Carbon benefits from forested protected areas for biodiversity conservation in the conterminous United States

Background/Question/Methods

Conversion of forests to other land uses or land covers not only releases the carbon stored in the forests, but also reduces carbon uptake. Designating lands as protected areas, which exclude human occupation or exploitation for resources to conserve biological diversity, has been shown to have lower rates of forest conversion to nonforest than those without protection status. The overall goal of this study was to estimate the carbon benefits in the period 1992-2001 in forested ecosystems of the conterminous USA due to protected area designation. Although methodologies exist for estimating carbon benefits at the project-level, standard approaches at the state- or national-level are not available. Our approach included national-level datasets including the remote-sensing based National Land Cover Retrofit Change Map to calculate forest change, the most updated Protected Area Dataset of the United States to distinguish protected forestlands from unprotected forestlands, and forest carbon density and growth tables based on USDA Forest Service Forest Inventory and Analysis field data to convert forest change to carbon change. We compared and statistically tested forest change rates in protected areas to forest change rates in unprotected areas by state, and estimated associated carbon changes.

Results/Conclusions

Approximately 10% (207,000 km²) of the forestland in the conterminous USA was designated as protected areas for biodiversity conservation (codes 1 and 2), with 60% of protected area forests in the eleven western states. The area-weighted net forest area change rate in the conterminous USA’s unprotected forests was about -4.1% for the 9 year-period, as compared to -0.74%  for protected areas.  These rates differed significantly (Friedman test chi-squared=55.29, df=2, p<0.0001). Pairwise comparisons by state showed highly significant differences (p<0.0001) between protected and unprotected lands. Conversely, afforestation rates are lower in protected areas, with a forest area change of 0.76% rate as compared to 2.25% over the 9 years.  Overall, these area change differences have resulted in forest ecosystem carbon benefits of 7.4 TgC per year during 1992-2001 due to protected area designation as compared to unprotected forests. These results indicate that protected areas for biodiversity conservation in the temperate forests of the conterminous USA significantly reduce forest conversion and contribute measureable forest carbon benefits.