Historical Range of Variation for Potential Natural Vegetation Types of the Southwest
Descriptions of the Historical Range of Variation or Variability (HRV) characterize the change over time and space in the condition of the Southwest’s major vegetation types and the ecological processes that shape those types. HRVs enable land managers and the public to understand the drivers of change in our region’s major vegetation types.
Subject Tags
- Desert
- Forest
- Grassland
Abstract
The Historical Range of Variation (HRV) describes the natural changes in ecological conditions and processes over time and space for potential natural vegetation types (PNVTs), particularly in Arizona and New Mexico's National Forests. It serves as a reference framework based on empirical data to understand vegetation composition, structure, disturbance regimes, and human impacts. HRV distinguishes between pre-settlement conditions (1000–1880) and post-settlement disturbances after 1880, which significantly altered ecosystems through grazing, logging, damming, and fire suppression. By comparing current conditions with historical variability, HRV helps land managers set realistic ecological goals, evaluate management outcomes, and integrate appropriate disturbance regimes. It also acknowledges that some historical conditions may no longer be attainable due to climate change and landscape fragmentation, emphasizing adaptive management and predictive modeling. Temporal and spatial scales critically influence HRV assessments, affecting how ecological processes like fire and species richness are measured and interpreted.
Citation
Schussman, Heather and Ed Smith. 2006. Historical Range of Variation for Potential Natural Vegetation Types of the Southwest. Prepared for the U.S.D.A. Forest Service, Southwestern Region by The Nature Conservancy, Tucson, AZ. 345 pp.
Media Contacts
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The Nature Conervancy