Forests help regulate Earth's climate in part by sequestering carbon from the atmosphere (Bonan 2008, Pan et al 2011a, Anderson-Teixeira et al 2012), yet tree mortality from disturbance accelerates carbon transfer from these ecosystems back to the atmosphere (Kurz et al 2008, Baccini et al 2012, Brinck et al 2017). Forests globally store a similar amount of carbon as the atmosphere (Pan et al 2011a, Houghton 2013), with much of the carbon (~42%) held in the biomass of living trees (Pan et al 2011a). Disturbances such as forest fires, insect outbreaks, and timber harvest can kill trees over large areas each year (Goetz et al 2012, Meddens et al 2012, Kautz et al 2016, Williams et al 2016) and thus contribute to increased regional carbon emissions as tree biomass subsequently decomposes or is rendered into wood products that have finite longevity (Harmon et al 1990, Harmon et al 2011, Ghimire et al 2015). Carbon emissions from forest disturbance can challenge efforts to meet greenhouse gas (GHG) emission targets (Gonzalez et al 2015), but are also highly uncertain in many parts of the world (Pacala et al 2010). Both the growing global demand for wood products (FAO 2017) and the increase in forest disturbance due to ongoing climatic change (Allen et al 2010, Williams et al 2012, Kautz et al 2016) underscore the need to better understand carbon implications of tree mortality from disturbance.

Forest disturbance by fires and insects increased during recent decades in the western contiguous United States as the regional climate became warmer and more arid (Westerling et al 2006, Williams et al 2012, Dennison et al 2014, Hicke et al 2015, Abatzoglou and Williams 2016). Regional mean annual air temperature increased 0.8 °C–1.1 °C from 1895 to 2011, with most of the warming having occurred in recent decades (Mote et al 2014, Walsh et al 2014). Higher temperatures contributed to higher atmospheric vapor pressure deficits (Abatzoglou and Williams 2016), reduced mountain snowpack (Mote et al 2005), and more frequent and severe drought (McCabe et al 2004, Diffenbaugh et al 2015). For instance, the western US recently experienced its most severe drought (2000–2004) in the past 800 years (Schwalm et al 2012), with hot and dry conditions then prevailing through the 2000s (Diffenbaugh et al 2015, Abatzoglou and Williams 2016). These conditions contributed to extensive forest disturbance by fires and bark beetles relative to recent decades (Williams et al 2012, Creeden et al 2014, Abatzoglou and Williams 2016). High temperatures and drought increase regional forest fire occurrence (Littell et al 2016) and the likelihood of post-fire tree mortality (i.e. increased fire severity; van Mantgem et al 2013), while also increasing beetle populations and the vulnerability of drought-stricken trees to beetle attack (Raffa et al 2008, Creeden et al 2014, Hart et al 2014). Projections indicate that regional temperatures could rise another ~3.8 °C–5.5 °C by the end of the 21st century and that much of the region, particularly the Southwest, could become increasingly arid and prone to drought under a high GHG scenario (RCP 8.5; Kunkel et al 2013, Walsh et al 2014, Cook et al 2015). These changes in regional climate could further accelerate tree mortality (Adams et al 2009, Allen et al 2015) and increase carbon emissions from forest ecosystems (Spracklen et al 2009, Jiang et al 2013, McDowell et al 2015).

Several states in the western US have GHG reduction targets (e.g. Oregon, California) and would thus benefit from information on the magnitude and primary causes of recent tree mortality. Prior studies have shown that fires, bark beetles, and timber harvest are important causes of tree mortality in this region (Masek et al 2011, Meddens et al 2012, Hicke et al 2015). In this study, we asked 'What was the magnitude and relative contribution of mean annual tree mortality from fires, bark beetles, and timber harvest from 2003–2012 both regionally and among the 11 western states?' Tree mortality can be quantified over large areas in terms of carbon using remote sensing estimates of tree aboveground biomass (AGB) together with information on the carbon content of AGB, as well as disturbance extent and severity (Baccini et al 2012, Hicke et al 2013). Here we quantified tree mortality as the amount of carbon stored in tree AGB (AGC) killed by disturbance (e.g., Mg AGC ha yr−1 or Tg AGC state yr−1). Specifically, we developed spatially explicit estimates of annual tree mortality from fires and bark beetles across regional forestland building off of a remote sensing framework from an earlier study (Hicke et al 2013). In addition to the remote sensing analysis, we estimated mean annual tree mortality from timber harvest for each state using harvest statistics from the US Forest Service (USFS; Smith et al 2009, Oswalt et al 2014). The USFS recently recommended that metrics related to fire and insect effects be used to track national climate change impacts (Heath et al 2015), further underscoring the importance of quantifying the magnitude and regional variation in tree mortality from these types of disturbance.