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Window into a watershed: by taking the pulse of its lifeblood - the hydrologic system - scientists are learning to better gauge the health of the forest ecosystem.

Heavy rain squalls

descend upon

northern hardwood forests. Plummeting drops are halted abruptly-scattered-by impact with a verdant and profuse canopy. The rain's pounding force is effectively diminished, and life-giving water can enter the forest's lower reaches at a more amenable rate.

The rainwater, of course, is on its usual peregrination through the hydrologic cycle. In its path across the northeastern U.S. are the exquisite inner workings of a northern hardwood ecosystem, an intricate mantle of arborescence that has become the land. Limbs reach out toward the light, gathering energy to send to their subterranean counterparts, providing the strength to literally bind life-giving soil in place.

These are parts of the system we can most easily comprehend. But equally important to how an ecosystem functions are many things not so readily scrutinized, factors requiring more than a casual musing on the beauty of a mountain forest. Scientists, by studying rain that has coursed through the viscera of a forest ecosystem, can take the pulse of the system. They can gauge its health, or its state of development, much the way a physician would use a blood sample to evaluate a patient's condition. Water, after all, is the life fluid of systems at all levels.

Hubbard Brook Valley in New Hampshire's White Mountains is one place where forest scientists have set up shop in order to get a fix on a little patch of our planet. Known more formally as the Hubbard Brook Experimental Forest, it has been a source of valuable research information in forest ecology for more than two decades.

Three prominent researchers who have done extensive investigation at Hubbard Brook are: Dr. F. Herbert Bormann, Oastler Professor of Forest Ecology at the School of Forestry and Environmental Studies, Yale University; Dr. Gene E. Likens, Director of the Institute of Ecosystem Studies, Millbrook, New York; and Dr. Robert S. Pierce, Project Leader of the Hubbard Brook Experimental Forest, U.S. Forest Service. Much of the important research arising from the Hubbard Brook studies results directly from their efforts, funded almost since the project's inception in 1963 by grants from the National Science Foundation.

Nature's secrets must be coaxed from the land, and as with many other endeavors, technique is all important. A study technique developed by the Hubbard Brook researchers, known as the "small watershed technique," has proven most effective in gleaning information from the Hubbard Brook ecosystem.

The basic principle of the small watershed technique can be described readily. A watershed is an area of land drained by a particular water system. If you follow a stream's course upcurrent, you encounter smaller tributaries at varying intervals, each patterned after the main stream but arranged in a divergent fashion. Exploration of each reveals a smaller counterpart of the main stream, perhaps with still smaller branching tributaries of its own.

Eventually you reach the headwaters. These are, collectively, the uppermost divides between the small tributaries leading to the watershed under consideration, and an adjacent watershed. The divide can usually be recognized on the land as the crest of a hill or ridge. Unless subsurface features interfere, rain falling on the side of the stream in question will flow into that stream. On the opposite side of the crest, water will generally enter a separate watershed and flow in another direction. Following each tributary and subtributary of a stream upstream to its divide yields a series of terminal points corresponding in pattern to the twig tips of a tree (this watershed form is termed dendritic" due to its treelike form).

Such a watershed, by virtue of its structure, present a natural unit for study. As in other kinds of systems, there are inputs and outputs to be considered. Inputs include solar energy' precipitation, wind, and other materials carried in air and water that enter the watershed. Among the outputs are gases that escape into the atmosphere, and liquid water with its associated load of impurities (either dissolved or suspended) that exit through stream flow.

The "basement" of the watershed is assumed to be watertight so that construction of a weir at the watershed's mouth brings all liquid water output over that structure. Two further important assumptions are these: the geologic makeup of material underlying the forest is uniform; and the small watershed is part of a larger uniform northern hardwood forest type. These assumptions allow changes in measured variables, such as streamflow, to be attributed to conditions within the watershed itself.

Water, as a basic resource of living systems, performs many vital functions. First, it enters the ecosystem through meteorologic input. But water doesn't necessarily travel alone. Frequently, if not always, precipitation carries a load of natural impurities and pollutants. just as water brings an influx of materials to the watershed, some of these materials (nutrients such as nitrogen compounds, for example) are detained or retained as water infiltrates and moves through the system. And some pre-existing nutrients are inevitably carried out of the ecosystem as water continues its unrelenting cycle. The beauty of the small watershed technique is that many ecosystem variables can be analyzed by studying water that has passed through. A reasonable analogy can be drawn by comparison to a bathtub in which a terrarium has been established. Turn on the showerhead, and the "watershed" receives "rainfall." The water is used by the plants and other living organisms in the terrarium. Some water is lost through evaporation, and some percolates into the " soil. " The excess enters the drain after having run through the system. If drain water can be collected for analysis and compared to an analysis of water captured as it falls, something can be learned of how the watershed system (terrarium) is functioning. And the effects of any experimental treatments given to the terrarium can be ascertained, again by analysis of the drain water.

Characteristics of the watershed vegetation exert a great deal of influence over the quantity and quality of water exiting the ecosystem. Hubbard Brook researchers have found that streamwater leaving a recently clearcut watershed takes with it many valuable nutrients. This leads to the crux of how these northern hardwood ecosystem studies were carried out.

It began with a series of adjacent watersheds forested with secondgrowth northern hardwood cover. (The predominant second-growth trees at Hubbard Brook are American beech, sugar maple, and yellow birch.) Some watersheds were monitored with cover intact. Others were subjected to various experimental treatments. The one of primary emphasis was deforested.

It was found that after clearcutting, the forest ecosystem shows a period of instability lasting one to two decades. Bormann and Likens term it the Reorganization Phase. During this phase, soil temperature increases in the summertime as does soil moisture (due to a reduction in the transpiration rate). And water yield from the watershed increases as does the load of particulate matter and dissolved nutrients. Nitrification-the bacterially mediated process by which ammonia is oxidized to form nitrites and nitrates-is more pronounced in the deforested watershed. These nutrients are lost to the ecosystem at accelerated rates. All of these factors indicate a system that appears to be -out of control. " But according to Bormann and Likens, it is during this phase that vegetation is gearing up for more orderly times to come. And they suggest that generalizations can be drawn from these studies that apply to commercially clearcut forests. One of the main contributions of the Hubbard Brook data to the practice of forest management concerns nutrient loss. Dr. Bormann says, ". . . there's probably no National Forest in the United States today where cutting would be proposed without the managers first trying to evaluate whether or not there was going to be nutrient loss from the forest ecosystem in streamwater." Prior to the Hubbard Brook studies, this important factor did not receive much attention from forest managers. Aggradation, the next phase of ecosystem development, is a period of time in excess of 100 years when total biomass-the sum of all living and once-living matter, from tree limbs to termites-is being built. It is this biomass development that is responsible for the strict control imposed on the ecosystem (in contrast to the Reorganization Phase). One example of how this control is exerted is given by considering the process of evapotranspiration.

Evapotranspiration is an energy-demanding process by which water vapor leaves the ecosystem. A portion of this water passes through the vascular system of plants and exits through the stomata of their leaves (transpiration), and a portion evaporates directly from other surfaces such as leaves, the soil, and bodies of liquid water. Under the conditions of a deforested ecosystem, a greater proportion of water exits the system in liquid form with its associated eroding power. And at the same time, the altered ecosystem is more susceptible to erosion.

Nutrients and particulate matter are retained in the watershed ecosystem to a greater degree as revegetation occurs. Further inquiry into the nature of transpiration and its effects reveals that it alone is an extremely important regulating factor as the ecosystem develops. As Bormann and Likens note, "Transpiration powers' a large part of the circulation of nutrients within the ecosystem. Some nutrients are drawn to root surfaces in the mass flow of water immediately after rainfall, and some are lifted to the canopy by transpirational pull.' In contrast to streamflow, which carries nutrients out of the system, transpiration may be viewed as a distillation' process in which nutrients are left behind in the leaves, where they are eventually recirculated by leaf drop, resorption, or leaching."

After aggradation, a transitional phase marked by reduction of total biomass is projected by Bormann and Likens. Finally, after this Transition, comes what they've termed the "Steady State," a period of fluctuation in biomass developing over several centuries. Here is where the Hubbard Brook findings become more speculative. But the general lesson is clear: as the ecosystem develops, biomass accumulates in all the various forms you might expect (living and dead plants and animals). This accumulation, besides having a stabilizing effect on the immediate ecosystem, helps in sequestering carbon from the atmosphere where, in the form of carbon dioxide, it contributes to global warming. t is important to keep in mind that this information represents scientific abstraction, and inevitab]y oversimplification, of a set of integrated natural processes. When we are in the forest-the White Mountain National Forest in New Hampshire or the back woodlot-much is going on around us. It is a continuous phenomenon, sometimes punctuated by a violent or cataclysmic burst, such as a lightning fire, a hurricane, or a timber harvest, but more often a profoundly still and quiet actuality, one that could easily go unnoticed if it were not for the probing nature of the human mind.

Whether in tropical forests, arctic tundra, or somewhere in between, our task is unambiguous: we must learn as much as we can about how our planet functions, and use that knowledge to cultivate not only forests, but an integrated global commitment as well. The Hubbard Brook researchers have helped lift the blind on one window to greater awareness. As more research is done, more of the scene will be clarified, contributing to a precious global data base. What the rest of us stand to gain from these efforts is better understanding and, ultimately, some timely advice on how best to move toward a sustainable future for our planet.
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Author:Miller, D. Glenn
Publication:American Forests
Date:May 1, 1989
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