Factors Affecting Distribution

Many of the photo pairs reveal changes in vegetation, whether complete loss of vegetation due to human development or development of a new pattern of vegetation such as with changing treelines.  But what is it that determines where the plants grow to begin with?  For example, have you ever thought about why trees don’t seem to grow above a certain elevation?

In Denali, it is primarily the variation in temperature, moisture, soil conditions, and other characteristics of the physical environment that control vegetation establishment and growth.

Every plant species has a preferred habitat.  The physical structure of the plant (what it looks like) and the physiological functioning of the plant (what it does to live) determine what kind of habitat it prefers.  However, not all plants get to live in their preferred habitat.  In the subarctic, many plants are living at the edge of their habitat limits, and they must compete against other plants for the more preferred sites.  Some plants have adapted to this by evolving better competition strategies, while other plants have adapted by developing physical structures that allow them to persist in less than desirable habitats.
Astragulus on gravel bar
NPS Photo / Carl Roland
Here are some examples:

  • Cold temperatures and short growing seasons that prevail on alpine mountain tops prevent tree species from growing there.  However, smaller-statured tundra plants, which cannot compete with larger plants because of their small size, have instead adapted to withstand the harsh conditions that tree species cannot.
  • Plants in the pea family (Fabaceae) and large alder shrubs (Alnus sp.) have paired up with nitrogen-fixing bacteria in symbiotic relationships which make them better able to colonize bare, nutrient-poor surfaces.  Being able to colonize gravel bars is a valuable skill in Denali as the active glacial rivers are constantly depositing sediment to create what these plants view as prime real estate!
  • The structure and function of aspen trees (Populus tremuloides) require them to grow in relatively warm soils but allow them to persist on dry sites.  In Denali these warm and dry conditions are most often found on steep, south-facing bluffs and therefore you will rarely, if ever, find an aspen forest on a north-facing slope or in a wet depression.

The set of plants (or plant community) that occurs in each landscape position – that is places on the landscape where orientation to sun, water, and other large scale features create particular physical conditions – are indicators of the physical environment that exists there.  Therefore, as physical attributes such as temperature or precipitation change, the patterns in plant communities will also change.  It is in this way that vegetation distribution mirrors the physical landscape.

man walking into wetland
NPS Photo / Sarah Stehn

Continue on to Successional Dynamics…