Slide Number 1Slide Number 2Slide Number 3Slide Number 4Historical Range of Variability Revisited(Unpublished m.s. Hayward, Veblen, Suring)Definition:Geog 4430HRV of ecological conditions can be defined as the variation of historical ecosystem characteristics and processes over time and space scales that are relevant to land management decisions. This definition emphasizes that HRV describes a body of knowledge about historical ecological conditions without any explicit prescription for how that body of knowledge should be applied to land management decisions. HRV concepts are applicable to most management goals and outcomes because of three premises that are well supported by theory and practice:1. Knowledge of past natural variability is an essential reference for evaluating impacts of modern land-use practices such as grazing, fire suppression, and logging on current ecosystem conditions and processes (Swanson et al. 1994, Landres et al. 1999). Evaluation of the ecological effects of such land-use practices depends on understanding how natural disturbances and climate variability have also affected ecosystem dynamics in a particular landscape. For example, contrasting effects of land-use practices on contemporary patterns of fuel types in ponderosa pine ecosystems demonstrate the necessity of developing HRV assessments for particular geographic areas as opposed to extrapolating conclusions from studies executed in other localities (Sherriff and Veblen 2006, Klenner et al. 2008). Such studies inform management decisions related not only to restoration goals but also to management outcomes aimed at reducing societal vulnerability to severe fire activity (Platt et al. 2006).2. Past natural disturbances have played key roles in structuring contemporary ecosystems, and will continue to do so in the future (White and Jentsch 2001, Long 2009).According to this premise, broad-scale legacies from past disturbances account for much of the spatial heterogeneity of landscape structure which in turn strongly influences the spread and severity of disturbances such as wildfire and insect outbreaks (Turner et al. 1993). For example, retrospective studies have revealed the importance of natural disturbance events occurring in the 19thcentury that created landscape templates strongly influencing the spread and severity of late 20thcentury bark beetle and wildfire activity (Veblen et al. 1994, Bigler et al. 2005).3. Hypotheses about the drivers and mechanisms of contemporary and future ecological change can be developed and tested with historical ecological data (Swetnam et al. 1999, Keane et al. 2009).Historical ecological studies document how changes in land use, such as grazing influences on fuels or elimination of fires set by aboriginal populations, have affected fire regimes of particular ecosystem types in the past (Gruell 1985, Savage and Swetnam 1990). Likewise, retrospective studies have revealed differential wildfire responses to major ocean-atmosphere oscillations and their teleconnections to climate across different forest ecosystem types from low elevation dry ponderosa pine woodlands to cool, mesic subalpine forests (Swetnam and Betancourt 1990, Sherriff and Veblen 2008). Such retrospective studies are essential for developing a mechanistic understanding of ecological changes which in turn supports the development of simulation models of future landscape dynamics driven by climate variability and changes in land-use practices (Flannigan et al. 2009, Keane et al.