Plant Community Responses to Disturbance by Mechanized Military Maneuvers
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The effects of 10 yr of military training exercises on vegetation structure were assessed across plant communities that differed in physiognomy and soil texture at Pinon Canyon Maneuver Site (PCMS), Colorado, after release from previous grazing management. Covariate analyses aided in separating temporal trends due to both release from grazing and imposition of training disturbance from the direct effect of training. The shift in land use had both synergistic and antagonistic impacts on successional trajectories of communities, and on horizontal and vertical structural heterogeneity. Vegetation basal cover declined with increasing intensity of disturbance by tracked vehicles, but release from grazing acts additively in this ecosystem. Litter cover increased following release from grazing, even though it declined with increasing levels of disturbance. Vehicular maneuvering generally reduced woody life forms in tall-height classes to a greater extent than short-height classes. Low growing cacti were susceptible to crushing. Species and functional group responses to vehicular disturbance were sometimes dependent on community type. Long-lived perennials declined, but were replaced by short-lived perennials in only the shrub-grassland community. Annuals and exotics did not show relationships with intensity of disturbance, though some weed species increased. Community-wide species dissimilarity did not show large shifts, and patterns in species diversity or richness were not related to intensity of disturbance. The PCMS appears to be in a transient stage where release from grazing has had as much or more impacts as did the imposition of military training. Fine textured soils may be more susceptible to the cumulative effects of vehicular loads.
