Introduction: Forest Carbon Storage Modeling Timely and Important

Policy makers at every level recognize the importance of forests in the global carbon cycle, and there is a growing consensus that protecting forestland and enhancing its carbon stores will be an important component of any attempt to mitigate climate change.  Whether it's Congress considering a national cap-and-trade system, the Regional Greenhouse Gas Initiative (scheduled for the northeastern U.S. in 2009), or the California Climate Action Registry Forest Project Protocol, forests will play an important role in carbon storage and mitigating climate change.  As these efforts unfold, it's important to develop accurate measures of the carbon stored in forests and the changes in those stores over time.

Existing Modeling Tools

To their credit, U.S. Forest Service researchers, working from Forest Inventory and Analysis (FIA) data designed primarily to track timber supplies, have put together the available information and have begun to provide carbon estimation tools to the general public via published reports and user-friendly web access.  The four modeling tools developed to-date represent an important step toward a better understanding of forest carbon storage and can be used to make broad comparisons across large regions.  However, they lack the calibration and resolution required for the purpose of registering or selling carbon credits.  Individual offset projects require field sampling that reflects the special characteristics of each forest.

Five Important Trends - Carbon Stored, Value of Public Lands, Old Growth

While the existing modeling tools can be improved, several broad trends are already apparent. 

• One, American forests store a tremendous amount of carbon, and forests in the contiguous states alone hold carbon reserves equivalent to more than 20 years of current U.S. greenhouse gas emissions from industrial and other sources.  Maintaining and enhancing these carbon stores is an important component of any policy to address climate change.  While it is important to protect all forests, carbon stored in forest lands is unevenly distributed across the landscape (See pages 7-8 and Figure 1).
• Two, on average public forest lands such as National Forests and State Forests appear to hold more carbon per acre than private lands (See Figure 3, page 9).
• Three, reserved forest lands, where timber harvest is prohibited such as in Wilderness, National Parks, and National Monuments, typically hold more carbon per acre than non-reserved lands (See Figure 4, page 9).
• Four, distribution of carbon varies across the landscape, but, in general, the amount of carbon stored above ground in trees - what we can measure with the most confidence - is less than half the total (See Figure 2, page 8).
• Five, because existing carbon measurement tools are based on models of intensively managed forests, they may well underestimate carbon stores on older or unmanaged forests.  They also do not account for old growth forests often being more resilient in the face of climate change-related stresses (See pages 14-16).

Limits on Forest Carbon Modeling

Highly precise measurement would require cutting down trees, digging up roots, collecting rotten logs and understory vegetation, extracting carbon from soils, etc., so carbon accounting necessarily relies on estimates.  Estimates based on field sampling would be ideal, but at a national level direct sampling across all regions and forest types would be a daunting task.  Hence, researchers developing national or regional estimates rely on existing national timber inventories such as the USDA Forest Service Forest Inventory and Analysis system to approximate forest carbon measurements. 

Some general limitations apply to all national estimates of forest carbon (See pages 9-13):

• One, the parts of the forest, from soil to dead trees, that together store most of the carbon are characterized with the least data.  Assumptions in the models used to fill data gaps such as these strongly influence the estimates.
• Two, the uncertainty and time lags involved with estimating forest carbon stores at a national level make it difficult to know how much carbon is added to those forest stores each year.  If we do not know for certain how much additional carbon our forests are storing each year, there is a danger of overestimating the forests' contribution, which could reduce the drive to limit fossil fuel use.  On the other hand, if we underestimate the carbon fixed by old growth forests we may fail to give them adequate protection.

Recommendations

The current modeling tools for regional and national forest carbon estimates come with significant limitations that must be understood by both policymakers and the public.  As the United States moves rapidly toward a national cap-and-trade system, which will likely incorporate the trading of forest-based offsets, land management agencies and regulators must:

• One, clearly explain the limitations of available tools and models that estimate forest carbon, especially when applied to individual properties.
• Two, require rigorous field sampling to document increases in forest carbon stores credited as offsets to regulated emissions sources.
• Three, accelerate the phase-in of the new national FIA survey design to improve information on the many different forms (such as standing dead, fallen trees or soils) in which forests store carbon, and incorporate this new information into forest growth and carbon models.
• Four, support research that investigates the role of older forests in storing long-term carbon, assesses the potential to enhance forest carbon stores through reserves or active timber management, and evaluates strategies to help forest systems weather future climate stresses by harboring a diversity of species and creating stable microclimates.

Conclusion

Given the incomplete information about forest carbon, climate change policies should focus primarily on direct emissions reductions.  Forest carbon wealth should be protected as a natural and national asset along with the many other important services that healthy forests provide - from clean water to wildlife habitat.

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