Environments
The biomes and habitats of North America provide a guide as to where the many species of common trees are found. Each of these habitats is characterized by certain combinations of environmental conditions for which each species of tree has evolved and adapted. In some cases, entire lineages appear to be adapted to certain types of environments. An example is the conifers, which are found most often in cooler climates. In other cases, lineages with many species have diversified over time to adapt to a wide variety of environmental conditions. The oaks and the maples, for example, have species in cold and warm climates, wet and dry soils, or undisturbed as well as highly disturbed places. Some of the primary environmental factors that affect species distributions include climate, elevation, topography, water availability, soil composition, disturbance, and pests and diseases. 2
Climate.
The elements of climate that have the greatest impact on trees are the amount of moisture in the air, the range of daily and nightly temperatures, and seasonality. These factors are interdependent. Environments may be quite dry and arid or wet and humid or in between. Temperatures fluctuate between day and night, but the range of these fluctuations is usually quite stable and predictable and varies from arctic, alpine, and boreal to montane and temperate to subtropical and tropical. And both moisture and temperature can be more or less invariable, with little seasonal change, or vary across the year in seasonable climates, such as hot and dry in summer versus cool and moist in winter, or vice versa.
Elevation.
In Alexander von Humboldt’ s classic 1807 work Essay on the Geography of Plants, he described how changes in elevation up the slopes of Mount Chimborazo in Ecuador mimicked changes in climate and vegetation as one traveled across latitudes from equatorial to polar. 3 This holds as true for the slopes of the Rockies of North America as for the Andes of South America.
Topography.
Flat coastal plain or flood-plain habitats pose different challenges to tree growth than mountain slopes. In the mountains, north-facing slopes can be cooler and wetter than south-facing slopes, even over short distances. Montane valleys harbor quite different vegetation than exposed ridges because of the contrasting climates. Within the flatter plains, slight dips in the terrain can create much wetter environments containing swamps or standing water. All of these topographical variations require adjustments in strategies if trees are to survive and grow.