Forests and woodlands are vegetation communities dominated by trees. Forests are composed of trees whose crowns touch and form a continuous canopy, while in woodlands the tree crowns are typically more spreading in form and do not touch to create a closed canopy. The most common forest type in the American Southwest is the pinyon-juniper woodland, defined by the presence of one or more species of pinyon pine (Pinus spp.) and juniper (Juniperus spp.). Most pinyon-juniper woodlands are found in areas subject to temperature extremes and limited moisture availability. Other forest and woodland types in the Southwest include ponderosa pine forests, mixed conifer forests (dominated, for example, by pines [Pinus sp.], spruces [Picea spp.], and firs [Abies spp.]), temperate deciduous forests (containing, for example, gambel oak [Quercus gambellii], trembling aspen [Populus tremuloides], and maples [Acer spp.]), and the unique yucca woodland of the Chihuahuan Desert. The many valuable ecological functions of these communities can be affected by changes in climate and fire frequency, and outbreaks of insects and disease.