The Time Factor in Succession
The key to understanding forest succession is knowledge of the time factor, for ecological succession is basically an historical subject. The traditional approach of plant ecologists in acquiring a time framework is to observe or measure the separate elements or stands of a forest mosaic that are thought to represent the various stages in succession, perhaps leading to some type of climax. Thus the short-lived jack pine [Pinus banksiana Lamb.] and aspen [Populus tremuloides Michx.] stands of the northern Great Lakes Forest might be seen as only stages in a succession to shade-tolerant fir-spruce-cedar-birch forest in the absence of fire. Occasionally circumstances might permit repeated measurements at the same site over a period of many years, to provide a highly specific picture of succession, but such opportunities are localized and short-term in relation to the lifetime of the forest type. Centuries would be required to document directly the complete forest successions in most regions, and such work is simply not being accomplished anywhere in the world.
The time dimension in studies of the elements of a mosaic can be most accurately determined when a definite starting time for succession can be identified. In the case of colonization and succession on abandoned river bars or on a recently deglaciated landscape, the starting time may be distinct, and the sequence from pioneer plants through various successional stages may be easily recognized. In the case of the complex mosaic that characterizes a fire-dependent forest, the starting times of the individual elements of the mosaic cannot easily be determined. Many elements may date from hundreds of years ago. Even when a severe fire occurs, some plants survive, so not all of the new growth starts from seed. Not only might single tall trees of red or white pine survive, but the root systems of other trees are adapted to send up shoots that grow profusely after a fire. In these cases, one should think not of a simple starting time but rather of a renewal of forest growth after an interruption.
In any case, reconstruction of long-time successional events in a forest mosaic simply on the basis of the modern forest composition encounters difficulties. Additional historical methods are needed. For forest succession under the influence of repeated fire, two approaches are available—one covering the time range of a few hundred years, the other covering many thousands of years. Both perspectives are needed.
First, the trees themselves, in their annual rings, may hold a record of fire history. When a fire is sufficiently severe it may kill all the trees, and at the same time it opens the area to light and provides a concentrated supply of nutrients in the ash, thereby encouraging the germination of seeds and the growth of shoots, resulting in a new stand of trees. The uniform age of the trees in such a stand thus may record the approximate date of the last major fire. On the other hand, if a fire is not severe enough to kill a tree, it may leave a scar on the trunk that is covered by subsequent ring growth but visible on cut sections of the trunk. Some trees carry several scars, each of which can be dated by ring counting. Usually, the fire-scar record confirms the age-class record. By these methods, the fire history of old forests can be traced for hundreds of years.
Beyond 300–500 yr in most regions one must go to another technique—the stratigraphic analysis of the pollen and charcoal content of lake sediments, preferably annually laminated sediments, which can provide a precise chronology. Charcoal analysis shows that fire has been an element in the forest ecosystem for thousands of years. The pollen content of the same sediment, which records primarily the regional rather than the local pollen rain, indicates that the forest mosaic as a whole has remained largely unchanged since shortly after the land was uncovered by the wasting ice sheet, while slight shifts in dominant tree types reflect minor climatic changes or tardy immigration from distant glacial refuges. In detail, of course, the composition of individual elements comprising the forest mosaic changes abruptly with each severe fire, and the composition of other elements changes gradually by succession between fires—for example, from pine to spruce and fir.
Thus the historical perspective afforded by tree-ring analysis and by charcoal and pollen analysis adds new dimension to the concept of climax. In a fire-dependent forest one may consider equilibria on two scales—a long-range equilibrium represented by the entire ecosystem, in which fire is the principal environmental factor, and a short-range equilibrium, or climax in Clements’ sense, which is occasionally approached in local elements of the mosaic that have escaped fire for a long time, such as an island in a lake. In a fire-dependent forest, the Clementsian climax is rarely reached, although continuation of the present policy of fire suppression would make its attainment more likely.
Geomorphologists may recognize that this evaluation of the vegetational climax is similar in certain respects to recent views on the erosion cycle and the formation of peneplains. William Morris Davis, who dominated geomorphic theory from 1890 to 1935, visualized a landscape as developing through successional stages of youth, maturity, and old age under conditions of crustal and climatic stability (Davis 1899). He conceived of the peneplain as the climax landform to which all stable landscapes must develop. It would not be surprising to find that Clements derived some inspiration from Davis as he developed his climax theory for vegetation along the same lines. In fact, in a sense the two concepts were brought together by Lucy Braun (1947) and other plant geographers in the Appalachian region, who wrote of a relic flora of Tertiary age on the Schooley peneplain. In this picture, crustal uplift interrupted the cycle of erosion and caused dissection of the peneplain, thereby initiating a new cycle but leaving remnants. At the same time the vegetational climax was interrupted, leaving behind relics coincident with the peneplain remnants.
After Davis the peneplain theory suffered criticism and rejection by many geomorphologists. The disagreements were first manifested by general disbelief that the crust and climate were stable enough for the landscape ever to reach its climax of erosional development. The criticism, started on a theoretical basis by Walther Penck (1924), came into focus with John T. Hack’s well-reasoned presentation of a substitute theory of landscape development, in which he describes a “dynamic equilibrium” between the degradational forces (the weathering, soil creep, and stream erosion inherent in Davis’s erosion cycle) and the opposing forces of rock resistance and crustal uplift (Hack 1960). The resulting landforms maintain a steady state of balance that is independent of time.
In a comparable way the concept of vegetational climax has been criticized and rejected by many ecologists, partly because of an inadequate basis to project the successional development far into the indefinite future under conditions of a stable environment. In systems like the northern conifer forests under consideration here, the environment is simply not stable in all of its factors. These difficulties can be met in part by recognizing that the mosaic of forest types represents a long-range equilibrium that depends on the recurrence of a major type of perturbation—fire. Some elements of the forest mosaic may succeed toward a climax, from youth to maturity and even to old age, while other elements are interrupted in the sequence by fire. As the locus of fire shifts through time from one element to another, the forest composition as a whole remains much the same, thus maintaining a stable ecosystem. Such an equilibrium can prevail for thousands of years, until climatic change, species migration, or some other new external factor is introduced to the system. Then in reaction to this change the system will adjust to a new equilibrium with a different overall forest composition, again with its own mosaic elements changing in a dynamic pattern as the short-range factors such as fire and succession exert their local influences.