Physiognomy and Structure: Vegetation included in this minor alliance has a moderately dense to dense upper canopy of mixed deciduous broad-leaved and evergreen needle-leaved trees (5-20 m tall). The understory has a moderate to sparse short-shrub layer typically dominated by evergreen, scale-leaved or deciduous broad-leaved shrub species. A sparse tall-shrub layer, dominated by deciduous broad-leaved shrubs, may be present. A moderate to sparse herbaceous layer may also be present, dominated by perennial forbs or graminoids.

Floristics: Seral forests are included in this Rocky Mountain alliance. The open to moderately closed, mixed evergreen needle-leaved and deciduous broad-leaved tree canopy is composed of short to moderately tall trees and is codominated by Populus tremuloides and Abies lasiocarpa or Picea engelmannii. Several other species of conifers may be scattered within the stands, including Abies concolor, Picea pungens, Pinus contorta, Pinus flexilis, and Pseudotsuga menziesii. Younger stands typically have dense Populus tremuloides, with Abies lasiocarpa or Picea engelmannii mixed in. As the stands age in this typically seral forest alliance, Populus tremuloides is slowly reduced until conifers become dominant (Mueggler 1988). Mixed stands must have at least 25% relative tree cover of both aspen (deciduous) and conifers. The understory is typically moderately dense to sparse and becomes sparser as Abies lasiocarpa or Picea engelmannii become more dominant. The understory is typically dominated by a short-shrub layer, with sparse tall-shrub and herbaceous layers. The short-shrub layer is often dominated by Juniperus communis or Shepherdia canadensis. Other shrubs may include Mahonia repens, Paxistima myrsinites, Physocarpus malvaceus, and Rosa woodsii. If present, the tall-shrub layer often consists of scattered Amelanchier alnifolia, Acer grandidentatum, and Prunus virginiana. The relatively sparse herbaceous layer is a mixture of graminoids and forbs and is often dominated by Bromus carinatus, Calamagrostis rubescens, Carex geyeri, Carex rossii, Pedicularis racemosa, or Thalictrum fendleri. Other common graminoids include Achnatherum occidentale (= Stipa occidentalis), Bromus ciliatus, Elymus glaucus, and Elymus trachycaulus. Forbs such as Achillea millefolium, Arnica cordifolia, Chamerion angustifolium (= Epilobium angustifolium), Eucephalus engelmannii (= Aster engelmannii), Fragaria vesca, Geranium spp., Lathyrus spp., Lupinus spp., Osmorhiza berteroi (= Osmorhiza chilensis), Senecio serra, and Pseudostellaria jamesiana (= Stellaria jamesiana) are common. The exotic species Poa pratensis and Taraxacum officinale are common in livestock-impacted stands (Mueggler 1988). Annuals are typically uncommon.

Dynamics:  Stems of Populus tremuloides are thin-barked and readily killed by fire. It is a fire-adapted species that generally needs fire or some other stand-replacing disturbance to establish and maintain dominance in a forest. These mixed forests are seral and in the absence of disturbance will slowly convert to late-successional forests dominated by Abies lasiocarpa or Picea engelmannii (Mueggler 1988). Most of the stands sampled by Mueggler (1988) have had a history of livestock grazing as evidenced by relative abundance of the exotic plants Taraxacum officinale and Poa pratensis and the scarcity of grazing-susceptible plants (Mueggler 1988).