SCOPE AND LIMITATIONS
The results of our HRV analyses must be interpreted within the scope and limitations of this study. Most importantly, our analyses were designed to simulate vegetation dynamics under a chosen historic reference period. We chose the period from about 1300 to the late 1800s, representing the period from Ancestral Puebloan abandonment to EuroAmerican settlement, as the reference or benchmark. This period is often referred to as the period of indigenous settlement, in contrast to the period of EuroAmerican settlement that began in the mid to late 1800s (Romme et al. 2003). The period of several centuries prior to 1900 represents a time when broad-scale climatic conditions were generally similar to those of today, but Euro-American settlers had not yet introduced the sweeping ecological changes that now have greatly altered many Rocky Mountain landscapes -- through fire suppression, grazing, road-building, timber cutting, recreation, and other activities (Knight et al. 2000). Thus, the pre-1900 period provides a suitable reference condition against which we can compare current landscape structure and dynamics (Swetnam et al. 1999, Landres et al. 1999). An understanding of natural landscape structures and variability during this reference period also provides a basis for forest management policies that seek to mimic natural disturbance patterns in our logging, grazing, and other activities involving commodity production from public forest lands (Romme et al. 2000, Buse and Perera 2002).
It cannot be emphasized too strongly that the chosen reference period was not a time of stasis, climatically, ecologically, or culturally. For example, the “Little Ice Age” occurred during this time, and there were small shifts in the position of the upper timberline and in the elevational breadth of the forest zone on the middle slopes of the mountains (Petersen 1981). Local human inhabitants obtained horses and new technology and were affected by disease and displacement of other tribes brought about by European colonization farther to the east (Whitney 1994). Nevertheless, compared with some other periods in history, the period from about 1300 to the late 1800s was a time of relatively consistent environmental and cultural conditions in the region, and a time for which we have a reasonable amount of specific information to enable us to model the system. It should be emphasized, however, that our choice of reference periods does not suggest that it should be our goal in management to recreate all of the ecological conditions and dynamics of this period. Complete achievement of such a goal would be impossible, given the climatic, cultural, and ecological changes that have occurred in the last century. It also would be unacceptable socially, economically, and politically. Nor do we suggest that the reference period was completely “natural” or preferable in all ways to today’s landscape. However, the period of indigenous settlement does provide a good benchmark for evaluating current conditions, because it appears to have been a time when the ecosystem apparently supported rich biodiversity, conserved soils and nutrients, and ran sustainably on solar energy (Kaufmann et al. 1994).
Because our study relied on the use of computer models, it is imperative that the limitations of these models be understood before applying the results in a management context. Here, we discuss several important limitations, some general to the modeling approach employed here and some specific to how we parameterized these models for application on the SJNF.
First, our approach relies heavily on the use of computer models, and while it is important to recognize the many advantages of models, it is critical to understand that models are abstract and simplified representations of reality. RMLANDS, in particular, simulates several natural disturbance processes, but does not simulate all of the disturbance processes or all of the complex interactions among them that characterize real landscapes. Ultimately, the results of a model are constrained by the quality of input data. While RMLANDS utilizes a rich database, the data layers themselves are not perfect. For example, the vegetation cover layer is subject to human interpretation errors and objective classification errors, and is further limited by the spatial resolution of the grid. Thus, our results should not be interpreted as “golden”. Rather, they should be used to help identify the most influential factors driving landscape change, identify critical empirical information needs, identify interesting system behavior (e.g., thresholds), identify the limits of our understanding, and help us to explore “what if” scenarios.
Second, it is important to realize that RMLANDS and HABIT@ models require substantial parameterization before they can be applied to a particular landscape and species, respectively. To the extent possible, we have utilized local empirical data. However, due to the paucity of local data, we also drew on relevant scientific studies, often from other geographic locations, and relied heavily on expert opinion when scientific studies and local empirical data were not available. The source of information used to parameterize the models is fully documented and subject to review. Thus, our results should not be viewed as definitive, but rather as an informed estimate of the HRV based on our current scientific understanding. It is important to understand that our estimate of the HRV is subject to change as new scientific understanding or better data become available.
Third, this report (and RMLANDS) devotes more attention to upland vegetation types than to riparian or aquatic types; indeed, riparian and aquatic vegetation are covered only briefly. There are two reasons for this emphasis on upland vegetation in RMLANDS: (1) riparian and aquatic vegetation cover only a small (but ecologically critical!) portion of the total landscape, and (2) vegetation patterns and dynamics of riparian and aquatic vegetation are more complex, more variable, and more difficult to model in a straightforward fashion than are patterns and dynamics of upland vegetation. Additional research is needed to fully characterize the range of variability in riparian and aquatic ecosystems in this landscape. We note, however, that the patterns and ecological processes of surrounding upland vegetation have profound influences on aquatic ecosystems; thus, our results for terrestrial vegetation provide a partial basis for future assessments of aquatic HRV.
Fourth, this report (and RMLANDS) focuses on the effects of two major natural disturbances: fire and insects/diseases. Other kinds of natural disturbances also occur, including wind-throw, ungulate and beaver herbivory, avalanches, and other forms of soil movement, but the impacts of these other disturbances tend to be localized in time or space and have far less impact on vegetation patterns over broad spatial and temporal scales than do fire and insects.