Lecture 7
Topics
- Succession: the historical roots
- The ideas and the time scales
The Main Points
- We started by examining the historical roots of the concept of "succession." I presented work by ecologists studying in four areas of North America. (Ricklefs does a beautiful job of presenting these and other works in Chapter 28. In particular, check-out figures 28-1, 28-2, 28-3, 28-8, 28-10, 28-11, and 28-13).
- Henry Cowles blazed the trail with his work on the Indiana Dunes. Do yourself a favor and read his original paper [Cowles, H.C. 1899. The ecological relations of the vegetation on the sand dunes of Lake Michigan. Bot.Gaz. 27: 95-117, 167-202, 281-308, 361-391]. He was able to infer a time series of vegetation-types from the spatial patterns he observed along a transect from the lake shore toward the interior of Indiana. He summarized these inferences with three principles; that Vegetation:
- Changes through time.
- Change is directional.
- Change goes toward an equilibrium.
- Frederic Clements wrote abundantly on the vegetation patterns of the vast grasslands of the great plains. Building on the work of Drude who worked on the central plains of Europe in the 1890s, developed a theory of "succession" that made the following points [see Clements, F.E. 1936. Nature and structure of the climax. The Journal of Ecology 24: 252-84]:
- Vegetation develops toward a predictable and stable association called a climax.
- The climax is an equilibrium between the vegetation and the environmental forces at work.
- On an Optimal site, the climate ultimately regulates the nature of the climax association.
- On the average, over a large contiguous area, this association is the Formation or the Climatic Climax.
- On less-than-optimal sites, apparently stable associations develop that are edaphic climaxes.
- In the Piedmont of SE United States, the botanists of Duke and North Carolina worked out the progress of vegetation change on the "old fields" that were no longer farmed. They developed a time-scale from the dates the fields were last planted. One scale took on the following form:
| Bare Soil | Weeds | Shrubs | Pine Forest | Oak-Hickory Forest |
|---|
| Time 0 | 1-2 Yrs. | 3-20 Yrs. | 20-125 Yrs. | 125+ Yrs. |
- In the NE U.S. and SE Canada, the vast array of nutrient-poor bogs provided sites to study the dynamics of succession in small aquatic systems.
- I finished the discussion by presenting a kind of graphical summary of the vegetation continuum. Along an X-axis, we established a kind of gradient of edaphic conditions. Along the Y-axis we established a disturbance continuum. The intersection of each edaphic factor with a disturbance-type produced a particular "vegetation-type". We reprresented each of these vegetation-types with a box.
- We represented "successional change" with vectors across the matrix that would depict the magnitude and direction of changes in the "vegetation-types" in response to changes in conditions along either or both axes.
- In reality, the "particular vegetation-types" represented by the boxes don't exist as such. They merge with each other as we visualize intermediate conditions along each of the axes. This is the "continuum" and in our example is represented in only two dimensions. Could you think of another axis that would produce a three-dimensional continuum?