Forest stand dynamics

Forests change through time - this is a fundamental fact that is critical to incorporate into approaches to management. Even in the absence of management, forests are in a constant state of change. Throughout the world, the forests that occur today are influenced by the conditions of the past, including past land use activities by people. Unfortunately, people sometimes think that the forests we have today have always been this way and can continue to do so forever into the future without human intervention. Understanding forest dynamics can teach us about how contemporary forests came to be and how they may change into the future with different types of (or no) management.

  

A few key processes contribute to the way that forests change through time:

  • Establishment of new trees: the process of tree regeneration introduces new individuals to existing forests, although the process is complicated. The conditions under which trees regenerate and the factors that inhibit their success differ among species in many ways - some need lots of light, some do well in shade, some need mineral soil, some prefer organic matter, etc., etc., etc.

  • Growth rates: trees growth occurs in two directions - upward from the ends of branches and outward from the live tissue beneath the bark. The factors that affect the rate of growth in these to directions differs. For example, height growth is generally closely related to site quality (water, nutrients) but diameter growth is closely related to stand density. Growth rates differ among species due to physiological differences. In addition, external factors, including insects or pathogens, climatic patterns, and disturbances such as fire, wind, and ice, can interact in complicated ways to further drive patterns in growth through time.

  • Tree death: when trees die, space and resources become available for new trees to establish or for established trees to grow. Tree mortality is occurring all the time - while some trees can reach some extreme ages, many trees die throughout stand development. The causes of tree mortality are sometimes attributable to discrete events, such as lightning strikes or a strong wind event, but often the causes are interacting stressors that have an accumulated effect through time. Yes - it is complicated.

 

In many ways, the actions taken through forest management are deliberate manipulations of forest stand dynamics - changes in the outcomes of tree regeneration, the rates of growth, and the patterns of tree mortality. This works well when we understand it and anticipate the outcomes, and long-term sustainable management requires that we consider these changes comprehensively.     

The black and white photo was taken in 1956, at the start of a long-term prescribed burning study. Repeated prescribed fire affects stand dynamics by eliminating small-diameter trees, changing the regeneration conditions, and affecting tree health and rates of growth and mortality (among other things!). The picture to the top right shows the area with no fire (unburned control) and the picture on the bottom right shows effects of burning each year from 1950 through the present.