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Geog 4430
Historical Range of Variability Revisited
(Unpublished m.s. Hayward, Veblen, Suring)
Definition:
HRV 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 19th century that created landscape
templates strongly influencing the spread and severity of late 20th
century 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. 2009).