<P> Broad - leaved deciduous cottonwoods, alders, and willows, and aspens line streamsides and canyons . The herbaceous layer in riparian communities is often more diverse than upslope areas and adjacent forests . Riparian and canyon vegetation types are generally too thin or too small to be displayed on regional vegetation maps, but the habitat is extremely important in the arid West . </P> <P> In the southern Rocky Mountains, a transition occurs between about 1,800 and 2,500 metres (5,900 and 8,200 ft), where plains communities are accompanied by pinyon pines . Mexican pinyons and singleleaf pinyons are found in western Utah, alligator junipers and Rocky Mountain junipers grow to the south, and Utah junipers grow to the north . Many shrubs and grasses of the plains occupy the gaps between tree outcrops . Heavy livestock grazing is associated with the spread of junipers (by reducing competition from grasses), and fire suppression is partly responsible for their continued dominance . </P> <P> The appearance of ponderosa pine woodlands varies from scattered individuals in low - elevation or rocky areas to dense forests at higher elevations or on deeper soils . Although ponderosa pines dominate the biomass of this community, other tree species such as Douglas - fir and Rocky Mountain juniper, shrubs (for example, raspberries, big sagebrush, gooseberries, currants, bitterbrush), and herb layers (such as mountain muhly, sedges, and sagebrushes) can develop . Typical intervals between natural fires are less than 40 years in most ponderosa pine forests . </P> <P> In geological time, ponderosa pine ecosystems are relatively new to the foothills of the central Rocky Mountains . An even newer addition to the ecosystem, European - American settlers, devastated the ponderosa pine forests through logging for houses, fencing, firewood, mine timbers, and railroad ties, and with fire . The ponderosa pine forests were close to the developing population centers at the forest - prairie edge . The scale of the loss of ponderosa pine habitat is demonstrated best in several hundred paired photographs from the early 20th century and 1980s . However, nearly all the paired photographs also reveal that the most important feature of the ponderosa pine ecosystem is its resilience . Ponderosa pine seedlings establish quickly in disturbed sites . Research in the Front Range of Colorado shows a tenfold increase in ponderosa pine biomass since 1890 in many stands . This regeneration has restored habitat for many wildlife species but has also led to unnatural forest densities in many areas . </P>

What is the natural vegetation of the rocky mountains