Chapter 3 Vegetation Diversity 3

Chapter 3 Vegetation Diversity Vegetation Diversity INTRODUCTION Biodiversity has been defined as the variety of living organisms; the genetic differences among them; and the communities, ecosystems and landscapes in which they occur (Noss 1990, West 1995). Biodiversity has leapt to the forefront of issues due to a variety of reasons; changing societal values, accelerated species extinctions, global environmental change, aesthetic values, and the value of goods and services supplied (West 1995). Maintenance of ecological functions, processes, and disturbance regimes is as important as preserving species, their populations, genetic structure, biotic communities, and landscapes. Hence ecosystem-level processes, services, and disturbances must be considered within the arena of biodiversity concerns (West and Whitford 1995). The biological diversity that is supported by a particular area is generally a positive function of the degree of environmental heterogeneity occurring over space and time within that area (Longland and Young 1995). Vegetation is a cornerstone of biological diversity. Vegetation exerts its influence into almost every facet of the biophysical world. Many biophysical processes and functions depend on or are connected to vegetative conditions. Vegetation is an integral part of ecosystem composition, function, and structure. Vegetation shapes and in turn, is shaped by the ecosystems in which it occurs. It provides plant and animal habitat, and determines wildfire and insect hazards. Leaves, branches, and roots contribute to soil productivity and stability. Large wood in streams increases physical complexity, providing more habitat diversity. Vegetation shades streams, helping to maintain desirable water temperature, and also acts as a physical and biological barrier or filter for sediment and debris flowing from adjacent hillsides toward streams. Indeed, vegetation provides so many different aspects of ecosystems that it is impossible to list them all. For many resources, vegetation condition is the single most important component that determines effects. Vegetation is important to humans not only because of our use of products such as timber and forage, but also through other experiences such as camping, hiking, or viewing scenery. Vegetation plays a major role in ecosystem process and function; hence it plays a major role in the diversity of living organisms. Conservation of biodiversity is important at the genetic, species, and ecosystem levels of organization, and vegetation unites many of these components and processes. Systems thinking involves studying ecological and human processes holistically. It builds on detailed knowledge about composition, structure, and function. A holistic analysis often draws conclusions different from a summing of the parts (Purvis 1996). Landscape mosaics are mixtures of natural and human-managed patches that vary in size, shape, and arrangement (Forman and Godron 1986). Ecogroup vegetation management strategies are aimed at providing ecological components, patterns, and processes operating at several scales in landscapes; this is the coarse filter approach, which seeks to provide for the full range of biological organisms in each ecosystem. Implementation of the coarse filter approach presents some risk because it requires that managers understand the consequences of their actions. Several studies have suggested that the landscape has critical thresholds at which ecological processes will change 3 - 414 Chapter 3 Vegetation Diversity qualitatively (Turner 1989). The more we learn about ecosystems, the greater the likelihood that our assumptions about ecosystem response will improve and we will achieve the conditions we desire. A coarse filter management strategy would not be complete without its complement, the fine filter approach, which provides a necessary species-specific management strategy. This fine filter approach is discussed throughout other sections in this chapter, most notably Botanical Resources, Soil, Water, Riparian, and Aquatic Resources, and Terrestrial Species and Habitat. Coarse filter units are described here with classification systems that consider groups or communities of vegetation, appropriate for mid-scale planning. The Forests have traditionally used cover type, strata, habitat type, and community types to classify these vegetative variations on the landscape. Over the past several years, large-scale disturbances such as wildfires and insect epidemics have prompted land managers to evaluate whether the current vegetative conditions are sustainable. Additional issues have centered on how vegetative conditions affect biodiversity, plant, animal, and fish viability, and ecosystem processes and functions. Historical range of variability (HRV) concepts were developed in part to better understand how disturbances, vegetation, and other ecosystem components interact, and in turn how interaction affects biophysical characteristics, such as plants, animals, fish, soil and water resources, and numerous other resources. Historical perspectives increase our understanding of the dynamic nature of landscapes and provide a frame of reference for assessing modern patterns and processes (Swetnam et al. 1999). Underlying this concept is the assumption that ecosystems operating within their historical range have evolved within the influence of disturbances, such as insects, disease, and fire. Insects, disease, and other disturbance agents generally operated at endemic or characteristic levels within historical landscapes (Harvey 1994). Over the last century, shifts in species composition and density have created vegetative conditions where insects, disease, and wildfire may operate at epidemic or uncharacteristic levels. Disturbances operate in a heterogeneous manner in the landscape; gradients of frequency, severity, and type are often controlled by physical and vegetative features. The differential exposure to disturbance, in concert with previous history and edaphic conditions, leads to the vegetation mosaic observed on the landscape (Turner 1989). Historically, fire regime was the principal factor determining the mosaic of different stand ages across the landscape (Lesica 1996). The concept of ecosystem ranges of variability (Morgan et al. 1994) has been suggested as a framework for coarse filter conservation strategies (Hunter 1990). Natural variability is defined as the ecological conditions, and the spatial and temporal variation in these conditions, that are relatively undisturbed by humans, within a period of time and geographical area appropriate to an expressed goal (Landres et al. 1999). A coarse filter conservation strategy seeks to preserve biological diversity by maintaining a variety of naturally functioning ecosystems across the landscape. If it is possible to produce or mimic the historic ranges in stand size, composition, and connectivity by forest type on current and future landscapes, then much of the habitat for native flora and fauna should be present. Mimicking the historic ranges of snags and coarse woody debris should also help these conservation strategies. Although coarse woody debris is an important structural component of forest ecosystems, managing for maximization of coarse woody debris, or having uniform standards across historically variable landscapes, is a fine-filter strategy that can literally backfire. The use of coarse woody debris levels characteristic of historical disturbance regimes is recommended as an 3 - 415 Chapter 3 Vegetation Diversity alternate system more likely to be sustainable (Edmonds and Marra 1999). Fine-filter strategies, such as individual species plans or snag retention, might still be needed, but most species and ecosystem elements should be present if natural ranges in habitat are provided (Haufler et al. 1996). The current Forest Plan revision effort uses a combination of these approaches to describe past, present, and future vegetative conditions. For the purposes of organization and clarity, vegetation diversity has been divided into three subsections: (1) forested vegetation, including forestlands, snags, and coarse woody debris, (2) non-forested vegetation, including woodlands, shrublands, and grasslands, and (3) riparian vegetation, including riverine (forested) riparian areas and deciduous riparian areas. Forested Vegetation The key to a healthy ecosystem is structural and functional diversity across forested landscapes (Franklin and Forman 1987). The achievement of multiple-use objectives dictates that Forest managers maintain biological diversity. A diversified forest provides a greater array of products, biological organisms, and greater inputs to soil organic matter and nutrients. The increased genetic diversity contributes to sustained productivity because the loss of trees to pathogens, climatic change, or pollutants is less (Franklin and Maser 1988). The variety of vegetative species that occur within ecosystems contributes to processes and functions in different ways. Some species, such as ponderosa pine or western larch, are long- lived and can persist on the landscape. Others, such as aspen, are shorter-lived and, in the absence of disturbance, are sometimes quickly replaced by more shade-tolerant conifers. Different species host different insect and disease agents, which in turn influence wildlife uses. The decaying fungi introduced by bark beetles facilitate excavation by primary cavity nesters (Bull et al. 1997). Other species like grand fir, which is often infected with heart rotting

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