Military lands are important ecological reserves because they often encompass large tracts of land that are protected from intensive agriculture and urban development (Boise 1997, Ripley and Lewis 1997a, 1997b, Lillie and Ripley 1998). Furthermore, some of the finest examples of fire-maintained ecosystems within the southeastern United States are found on military bases in and adjacent to artillery ranges where frequent fires are assured and unexploded ordnance provides protection from development (Peet and Allard 1993). But how do military training activities compare to the natural disturbance regimes? And how might military disturbances interact with land management activities on military bases?
Disturbances from military missions may enhance or exacerbate their effects on ecosystem components. In general, military training in terrestrial environments can be broadly categorized into two major types of disturbances—ground maneuvering (tracked and wheeled vehicles) and air-to-ground impacts. Military installations subject to usage by the U.S. Army are often subject to additional impacts from training exercises. Typically, maneuvers on Army installations involve large vehicles that can cover large areas in a single training exercise. The available land base for training has a strong influence on the intensity and frequency of usage (Demarais et al 1999) and thus on the disturbance effects.
Large-vehicle maneuvers are widely used and are consistently shown to have negative effects across ecosystems. These repeated human-induced disturbances have no natural analog. The negative effects of ground maneuvering training have been studied in California (Lathrop 1982, Prose 1985), Colorado (Milchunas et al. 1999), Georgia (Dilustro et al. 2002), Kansas (Quist et al. 2003), Washington (Severinghaus and Goran 1981), Wisconsin (Smith et al. 2002), Texas (Severinghaus et al. 1981), Manitoba (Wilson 1988), and western Europe (Vertegaal 1989). Although studies have been conducted across a variety of ecosystems (e.g. deserts, prairies, pine-oak forests, etc.) several generalizations have emerged. In particular, it is the cumulative effect of repeated military disturbances that ultimately results in reduced abundance of perennial species, overall losses of native species, increased numbers of introduced species, and increased amounts of bare and compacted soil.
While most studies have focused on effects of large vehicles, the observed results probably also include the effects of other vehicular disturbances as well (i.e. off-road vehicles) that often occur in conjunction with tracked vehicle maneuvering activities. Road-like features, including active and remnant trails and vehicle tracks, are the most prevalent disturbance features at installations with high-usage maneuvering areas (Dilustro et al 2002, Quist et al 2003). These disturbance features act to increase fragmentation of the landscape, which can in turn affect ecosystem-level processes (i.e. spread of fire, flooding, drainage, etc.).
In native grasslands where maneuvering has been examined, at least one study (in Central plains grasslands at Fort Riley Military Reservation in northwest Kansas), has shown increased bare soil, reduced total plant cover, and compositional shifts in plant communities (Quist et al 2003). Reduced cover of the perennial, matrix-forming grasses and native species, and increased cover of annual and introduced species were also associated with high-usage maneuvering training activity. Quist et al. (2003) also reported high-usage maneuvering associated with increased sediment and reduced abundance of benthic insectivores, herbivore-detritivores, and silt-intolerant aquatic species. Watersheds with high military maneuver usage also were characterized by an abundance of trophic generalists and disturbance-tolerant species. Overall, the Quist study suggests that high-usage maneuvering areas had significantly altered the nature of terrestrial and aquatic ecosystems, making them less resilient to future disturbances. To prevent significant degradation of training areas and to provide a coordinated assessment and monitoring of these impacts, the U.S. Army has implemented an Integrated Training Area Management (ITAM) program. This program emphasizes monitoring of military impacts (erosion, siltation, soil compaction, loss of native plant cover, hydrologic alterations, etc.) on training lands.
In contrast to ground maneuvering activities, air-to-ground missions are capable of mimicking natural disturbance regimes in some ecosystems. This is particularly true when active bombing and gunnery ranges exist within fire-evolved ecosystems like prairies, savannas, and some wetland types. Aerial bombing and gunnery ranges used by fighter and bomber aircraft, and artillery and mortar gunnery from ground-based weapon systems can provide the ignition sources in fire-evolved ecosystems. Some air-to-ground ranges dating back to pre-World War II contain remnant fire-maintained plant communities no longer found in the surrounding fire suppressed landscape.
An impact area on Avon Park Air Force Range in central Florida known to receive over a thousand high explosive rounds and several thousand non-explosive rounds strikes annually (Delany et al. 1999) has created a long history of frequent mission-caused wildfires that in turn have provided some of the variation inherent under a natural fire regime. Ordnance-ignited wildfires in this impact area are frequent (>1/yr), may occur year-round, and have occurred since the 1940s. As a result, the vegetation within the impact area has never been fire suppressed. Despite bomb craters created by high-explosive munitions, portions of the impact area with native vegetation support endangered birds, numerous rare plant populations, and some of the highest natural-quality examples of fire-maintained plant communities found in central Florida (Orzell 1997). Similar native species-rich plant communities, often containing enclaves of rare plants, have been recorded elsewhere in or near active air-to-ground impact areas in the southeastern United States (Peet and Allard 1993, Sorrie et al. 1997).
Management implications
Management should be guided by ecological principles and strive to approximate ecologically appropriate disturbance regimes, while never neglecting the overarching need to support the military mission. In many cases, restoration of natural disturbance regimes has a positive long-term effect (Van Lear et al. 2005). Special care must be taken, of course, if there are threatened and endangered species involved. Restoration of ecosystems that have long been modified by anthropogenic activities or invasion of exotic species may not necessarily have the intended result or immediately positive consequences. For example, Varner et al. (2000) found that re-introduction of fire to a longleaf pine forest after many years of fire exclusion and organic matter buildup led to an unforeseen high mortality of large longleaf pines. In areas long degraded by fire suppression, repeated burns may be necessary (Heuberger and Putz 2003).
Another challenge for land managers is simulating natural disturbances on small parcels of land in a highly fragmented and human-dominated landscape—although one advantage of military installations may be that fragmentation and development are less of a problem than on surrounding, non-military lands. Incorporating disturbance regimes that approximate historic natural disturbances into management schemes should help to improve and maintain structure and function of the disturbance-dependent communities. Doing so, however, may be controversial and demands a great deal of planning and forethought.
Next Page: Resources
Author
Patrick Comer, Ph.D., Chief Ecologist
NatureServe
William Platt, Professor
Louisiana State University
Steve Orzell, Botanist/Ecologist
Avon Park Air Force Range, United States Air Force
Understanding landscape and ecosystem dynamics
8.1. Understanding landscape and ecosystem dynamics
Box 8.3: Conceptual ecological models to understand ecosystem dynamics