Western Riparian Woodland - Code: Ne3D

In the open expanses of the Great Plains and western shrublands and deserts, this habitat often appears as an oasis in the distance: isolated ribbons of impressively tall trees growing along the banks of rivers and streams as they traverse the landscape. This habitat spans a huge area of the United States, from the cold prairies of Montana to the sunbaked landscapes of New Mexico and California and extends into s. Canada and n. Mexico. Despite its large range, the structure and nature of western riparian woodland remain consistent throughout—islands of dense, distinctly layered, deciduous broadleaf woodlands in a sea of other habitats. Taller forests occur in flatter areas with a wider floodplain. Steep, narrow or largely dry waterways may have few or no large trees and include only the shrub layer.
Although there are few unifying climatic characteristics, the plant composition is relatively consistent throughout. The largest and most dominant species in the canopy are Fremont (Populus fremontii) and Plains Cottonwoods (Populus deltoides), which are separated by the Rocky Mountains. These trees form a dense canopy, usually greater than 60% coverage, and grow to a height of 65–80 ft. (20–25m). Under the dense canopy, the woodland can be open and park-like, with little understory. Along with these primary trees, Arizona Walnut (Juglans major), Goodding’s Willow (Salix gooddingi), and California (Platanus racemose) and Arizona (Platanus wrightii) Sycamores are present and sometimes co-dominant in the southern half of the range. In the northern parts of the range, alders (Alnus spp.), Black Cottonwood (Populus trichocarpa), Box Elder (Acer negundo), and Quaking Aspen (Populus tremuloides) contribute to the canopy.
The shrub layer is diverse, dense, and variable. Generally, it forms a dense thicket at the water’s edge and thins as it is shaded out by canopy trees. In areas where there are no large cottonwoods, these thickets can form a dense woodland and grow up to 30 ft. (10m) tall, though heights of 5–15 ft. (1.5–4.5m) are more typical. Willow (Salix spp.) is almost always the primary component or at least co-dominant. There is a large array of other shrubs, including elderberries (Sambucus spp.), serviceberries (Amelanchier spp.), dogwoods (Cornus spp.), poison oaks (Toxicodendron spp.), mesquites (Prosopis spp.), and various cherry species (Prunus spp.). In arid areas, the invasive Tamarisk (Tamarix aphylla) is common component and can  completely supplant native riparian vegetation. The herbaceous layer is strongly dependent on distance from water, successional stage, and geographic range. Some grass is almost always present, especially close to the water’s edge or on most recently exposed banks. In particularly wet areas, there are Nearctic Reedbed Marshes and Nearctic Sedge and Grassy Marshes.

The Western Mesquite Bosque subtype occurs in the southwestern United States and NW Mexico within the Sonoran Desert. This subtype is notably shorter than most Western Riparian Woodlands and the canopy is entirely dominated by Velvet (Prosopis vetulina) and Honey (Prosopis glandularis) Mesquites with relatively sparse understory. The waterways in this subtype frequently lack surface water and can remain as a sandy channel for years at a time.
This is a dynamic ecosystem that is dependent on disturbance. 

The Lower Rio Grande and Tamaulipan Riparian Woodland subtype is also quite distinctive. This habitat has little vegetative overlap with other Western Riparian Woodlands and instead is comprised largely of Texas ebony (Ebenopsis ebano), anacua (Ehretia anacua), Mexican ash (Fraxinus berlandierana), tepeguaje (Leucaena pulverulenta), Montezuma cypress (Taxodium mucronatum) and cedar elm (Ulmus crassifolia).  Dense, swampy stands of Mexican Sabal Palm (Sabal mexicana) are another endangered but important feature of this habitat in the Rio Grande delta. Small oxbow wetlands and resacas are also common along the floodplain of this habitat. Farther south in forested parts of Mexico, riparian corridors often have extensions of habitats found in wetter climates.  

The structure and continued survival of western riparian woodland depends on the waterways themselves. Changes in peak and minimum flows, the duration for which surface water is present, and the period between floods drastically affect the species composition and health of this habitat. Regular floods are important for creating successional zones, depositing fresh nutrients, and germinating dormant seeds. If floods are too intense, the entire area can be scoured clean and in the process robbed of nutrient-rich soils that would normally allow for regeneration.