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:: Home > Watersheds > FAQ: Forested areas in otherwise treeless places

What type of forest grows along a watercourse in a region otherwise devoid of trees??

Sometimes we forget that, even if a landscape is mostly treeless, there may be relatively small areas of land that are chock-full of trees, other plants, and animals that depend on these trees and plants for survival. The limiting factor in a landscape is often the lack of water available to plantlife. The greenest areas in such a landscape are often found near water sources like streams and lakes.

The misconception that dry areas, like deserts and arid grasslands, have no forests is frequently supported by maps of forest cover types. When a map of "forest types" is created, it shows so much land area (an entire state, for example) that one can't see individual streams. As a result, large swaths of land appear to be devoid of water-loving plant life.

Even in landscapes that look like deserts, subsurface water can give life to refuges of plantlife— like an oasis in the Saharan Desert!

Nothing could be further from the truth. In many seemingly arid areas like central and southeastern Oregon, the land is engraved with corridors packed with water-loving plants. The corridors are created by streams or by standing water, like lakes and reservoirs. The water-loving plants may exist mere feet from a stream, sometimes in it, in an area watered by the underground water of the stream. The underground water of a stream exists in a space scientists call the hyporheic zone (1)— hypo meaning "injected into," and "rhe" referring to "flow"; thus, the flow of water injected in the ground.

Even farther from the hyporheic zone is a place below-ground where there is ground that is permanently wet, but the water doesn’t “flow;” this area is called the phreatic zone (2). Some plants, like the mesquite tree of the desert southwest, can exist in arid areas by sending long tap roots down through the ground in order to tap the water in the phreatic zone. The mesquite doesn’t need to live near streams because of this adaptation. Plants like the mesquite tree are called phreatophytes (3).

Now we travel back to the streams and lakes. The moisture provided by streams, rivers, and lakes, creates a relatively small area suitable for all kinds of plants to survive. This is called a microclimate (4), and this microclimate is characterized by:

  • Higher soil moisture than surrounding areas,
  • Higher air humidity,
  • More moderated air temperatures— cooler air in otherwise hot areas, and warmer air in otherwise cool areas.

The microclimate I describe above is found in the riparian zone (5), or the area of land adjacent to water. Because of the microclimate effect in riparian zones, an amazing variety of water-loving plants can grow. The plants need to be water-loving however, and they need to be able to survive flooding.

In the book Forest Cover Types of the United States and Canada (Society of American Foresters, 1980), a riparian forest cover type is described for the west, in various regions. The book describes a forest in the low-elevation interior on North America that “occurs along rivers and streams and around ponds,” and is dominated by trees like cottonwoods and willows. In my area, the Willamette Valley in western Oregon, a riparian area can also contain Oregon ash, valley ponderosa pine, and white alder.

Each area may include different trees. And remember that “this [forest] type is a climatic anomaly since [they] are normally found in cooler and wetter climates than prevail in these areas (page 113).”

Words to Know:

  • Hyporheic zone
  • Phreatic zone
  • Phreatophyte
  • Micromclimate
  • Riparian Zone

For more information: Society of American Foresters. 1980. Forest Cover Types of the United States and North America. FH Eyre, Editor.