 28.7% soil organic carbon, 29.1% live above-ground.
Figure 1: Percentage of Total Forest Carbon by Pool in Colorado. Click on image to enlarge.

Carbon Stocks by Forest Type

The forest type with the largest storage of carbon in Colorado is spruce-fir, followed by aspen and piñon-juniper woodlands (Fig. 2). In the American Southwest, water availability is a primary factor in forest productivity. Species including Engelmann spruce, subalpine fir and aspen are well adapted to higher elevations where there is generally more available soil moisture. Shorter growing seasons and typically longer periods between stand-replacing disturbances create conditions for large amounts of carbon storage within spruce-fir forests. Aspen are pioneer species that typically occupy sites quickly after disturbance and demonstrate faster growth rates and therefore elevated rates of carbon sequestration. However, some might be surprised to learn that piñon-juniper woodlands rank third in terms of total carbon sequestered, given that they thrive in arid environments. Although piñon-juniper woodlands are less productive, they cover the greatest area of any forest cover type in the state, at approximately 6.48 million acres. There are approximately 4.66 million acres of spruce-fir and 3.5 million acres of aspen in Colorado. Despite the considerable differences in acreage between these forest types, the increased water availability at higher elevations and genetic growth potential allows spruce-fir and aspen forest types to lead in terms of total carbon storage.

Histogram with Tons Carbon on y axis and types of forests listed on x-axis.
Figure 2: Total Carbon by Forest Type in Tons (US) in Colorado. Click on image to enlarge.
horizontal bar graph that shows the carbon density by forest type with the types of forests listed on the y axis and the tons carbon/acre on the x-axis.
Figure 3: Carbon Density by Forest Type in Tons (US)/ac in Colorado. Click on image to enlarge.

Carbon Density by Forest Type

Differences in carbon dynamics between each forest type can be further understood by assessing the carbon density of each (Fig. 3). Within the consolidated forest types of this factsheet, spruce-fir forests have the highest density at 67.35 t (US)/ac. Aspen forests are the second most dense at 65.64 t (US)/ac, followed by mixed conifer forests at 61.19 t (US)/ac. Despite being the forest type with the third largest amount of carbon stored, piñon-juniper woodlands rank low among the forest cover types in terms of carbon density at only 28.40 t (US)/ac, further highlighting the climatic conditions of piñon-juniper forests. Across all forest types, Colorado averages 47.63 t (US)/ac on forestlands, which is similar to Wyoming and higher than any of our bordering states.

Summary

Forests function as both carbon stocks (sinks) and carbon sources simultaneously. A carbon sink is a pool that sequesters carbon from the atmosphere, while a carbon source releases it. Net primary productivity (NPP) is a means to measure this. It represents the difference between productivity (i.e., photosynthesis and carbon fixation) and respiration.

Disturbances can shift forests from being carbon sinks to carbon sources, or vice versa, and disturbances can be either natural or anthropogenic. Events such as insect outbreaks, disease and windthrow transfer carbon between the five pools mentioned prior and reduce NPP. Fires chemically transform and release carbon in addition to shifting carbon between pools and reducing NPP. Anthropogenic disturbances, such as harvesting, can serve to store carbon in long-lasting products and improve the stability of remaining stocks, but unavoidably shift carbon between pools and temporarily reduce NPP. In the case of a mountain pine beetle outbreak, it can take forests 5-40 years to recover to pre-disturbance levels (Hansen, 2013). Severe disturbances can have lasting consequences for carbon storage and sequestration.

There are many ways in which carbon dynamics, storage and sequestration can be analyzed and assessed. This factsheet is but a small sample. Based on a study by Domke et al. (2020), Colorado’s forests emit more carbon than they store. Climatic changes in precipitation and temperature and associated shifts in typical fire regimes, along with outbreaks of forest pests and diseases, will continue to negatively affect forest health in the state. However, gaining a better understanding of the complexities of carbon can help inform management strategies, identify priority areas, and promote climate mitigation and adaptation. The resulting efforts can work to conserve existing carbon stocks while producing forest products and maintaining or even improving net primary productivity.

Clarifications             

Forest Type Descriptions

  • Other Conifer includes the Douglas-fir, southwestern white pine and white fir forest types.
  • Other Hardwoods includes the sugarberry/hackberry/elm/green ash and other exotic hardwoods forest types.
  • Woodland Hardwoods includes the Cercocarpus (curl-leaf mountain mahogany) woodland and deciduous oak woodland forest types.
  • Cottonwood includes both the cottonwood and cottonwood/willow forest types.

References

Burill, E.A., Wilson, A.M., Turner, J.A., Pugh, S.A., Menlove, J., Christiansen, G., Conkling, B.L., & David, W. (2018). The Forest Inventory and Analysis Database: Database description and user guide version 8.0 for Phase 2. U.S. Department of Agriculture, Forest Service. Retrieved from http://www.fia.fs.fed.us/library/database-documentation

Domke, G.M., Walter, B.F., Nowak, D.J., Smith, J., Ogle, S.M., Coulston, J.W., & Wirth, T.C. (2020). Greenhouse gas emissions and removals from forest land, woodlands, and urban trees in the United States, 1990-2018. Resource Update FS-227. Madison, WI: U.S. Department of Agriculture, Forest Service, Northern Research Station. https://doi.org/10.2737/FS-RU-227

Hansen, E.M. (2013). Forest development and carbon dynamics after mountain pine beetle outbreaks. Forest Science, 60(3), 476-488. http://dx.doi.org/10.5849/forsci.13-039

Pan, Y., Birdsey, R.A., Fang, J., Houghton, R., Kauppi, P.E., Kurz, W.A., Phillips, O.L., Shvidenko, A., Lewis, S.L., Canadell, J.G., Ciais, P., Jackson, R.B., Pacala, S.W., McGuire, A.D., Piao, S., Rautiainen, A., Sitch, S., & Hayes, D. (2011). A large and persistent carbon sink in the world’s forests. Science, 333(6405), 988-993. https://doi.org/10.1126/science.1201609