Abstract

Pinon-juniper (Pinus edulis - Juniperus osteosperma) woodlands and adjacent sagebrush (Artemisia spp.) shrublands and grasslands on the Uncompahgre Plateau of western Colorado were subjected to intense land uses after EuroAmerican settlement (A.D. 1881). It is thought that livestock grazing and suppression of frequent, low-severity, surface fires have increased woodland density and severe disturbances (e.g., stand-replacing fire). After fire, these ecosystems are vulnerable to invasion by cheatgrass (Bromus tectorum), a non-native that can dramatically alter ecosystem function. Restoring these ecosystems to within their ranges of natural variability requires an understanding of ecosystem structure, composition, and dynamics.

To aid this understanding, three research questions were addressed: (1) How have woodland stand age structure and disturbance dynamics changed since EuroAmerican settlement? Stand age structures of piñon-juniper woodlands were assessed by collecting tree age data using dendrochronology techniques, and these data were analyzed in relation to disturbance events, environmental gradients, and climate records. (2) How has land use affected natural plant communities? Floristic data were collected within lightly used reference areas and compared to a regional floristic survey. (3) What factors drive cheatgrass invasion after fire? Post-fire cheat cover was analyzed in relation to environmental and climatic variables over time, to determine if predictive relationships exist.

Results indicate that woodlands on the Uncompahgre Plateau were shaped by infrequent stand-replacing fires, drought, and patchy biological disturbance events, rather than frequent low-severity fires that maintained open, spatially restricted woodlands. However, livestock grazing and wet periods likely increased extant bee densities. Livestock grazing also significantly reduced native plant diversity, perennial grass cover, and biological soil crust cover on degraded areas compared to reference areas. After fire, cheatgrass cover increased with time, more so in sagebrush than piñon-juniper ecosystems, while it decreased with pre-fire species diversity and soil crust cover. Temporal patterns of moisture availability also affect cheatgrass cover. Post-fire seeding was not significantly correlated with cheatgrass cover. Thus, restoration of the Uncompahgre Plateau will require multiple approaches, including improved grazing management, limited thinning of young trees in dense stands, restoration of native plants in degraded areas, and pre- and post-fire rehabilitation of burned areas to prevent cheatgrass dominance.