Trees are single-stemmed, woody plants that are generally taller than 10 m (~30 ft) when mature. In the boreal forest, trees are often stunted and dwarfed.
Trees tend to thrive where there is sufficient warmth and nutrients. In these areas, trees are able grow taller than their neighbors and can therefore capture more sunlight, shading and out-competing their neighbors. However, because trees must invest a lot of energy into building the structural tissues (wood) that will give them this competitive advantage for light, they are sometimes put at a disadvantage where resources (such as nutrients and radiation) are very limited. In these “stressful” environments, plants whose bodies are constructed to take maximum advantage of the limited light, radiation, and nutrients that are available are favored by evolution. This often means not investing in energetically “expensive” structural tissues such as wood. Most low-growing, non-woody plants have a higher percentage of their bodies directly devoted to photosynthesis and uptake of nutrients, and thus can be more “efficient” in more stressful growing environments with short, cold growing seasons.
You can observe the results of this dynamic competition between different kinds of plants very clearly on the landscape of Denali. Trees tend to grow best in the lowlands where it is warmer and sunnier and along rivers where the flowing water has melted the permanently frozen ground. In fact, trees dominate the landscape in many lowland areas of the Park. Forests blanket the river plains and lower hills in the Park where the growing season is longer and warmer, and conditions are more favorable for plant growth. As you start to climb out of the valleys up to the ridges where it’s often cold and windy, you will notice that trees (and even lower shrubs) become fewer and farther apart, and the vegetation is lower to the ground with fewer woody species and many more grasses and forbs.
Denali is situated north of sixty-one degrees north latitude. Because of this, there are only a handful of tree species that are able to grow and reproduce naturally this far north. In fact, if you go a few hundred kilometers even further to the north, you will reach the arctic tundra biome, where there are virtually no trees at all. There are eight species of trees that occur in Denali National Park and Preserve, six of which are included in the atlas. Three of these are coniferous (cone-producing) plants from the pine family (white spruce, black spruce, and larch) and the other five are flowering plants from the willow and birch plant families (quaking aspen, balsam poplar, black cottonwood, Kenai birch and Alaska birch). Also note that there are some tall shrubs, primarily willows and alders, which could be mistaken for trees.