CLIMATE CHANGE MITIGATION AND ADAPTATION IN FOREST SYSTEMS

Natural disturbances in part determine the carbon carrying capacity of a given forest by limiting the amount of carbon that can be sustained (Keith et al. 2009). Wildfire is a natural disturbance that contributes the equivalent of 4-6% of annual anthropogenic emissions in the United States (Wiedinmyer and Neff 2007) and has legacy effects on carbon sequestration that can persist for years (Dore et al. 2008, Meigs et al. 2009). Fire severity and the resultant emissions can be influenced by forest management (Hurteau et al. 2008, North et al. 2009) with treatments that improve the resistance of forest carbon to loss by fire, thereby increasing the permanence of the forest carbon stock (Hurteau et al. 2008, Hurteau and North 2009, North et al. 2009). However, the carbon balance of wildfire risk mitigation depends on forest type. In mesic forests of the Pacific Northwestern US, where historic mean fire return intervals are long, fuels reduction treatments yield a net reduction in forest carbon stocks (Mitchell et al. 2009). In dry temperate forests, such as ponderosa pine or Sierran mixed-conifer forest, fuels reduction treatments can actually increase carbon storage by reducing wildfire emissions and reducing tree mortality (Finkral and Evans 2008, Hurteau and North 2009, Mitchell et al. 2009). Regardless of forest type, wildfire risk mitigation treatments incur immediate carbon costs. Tree removal and prescribed fire emissions are a function of the treatment implemented on a given site and represent the largest emissions sources, with equipment related emissions representing less than 1-3% of the forest carbon (North et al. 2009). While thinning does reduce carbon stocks, it can serve to consolidate the remaining carbon in fewer larger trees that continue to sequester carbon. Additionally, in some forested systems prescribed fire emissions can be quickly re-sequestered by the remaining trees on site (Hurteau and North 2010). In many of these dry forest types, past land management including grazing and fire suppression has fundamentally altered the forest structure. Although some research suggests that this has led to increased tree density and forest carbon stocks (Hurtt et al. 2002), recent studies comparing forest inventories from the 1930s with current forest inventory data in the Sierra Nevada found that while forest density has increased, carbon stocks have decreased (Fellows and Goulden 2008). If generally true, this suggests that structural manipulations targeted at restoring fire in these systems may sequester more carbon by favoring retention of fewer, larger trees that are resistant to loss from high-severity fire (Hurteau et al. 2008, Hurteau et al. 2009). This type of forest management, designed to reduce fuels and facilitate fire reintroduction, may best emulate natural disturbance patterns. Forest productivity is a function of many abiotic and biotic factors, including climate. Recovery from past disturbance, including land-use and land-use change, has yielded a net increase in the amount of forest carbon sequestered over the past century (Hurtt et al. 2002). The increase in net primary productivity has been further enhanced by anthropogenic emissions of carbon dioxide and nitrogen. As anthropogenic emissions continue to alter the climate, forest productivity and distribution is likely to be impacted. Since high-severity wildfire and other disturbances can reset the successional stage of a forest, these disturbances could act as a catalyst for landscape and regional change in the distribution of forests (Hurteau and Brooks 2011). Yet, there is uncertainty surrounding many aspects of how forests will respond to changing climate, disturbance regimes, and management actions. Natural disturbances in part determine the carbon carrying capacity of a given forest by limiting the amount of carbon that can be sustained (Keith et al. 2009). Wildfire is a natural disturbance that contributes the equivalent of 4-6% of annual anthropogenic emissions in the United States (Wiedinmyer and Neff 2007) and has legacy effects on carbon sequestration that can persist for years (Dore et al. 2008, Meigs et al. 2009). Fire severity and the resultant emissions can be influenced by forest management (Hurteau et al. 2008, North et al. 2009) with treatments that improve the resistance of forest carbon to loss by fire, thereby increasing the permanence of the forest carbon stock (Hurteau et al. 2008, Hurteau and North 2009, North et al. 2009). However, the carbon balance of wildfire risk mitigation depends on forest type. In mesic forests of the Pacific Northwestern US, where historic mean fire return intervals are long, fuels reduction treatments yield a net reduction in forest carbon stocks (Mitchell et al. 2009). In dry temperate forests, such as ponderosa pine or Sierran mixed-conifer forest, fuels reduction treatments can actually increase carbon storage by reducing wildfire emissions and reducing tree mortality (Finkral and Evans 2008, Hurteau and North 2009, Mitchell et al. 2009). Regardless of forest type, wildfire risk mitigation treatments incur immediate carbon costs. Tree removal and prescribed fire emissions are a function of the treatment implemented on a given site and represent the largest emissions sources, with equipment related emissions representing less than 1-3% of the forest carbon (North et al. 2009). While thinning does reduce carbon stocks, it can serve to consolidate the remaining carbon in fewer larger trees that continue to sequester carbon. Additionally, in some forested systems prescribed fire emissions can be quickly re-sequestered by the remaining trees on site (Hurteau and North 2010). In many of these dry forest types, past land management including grazing and fire suppression has fundamentally altered the forest structure. Although some research suggests that this has led to increased tree density and forest carbon stocks (Hurtt et al. 2002), recent studies comparing forest inventories from the 1930s with current forest inventory data in the Sierra Nevada found that while forest density has increased, carbon stocks have decreased (Fellows and Goulden 2008). If generally true, this suggests that structural manipulations targeted at restoring fire in these systems may sequester more carbon by favoring retention of fewer, larger trees that are resistant to loss from high-severity fire (Hurteau et al. 2008, Hurteau et al. 2009). This type of forest management, designed to reduce fuels and facilitate fire reintroduction, may best emulate natural disturbance patterns. Forest productivity is a function of many abiotic and biotic factors, including climate. Recovery from past disturbance, including land-use and land-use change, has yielded a net increase in the amount of forest carbon sequestered over the past century (Hurtt et al. 2002). The increase in net primary productivity has been further enhanced by anthropogenic emissions of carbon dioxide and nitrogen. As anthropogenic emissions continue to alter the climate, forest productivity and distribution is likely to be impacted. Since high-severity wildfire and other disturbances can reset the successional stage of a forest, these disturbances could act as a catalyst for landscape and regional change in the distribution of forests (Hurteau and Brooks 2011). Yet, there is uncertainty surrounding many aspects of how forests will respond to changing climate, disturbance regimes, and management actions.