BACKGROUND

In this classic paper on landscape-scale disturbance, Sprugel questions the simplistic notion of climax generally accepted in the 50's and 60's. In his introduction Sprugel says: "ecologists spent much time and effort searching for, describing, and classifying 'climax' ecosystems … even though there was often little or no evidence that stable systems of this sort would ever come into existence under natural conditions. In fact, many studies have indicated that natural disturbance plays a far more vital role in ecosystem dynamics than that attributed to it by the classical climax theory."

To address this issue Sprugel studied balsam fir communities, the uppermost tree zone in the northeast U.S. It was well known that in these high-altitude fir forests "waves" of crescent-shaped bands of dead trees were found in systematic patterns. The waves are areas of standing dead trees with mature and healthy forest surrounding them. From left to right the cross section in Fig.1 shows a mature forest an adjacent area of dead and dying trees, an area where dead trees are being replaced by fir samplings of successive age, and a second area of dead trees. The paper also includes several photographs of the fir waves.

Sprugel's main site was Whiteface Mountain in New York; Whiteface is the most northerly peak in the Adirondacks and in his study locale 99% of trees are balsam fir. He also worked in New Hampshire and Maine. Sprugel measured direction of tree die off by taking transects through the waves; here he also determined tree ages by coring them. For another part of the study he marked trees for several years and classified them into improved or deteriorated categories by examining browning of tips and overall browning.

Sprugel found that waves move in the direction of the prevailing wind. He next considered the cause of tree death and, using data on wind speeds in a conifer forest, reasoned that wind velocity at the edge of a tree canopy was over 50% higher than that within the forest. Rime-ice, ice formed when water droplets hit solid surfaces and immediately freeze, was a well known phenomenon on Whiteface Mountain. (The paper includes Weather Bureau statistics that riming occurs there on about 1/3 of days from October through April.) Rime accumulates more on trees exposed to wind.

Sprugel's conclusion is that trees at the leeward edge of the canopy opening in the wave are exposed to winds and die from loss of needles and branches due to heavy ice accumulation. He also describes winter desiccation and lowered rates of production in summer as a result of needle cooling. As these trees die, adjacent firs experience the same conditions and begin to die. The overall direction of the wave motion is therefore directly related to wind direction.

Regeneration of waves occurs at about 60 year intervals and thus all stages of regeneration and deterioration can be found in the forest. In this sense the system is steady-state.

Literature Cited

• Sprugel, D. G. 1976. Dynamic structure of wave-regenerated Abies balsamea forests in the north-eastern United States. Journal of Ecology 64: 889-911.