-i) HARVESTING EFFECTS ON SOIL AND WATER IN THE EASTERN HARDWOOD FOREST INFLUENCES ON SOIL. J. H. Patric Erosion

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1 -i) HARVESTING EFFECTS ON SOIL AND WATER IN THE EASTERN HARDWOOD FOREST are, of course, many things yet to learn, but in light of present-day concern for the environment, a summary of the known effects seems timely. Forest Service research on soil and water has been intensive, especially in the Appalachians. It began at North Carolina in 1933, spread to Pennsylvania in 1948, to West Virginia in 1951, to New Hampshire in 1954, and to Kentucky in The Tennessee Valley Authority has conducted research in forest-soil-water relations since Other federal and state agencies as well as universities have contributed important new knowledge, and a pattern of relatively consistent results emerges from this accumulation of information. While most pertinent to the mountain hardwood forests extending from Alabama far to the northeast, many of these results pertain to all eastern forests. Perhaps nowhere is environmental concern mirrored more accurately than in the."letters to the Editor" section of any conservation-oriented magazine. Some readers state beliefs that soil erosion and spring flooding increase after forest cutting, especially after clearcutting. Others argue that climax forests use more (or sometimes less) water than subclimax stands. Some worry that certain practices, notably clearcutting, will impoverish forest soils. Others propose that highly flammable fuel accumulations left over from logging can be prevented by composting the forest litter. By far the most common attitude is that water resources are best protected by commercially valuable stands of large trees. Despite these popular beliefs, much experience and research show that wood products can be harvested with adequate protection to soil and water. J. H. Patric Erosion INFLUENCES ON SOIL ABSTRACT. For the eastern United States, there is overwhelming evidence that neither the productivity of forest soil nor the quality afforest water are substantially lessened during or after responsibly managed harvest of wood products. Carelessness, however, damages both resources. The key is forest roads; they cause little adverse effect on soil or water given proper location, drainage, traffic control, and maintenance. The public must better understand that it bears much of the cost for these measures. Nowhere in the world is there a firmer factual record of how forestry practices affect soil and water than in the eastern hardwood region. There 66 There is no doubt that forest provides maximum protection against soil erosion. Nor is there doubt that careless logging, grazing, burning, and even recreation can accelerate erosion, sometimes to intolerable levels. But even under dense and undisturbed forest cover, some soil loss does occur. Erosion is a normal geologic process; and soil loss at rates up to 0.1 ton per acre per year must be accepted, even in the old-growth forest. Although tons per acre implies uniform sheet erosion, soil lost from undisturbed forest almost always originates in stream channels. These and other generalizations which follow apply to all land having a normally developed forest soil; they do not apply to land that is compacted by improper use or that is unprotected by a litter cover.

2 The key to holding soil loss close to geological rates is to disturb soil and vegetation minimally in the channel vicinity. Elsewhere, the kind, size, or density of trees has little influence on credibility as long as the forest soil structure is relatively undisturbed. Soil loss not more than double the geologic rate is all that should be expected, and that for only a year or two after responsible wood products harvest. Forest soil is virtually armored against erosion by surface accumulations of fresh as well as decomposing leaves and twigs, ranging from three or more tons per acre in the South to 12 or more tons per acre in the North. This organic cover is replenished annually by leaf fall at rates from one to two tons per acre. Decomposing organic matter also helps nourish the living plant cover. Surface accumulations of litter play the key hydrologic role. They absorb the entire kinetic energy of falling rain, preventing particle detachment and sealing of the porous surface, which often occur when rain pounds down on soil unprotected by litter. Furthermore, water infiltrates rapidly into soil lying just beneath the litter, at rates far exceeding rainfall intensity. Infiltration rates of 50 or more inches per hour are common, while rainfall intensities exceeding 2 inches per hour are uncommon in the eastern hardwood region. Therefore overland flow rarely occurs. Given these conditions, there is no mechanism to detach or to transport the litter-protected soil, with only the stream channels ordinarily providing sites for loss of soil particles. But water storage capacity in the soil, particularly during wet weather, often is insufficient to contain all of the infiltrated rain. Under these circumstances, the absorbed rain moves laterally through the porous soil to streams. Forest hydrologists generally agree that complete absorption of the rain and its subsequent movement downslope through the soil accounts for most forest streamflow, during as well as between storms. Site preparation for planting trees, as well as grazing, careless logging, and even recreation, often disrupt the litter cover, thus exposing the mineral soil to erosive action by rainfall. In addition, such forest uses compact mineral soil underlying the litter, greatly reducing infiltration rates. Under these conditions, measures to prevent erosion sometimes are needed because soil exposure and compaction can and do lead to overland flow and accelerated soil loss. Severe fires also expose large expanses of soil to erosion, but they are rare in the humid East. Research suggests that burning increases erosion somewhat over geological rates, but that accelerated rates seldom persist for more than a few years. Burning does, even under control, alter forest composition. More research is needed to understand the full role of fire, but it is increasingly clear that most burning in the eastern forest has little effect on soil or water, either during or between storms. There is no evidence that cutting trees even clearcutting accelerates soil erosion much above geologic rates. Tree cutting alone does not compact the soil, and it initially adds to the litter cover. Revegetation by seedlings and sprouts is so fast in hardwoods that a complete small tree and shrub cover regrows within two to three years. And the litter-production capability of these dense juvenile stands is only slightly less than that of mature hardwood forests. Heavy browsing by domestic animals or large deer herds can slow the rates of stand regrowth. Timber harvesting logging is another matter. Dragging logs across the forest floor disrupts the organic cover, exposing and compacting the mineral soil so that overland flow can and does occur. These effects are caused, not by dragging Figure 1. Twenty years ago, more than half of the world's calibrated watersheds were at the Coweeta Hydrologic Laboratory, a pioneer installation for forest hydrology research near Franklin, North Carolina. Although this kind of research is now widespread, Coweeta remains a reservoir of unequalled watershed experience. This 40-acre watershed was clearcut in 1939 and again in Greatly increased flow after each cutting was followed by rapid and complete reforestation that decreased flows to beforecutting levels. 67

3 Figure 2. Forest hydrology research in Kentucky is concerned with strip-mining effects on forest water resources. Coal mining such as this is a primary cause of flashy, acid, sediment-laden streams. Mining methods are being developed to minimize undesirable effects on soil and water. one or two logs, but by dragging many along a single track. Under these circumstances, the erosion hazard is increased most on steep slopes, in wet soils, and along streams. Highest rates of erosion usually occur on logging roads and skid trails. Where trucks and rubber-tired skidders are used, soil may be exposed and compacted on as much as 15 percent of the land surface, almost one of every seven acres. Many people believe that logging-accelerated erosion is avoidable by using silvicultural systems that don't involve clearcutting. This is not necessarily true. Indeed, it can be argued logically that more frequent re-use of roads under selection management affords a greater potential for accelerated erosion. Guidelines for logging and road manage- 68 ment that hold soil losses close to geologic rates are simple: 1. Do not operate machinery in streams. No other logging practice causes greater or more lasting soil losses. 2. Locate roads as far as practicable from streams. Except on steepest land, soil eroded from roads will not reach streams more than 100 feet away. 3. Skid logs up-slope to roads that parallel streams. This minimizes soil compaction and maintains high infiltration on stream-side soils. 4. Build as few roads as possible. This means advance planning, a measure all too often neglected by loggers.

4 Figure 3. Research at the Fernow Experimental Forest near Parsons, West Virginia, has emphasized timber-management effects on water resources. This 86-acre watershed was clearcut in except for a seven-acre strip left along the stream to maintain cool water. The two main skidroads were carefully located parallel to the shade strip, but even the less carefully located skidroads on ridges caused little sedimentation. Water quality differed little from that of streams draining uncut forestland. 69

5 Figure 4. The Tennessee Valley Authority owes much of its success in watershed rehabilitation to the conversion of marginal farmland to thrifty forest. Here, fields cultivated in 1935 (left) had reforested naturally by 1954 (right). These photographs, taken from slightly different angles, illustrate the rapid and complete reforestation characteristic of eastern hardwoods. 5. Apply erosion-control measures as needed. Grade limits, gravel, and adequate drainage can greatly reduce erosion during logging. 6. Revegetate roads as soon as possible. Water bars, outsloping, and grass seeding greatly reduce soil loss after logging. Two kinds of education are badly needed concerning forest soil erosion. (1) Some loggers are honestly unaware that their work can increase soil losses; others perceive little need to apply known measures to control them. (2) Recreationoriented conservationists need to place more weight on economic realities. Our society needs wood fiber, wood is produced on forestland, and its production is accompanied by some acceleration of soil loss. These losses can be held to negligible levels but and here is the rub at a not always negligible cost. Ultimately, all of the people that benefit from protecting soil and water must contribute to the costs of protection. It is unrealistic to assume that loggers, land managers, or wood processors bear the full costs. They are passed along to the public, either as higher taxes or as higher prices for wood products. Those who would harshly restrict forest use in the name of ecology must face this economic reality. Fertility We have learned far less about timber harvesting effects on forest soil fertility, a problem insufficiently researched to provide firm answers. Nitrogen and calcium losses amounting to 2 and 4 percent, respectively, of amounts contained in podzol soils have occurred two years after clearcutting in New Hampshire. Losses of this magnitude are not reported elsewhere, regardless of the timber-harvesting method used. These losses are replaced by rainfall and rock weathering long before the new tree generation is grown. But even in New Hampshire, it appears that accelerated loss of soil fertility after timber harvesting is a short-lived process. After perhaps five years of forest regrowth, the new generation of small trees seems as capable as former stands of preventing accelerated loss of nutrients from forest soils. Much remains to be learned about both short- and long-term effects of timber harvesting on soil productivity. New techniques are permitting widespread research on this subject. Limbs and other slash remaining on the ground after conventional timber harvesting provide important sources of nutrients for recycling into the regenerating stands. Whole-tree harvesting for pulpwood utilizes both of these nutrient sources. It is coming into vogue, and forests so managed must be watched closely for symptoms of nutrient deficiency. There is no evidence that forest soils have been impoverished by monoculture, although the failure of exotic species in plantations has been so interpreted, particularly where conversion from mixed hardwood to pure stands of conifers is widely practiced. Summary of Effects on Soil Erosion is a normal geologic process that is accelerated modestly and briefly by wood products harvest, even with careful practices. Overland flow seldom occurs in the carefully managed forest. Its occurance on logging roads is a common, though preventable, cause of accelerated erosion. 70

6 Figure 5. Research at Hubbard Brook, New Hampshire, has gained worldwide recognition by contrasting nutrient cycling on forested, deforested, and conventionally harvested forestland. After all trees were cut and regrowth was temporarily prevented on the right-hand watershed, some soil nutrients normally taken up by trees leached through the soils to the stream. This was deforestation, not clearcutting. Less nutrient leaching occurred after conventional timber harvest by cutting the left-hand watershed in strips. It has not been shown that conventional harvest of wood products decreases soil productivity. Careless logging and road building, especially in stream channels, can accelerate soil erosion substantially and for several years. Fire may modestly and briefly accelerate soil erosion. Quantitative INFLUENCES ON WATER Precipitation ranges from 35 to 70 or more inches annually over the eastern United States. About 5 to 7 inches never reach the soil under a mature hardwood forest. It is intercepted on foliage, litter, and branches, then evaporated from them. Of rain that does enter the soil, perhaps 12 to 24 inches is lost by transpiration, the evaporation of soil moisture through leaves of trees. Some rainfall seeps into bedrock, but the balance ordinarily becomes streamflow. Thus streamflow is the remainder from precipitation after all other losses of water have occurred. The actual amounts in this water-balance generally decrease from south to north, but the principles apply universally. When the forest is cut, both interception and transpiration losses decrease. The conservation-ofmass law applies and, as both losses decrease, streamflow increases accordingly. But the evaporative losses are never eliminated, so streamflow never increases to the full amount of evaporative demand. Streamflow increases are approximately proportional to the severity of forest cutting; and clearcutting causes maximum increases, reported to range from 8 to 16 inches annually. Streamflow always decreases as regrowing forests return evaporative losses to beforecutting levels, a process that may require only one year after partial cutting to more than 10 years after clearcutting. Flow increases, often measured on small (less than one square mile of watershed) streams, are seldom detectable when the far greater flows of large rivers are measured. There, the flowincreasing effects of cutting in some places are usually countered by the flow-decreasing effects of regrowth in other places. Forest cutting, blowdown, or fire of regional extent must occur before evaporative losses are sufficiently reduced to cause measurable flow increases in rivers draining many hundreds of square miles. Presumably, the largest flow increases should be realized by cutting those trees that evaporate most water. It now seems that permanently foliaged conifers intercept as well as transpire more water than deciduous hardwoods do, while grass uses a little less than coniferous or deciduous trees. To date research has not shown that annual water use varies significantly with species, size, or age of hardwood trees in a humid climate. (As used here, trees refers to vegetation larger than saplings.) The field of plant-water relations remains one of great and continuing research interest, with complete answers not yet available. Autumn leaf-fall marks the end of transpiration from the hardwood forest and the beginning of replacement of soil moisture after the growing season. After late autumn, soil moisture remains at near-capacity levels, so that most of the absorbed rain is stored in the soil or moves through it to streams. Under dormant-season conditions, there is little difference in stream behavior on forestland with or without trees if the litter 71

7 Figure 6. The extraordinary ability of forest soils to absorb and transmit enormous volumes of water makes them a promising disposal site for the nutrient-rich effluents of modern sewage-treatment plants. Pennsylvania State University and the University of Florida are exploring this use of forests to dispose of liquid wastes. Here freezing weather has delayed, but by no means prevented, the absorption of aerially applied effluent from University Park, Pennsylvania. cover is intact, if the underlying soil is uncompacted, and if all water reaches streams by flowing through the soil, not over the land surface. This generalization may or may not hold true during snowmelt. Soft, easily rutted logging roads are explained by these soil moisture conditions. When hardwood foliage reemerges in the spring, soil-plant-water relations are much changed. Both rainfall interception and transpiration greatly increase, with correspondingly decreased soil moisture. Under these typical growing-season conditions, most of the rain that enters the soil merely replaces previous losses to transpiration and is soon lost. Unless rainfall is very heavy, little of the soil moisture can reach a stream; so low flows, characteristic of the growing season, prevail. Rapid evaporation minimizes mud problems on logging roads. On cutover land, however, both evaporative losses and soil-moisture depletion are minimal. Most of the rain that falls still enters the soil, but whatever moisture has been lost is quickly replaced, with the absorbed rain moving rapidly through the soil to streams. With clearcutting or other substantial vegetation removal, growingseason flow is augmented by rain thus diverted from evaporative loss. For these reasons, growingseason flows are much increased by heavy tree cutting, but dormant-season flow is virtually unaffected. Generally higher levels of growing-season flow from cutover forestland can therefore increase channel scouring, but the added sediment is barely measurable, even on carefully observed experimental watersheds. Flooding The forest-flood relationship merits special attention. In their zeal to achieve the proven ability of forests to control soil erosion, foresters of several decades ago were wont to proclaim corollary, sometimes unfounded, flood-control benefits. 72

8 A more factual view of the forest role in flooding is possible now. Dormant-season floods in the hardwood region usually are caused by extensive frontal storms, and sometimes they are augmented when rain falls on melting snow. For reasons already stated, the presence or absence of trees has little effect on delivery of dormant-season stormflow to streams. And it has recently been concluded that forest management need not be considered as a means of protection from snowmelt floods. Overland flow from eroding logging roads conceivably augments stormflow. But, common as such roads are, they nevertheless are too few and scattered to aggravate flooding on a regional scale. The inescapable conclusion is that heavy rains cause floods, regardless of tree cover conditions. Conceivably, regionwide cutting could increase forest soil moisture enough to augment flooding during the growing season. Several factors, however, serve to undermine this possibility. The prevailing pattern of small ownership, coupled with the enormous diversity of forest conditions, precludes regionwide cutting. Frontal storms are not common in the growing season; high-intensity thunder storms produce most of the summer rainfall and sometimes they do cause local flash flooding. But even if the locations of high-intensity summer showers did happen to coincide with patches of heavy cutting, the resulting stormflow in headwater streams would soon be contained in the larger and unflooded channels downstream. When great regional storms do occur in the growing season (for example, the tropical hurricanes), heavy rain fills even the forested soils to capacity, and therefore stormflow from all forestland resembles that of the dormant season, with tree-covered and cutover lands generating stormflow similarly. There is little reason to believe that cutting can increase flooding on forestland as used today. Despite the preceding interpretations of research, there are records of increased flooding after destruction of the old-growth forest some 75 to 100 years ago. But this was regionwide cutting, followed more importantly by fire, grazing, and agriculture that destroyed the forest soil structure and retarded the natural regrowth of trees. A few of the affected areas have not regrown trees to this day. The all-important point is that regional and lasting deforestation, not timber harvesting, somewhat increased the flood hazard at the turn of the century. Qua//fa(/Ve There is little doubt that increased soil moisture and temperature do accelerate release of nutrients contained in the organic layers on cutover forest- land. Most minerals so released are promptly taken up by the regrowing trees, probably an important means of successful stand regeneration. Nevertheless, some of the released minerals will drain away in streams, resulting in a minor and short-lived enrichment. Given unwise logging and road practices, this enrichment can reach pollution levels causing damage to fish and recreation. When only parts of watersheds are logged or managed unwisely, dissolved minerals are diluted rapidly in flow from other parts of the watershed, and the enrichment becomes undetectable downstream. Should reduced tree growth or other nutrient problems develop, most are curable by modified cutting practices. A few are curable by fertilization, although this practice, too, may increase chemical loading in streams. Actually, some increase in the chemical content of sterile headwater streams may not be detrimental and even prove beneficial to fish, often limited in production by nutrient deficiency. In view of its increasing cost and of worldwide food and energy shortages, forest fertilization may never be practiced widely. Nutrient amendments such as sewage sludge may prove to be an acceptable substitute for conventional fertilization. The remoteness, steepness, and inaccessability of much forested land often preclude any form of fertilization as a realistic means of enhancing productivity. Water temperature usually increases when trees shading the channel are cut. Warming may range from 1 to 10 degrees or more. Heating dissipates quickly as warmed waters are diluted in cooler water downstream. Warming varies with channel exposure, latitude, season, and volume of streamflow. Cool water is restored as revegetation reshades the channel. The heating problem is avoided easily by keeping channels shaded. Summary of Effects on Water Tree cutting reduces evaporative losses with corresponding gains in stream volume. Wood products harvest in scattered blocks has negligible effects on river flooding. Wood products harvest temporarily accelerates release of nutrients from the forest floor, causing some enrichment of streams. Fertilization, too, enriches streams temporarily. Channel clearing increases stream temperature until regrowth restores shading. /. H. Patric is project leader in forest hydrology research, USDA Forest Service, Northeastern Forest Experiment Station, Timber and Watershed Laboratory, Parsons, West Virginia. 73