Vegetation Transition Model

Riparian

      Description.–Riparian vegetation structure and species composition are extremely variable at multiple scales, from broad-scale elevational gradients to very fine-scale patterns related to local variation in soil moisture, organic matter, and light conditions (Redders 2003b). At the broad scale, dominant woody species vary from Rio Grande cottonwood (Populus deltoids ssp wislizeni) and coyote willow (Salix exigua) at low elevations, to narrowleaf cottonwood (Populus angustifolia) and alder (Alnus incana ssp tenufolia) at middle elevations, to Drummond willow (Salix drummondiana) and Geyer willow (Salix geyeriana) at the highest elevations. Common herbaceous species at lower to mid elevations include baltic rush (Juncus balticus), reed canarygrass (Phalaris arundinacea), broadleaf cattail (Typha latifolia), cow parsnip (Heracleum lanatum), and bedstraw (Galium triflorum). Common herbaceous species at mid to higher elevations include tufted hairgrass (Deschampsia cespitosa), water sedge (Carex aquatilis), bluebells (Mertensia ciliata and fransiscana), elephanthead (Pedicularis groenlandica), bittercress (Cardamine cordifolia), Richardson geranium (Geranium richardsonii), strawberry (Fragaria vesca and virginiana), and arrowleaf groundsel (Senecio triangularis) at high elevations.

      Riparian vegetation is extremely variable in structure and composition, and it is beyond the scope of this report to characterize all of the interesting variation. Because the riparian cover type usually comprises only a narrow strip of vegetation along a perennial water source, and because structure and composition are so variable, we treat riparian vegetation as a single cover type for modeling purposes (RMLANDS). See Redders (2003b) for a more detailed treatment of riparian vegetation in the South Central Highlands Section.

      Distribution.–Riparian vegetation is found at all elevations along streams and pond margins, and other locations where moisture is available perennially near the soil surface (Figure-distribution map; Figure-elevation chart; Table-areal coverage). Similar communities are found throughout the Colorado Plateau and southern Rocky Mountain regions (Somers and Floyd-Hanna 1996, Johnston and Huckaby 2001).

      Stand conditions.–The riparian zone is generally very narrow, highly variable structure and composition, and subject to diverse post-disturbance successional trajectories. Attempting to incorporate all of this fine-scale variability into a landscape model like RMLANDS would likely obscure the broad-scale patterns that we are seeking. Thus, riparian was treated as a static cover type (i.e., no succession) and therefore did not have separate stand conditions.

      No Conditions (NC) – we did not recognize separate stand conditions in this cover type.

      Succession Transitions.–Although riparian areas undergo succession, we chose not to simulate succession in this cover type in our modeling work (RMLANDS) for the reasons mentioned above and because of its relatively small extent. Thus, for the purposes of our modeling work we chose to treat riparian areas as static.

      Disturbance Transitions.–Natural disturbances do occur in riparian areas, e.g., wildfire, flooding, and heavy grazing or browsing. In addition, many of the riparian areas in the region also have been dramatically altered during the last 100 years by heavy grazing, plowing, planting of non-native species, and invasion of non-native species (e.g., Tamarix chinensis and Cirsium arvense), but these disturbances were not considered relevant for the HRV scenario. Wildfire is the only one of these disturbance process we simulated in this application. As we did not recognize separate stand conditions, wildfire disturbances simply recycled the stand through the single stand condition:

NC – high mortality wildfire recycles the stand through this condition while a low mortality wildfire maintains the condition.

*Note, see Disturbance Rules for the formal implementation of these rules in RMLANDS.