EFFECTS OF CUTTING PRACTICES ON MICROENVIRONMENT IN ...

EFFECTS OF CUTTING PRACTICES ON MICROENVIRONMENT IN RELATION TO HARDWOOD REGENERATION Wayne T. Swank and James M. Vose Principal Plant Ecologist and Research Ecologist USDA Forest Service Coweeta Hydrologic Laboratory Southeastern Forest Experiment Station Otto, NC 28763

ABSTRACT Regeneration cutting results in significant changes in the microenvironment of the forest floor and soil. For example, light intensity, surface soil temperature, and soil moisture increase with removal of the forest canopy. These changes influence regeneration success directly through their impact on photosynthesis, heat and frost damage, and available soil moisture. Changes in the microenvironment also influence available nutrients because decomposition and nutrient cycling processes are strongly regulated by soil temperature arid moisture. Nutrient availability is further influenced by changes in dry deposition and leaching through the logging slash. The degree of changes in the physical, chemical, and biological properties of the forest floor and soil is dependent upon cutting intensity. Thus, silvicultural prescriptions should consider the microenvironmental requirements of the desired regenerating species.

Introduction Removal of part or all of a hardwood overstory canopy results in significant changes in the physical, chemical, and biological properties of the forest floor and soil. The establishment and growth of hardwood regeneration are regulated by these changes in the microenvironment since light, temperature, moisture, and nutrients are crucial to plant processes. The objectives of this paper are to examine effects of different hardwood regeneration harvest methods on the microenvironment and the potential influence of changes on the establishment and development of regeneration. We prefer to use the broader term "microenvironment" instead of "microclimate" because biological and chemical processes that take place in the plants and soil are strongly linked to site conditions. Major variables

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we plan to discuss are illustrated in Figure 1. Cutting alters solar radiation and hence the energy balance of the land surface. Consequently, variables such as temperature and moisture of the forest floor and soil are also altered. Changes in the physical factors produce significant changes in nutrient input, uptake, and biological processes such as decomposition and transformation of nitrogen to a form available for plant uptake. In this paper, our approach will be to discuss basic principles and suggest expected changes in the microenvironment. Emphasis will be placed on clearcutting because response information is most abundant for this harvest method.

Solar Radiation

Physical Responses

y Surface

Energy Balance

Chemical Responses \ Nutrient lnputs"Moisture a Temperature Nutrient Uptake! of Forest Floor a Soil

Biological Responses ' Decomposition Microbial Transformations

SOIL

Figure 1.

Illustration of major components of the microenvironment in response to regeneration cutting.

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Physical Responses Solar Radiation (Light) Alteration of solar radiation is the largest and most important change to the microenvironment that accompanies cutting practices (Wallace 1988). Removal of even a part of the forest canopy strongly affects both the intensity and quality of light reaching the forest floor. Kramer and Kozlowski (1979) conclude that tree growth is influenced more by changes in light intensity than by changes in light quality. Generalizations about the utilization of energy (light) in the ecosystems are difficult to make. Swift (1972) found that different energy components following clearcutting of hardwoods in the Southern Appalachians were dependent upon season and topographic position. However, he concluded that clearcutting reduced energy used for plant growth, water evaporation, and heating of soil and air (daily net j?adiation) on both north- and south-facing slopes about 350 to 460 Watts/m in the growing season. The magnitude of change in light intensity after removal of the forest canopy depend on the amount of canopy removed or the size of the opening (Wallace 1988). In full leaf, a closed hardwood stand reduces light intensity at the forest floor to about 90 percent of the light received in open areas. However, after leaf-fall, light increases to about 50 percent of values received in the open areas. Increases in light due to regeneration practices are not proportional to the amount of canopy removed because of repeated reflection and absorption of radiation in the remaining canopy (Reifsnyder and Lull 1965). Thus, the intensity of light available to seedlings and sprouts declines rapidly from clearcutting conditions to about 35 percent crown density. Thereafter, available light decreases more slowly with increasing crown density (Kramer and Kozlowski 1979)• Between 50 to 75 percent of a stand would have to be cut to increase summertime light intensity to half of that in the open (Reifsnyder and Lull 1965). The amount and type of reproduction associated with various silviculture methods are strongly dependent upon changes in light intensity reaching the forest floor after harvest. Rates of photosynthesis (plant growth) increase in proportion to light intensity until light saturation occurs; thereafter, photosynthesis is generally constant. Maximum photosynthesis for species such as white oak, red oak, and dogwood may occur at light intensities of about one-third of full light while light saturation of red maple is much lower (Kramer and Kozlowski 1979)In mixed oak stands of West Virginia, Carvell and Tryon (1961) found that oak regeneration density increased linearly over the range of 5 to 40 percent sunlight. The ability of species to respond to increased light intensity is not solely dependent upon maximum photosynthetic rates. For example, recent efforts have focused on photosynthesis and growth responses of seedlings to changes in available light for photosynthesis activity—400 to 700 nm wavelength

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(Wallace 1978)- Additionally, some species exhibit other physiological and morphological responses to increased light intensity that may allow them to grow in a variety of light conditions (Wallace 1988). Wallace and Dunn (1980) found that intermediate shade-tolerant species such as white oak, red oak, red maple, and ash had a range of physiological and morphological responses to changes in the microenvironment created by forest openings. In contrast, shade-intolerant yellow-poplar has rigid physiological and morphological characteristics that may restrict this species to sites with abundant light. At the present time, insufficient information is available to predict changes in light intensity produced by different regeneration methods. Moreover, specific light requirements, in combination with other physiological responses for the numerous species that occupy mixed hardwood stands, is not well known. Thus, the best practical guidelines for species light requirements are probably the long-established criteria of shade tolerance (Baker 193^)Moisture and Temperature Due to the influence of the remaining overstory, small openings (less than about 0.2 acre) in the overstory will change the moisture and temperature characteristics of the microenvironment less than light intensity. However, with large clearcut openings, significant alterations of both moisture and temperature occur. Examples of changes in moisture and temperature as related to clearcutting are illustrated by a long-term study at the Coweeta Hydrologic Laboratory in the Southern Appalachians. Beginning in January 1977. a 1^7-acre, south-facing watershed covered by mixed hardwoods was clearcut and logged with a mobile cable system. Removal of commercial sawlogs was completed in June 1977 and site preparation (clear-felling all remaining stems >1 inch) was completed in October 1977• A number of environmental responses to the clearcut treatment were compared to an adjacent undisturbed hardwood forested watershed. Beginning in August 1977, soil moisture was measured at biweekly intervals in the 01 and 0? (organic) litter layers of the forest floor and approximately 0 to s4 inches and 4- to 12-inch depths in the soil of treated and control watersheds. A summary of moisture data shows how changes in light intensity and temperature at the forest floor as well as reduced evapotranspiration (combined loss of water by evaporation from the soil surface and by transpiration from plants) influence litter and soil moisture (Table 1). In the first autumn following harvest, litter moisture content was 20 to 30 percent below that of the adjacent undisturbed forest (Table 1). With winter precipitation soil recharge and reduced evaporation, litter moisture content was similar on both watersheds. During the spring, summer, and autumn periods of the ensuing year, moisture content of the 0.. and

0 layers of the forest floor in the clearcut area was consistently 30 to 50 percent lower than values from the undisturbed watershed.

Table 1.

Quarterly forest floor and soil moisture averages (percent) for mixed hardwood forests during the first year after clearcutting (WS 7) and for a control (WS 2) at the Coweeta Hydrologic Laboratory.

Treatment and depth

1977 Aug-Nov

Year and Quarter 1978 Dec-Mar Apr-Jul Aug-Nov

Moisture content (percent by weight) Clearcut

1 0 to old leaves > twigs > branches > stemwood. Differences between tissue type are related to both tissue quality and surface area/mass relationships. Microbial Transformations. Most research on the impact of forest removal on microbial transformations has been concerned with the N cycle because (1) N most commonly limits tree growth, and (2) concern over the effects of increased mineral N on stream and ground water quality. Thus, our discussion will focus on the effects of forest removal on several important microbial processes in the N cycle such as: 1.

N mineralization—conversion of N from the inorganic form (ammonium)—available to plants.

2.

N immobilization—conversion of N from organic form—not available to plants.

3.

N nitrification—conversion of ammonium to nitrate—highly available to plants, but subject to leaching losses.

4.

N fixation—conversion of gaseous nitrogen to a plant-useable form.

5-

Denitrification—reduction of nitrate oxides—not available to plants.

to

the

organic

to

inorganic

to

gaseous

N

or

N

N mineralization is the microbial process whereby ammonium (NFL ) is released from OM as it decomposes. The impacts of forest removal on N mineralization processes are related to the effects of forest removal on microenvironment and the quality and quantity of the logging slash and organic matter. For example, at the Hubbard Brook Experimental Forest in the northeast, N mineralization increased substantially following forest overstory removal, resulting in a net N loss from the soil of 421 pounds per acre over a 3-year period (Bormann and Likens 1979). In contrast, only modest increases in total N mineralization (9 to 27 pounds per acre per year) were observed on a clearcut watershed in the Southern Appalachians

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(Waide et al. 1988). However, in the Hubbard Brook clearcut, no products were removed'and regeneration was prevented with herbicides. Thus, factors related to. the quantity of the logging slash and organic matter, improved microenvironment for decomposition, and inhibited N uptake by early successional species probably contributed to the high soil N losses at Hubbard Brook. Mineralization rates also depend upon the quality of the logging slash. In general, slash with low C:N ratios, such as fresh foliage and twigs, mineralizes faster than slash with high C:N ratios, such as branches and stemwood (Mattson et al. 1987)Additionally, many pioneer species have high N concentrations and low lignin contents resulting in increased N mineralization following forest removal (Vitousek 1981). The availability of mineralized N for seedling uptake and/or ground and stream water losses depends, for the most part, on immobilization and nitrification processes. Nitrogen is immobilized in the decomposition of material with high (>25:1) C:N ratios (Vitousek 1981). Eventually, decomposition reduces the C:N ratio of the logging slash-organic matter and mineralization occurs. The immobilization of ammonium (NH^ ) by soil microorganisms can be a very important nutrient conservation mechanisms in clearcut forests (Aber and Melillo 1982); however, immobilization makes less N available for seedling uptake. Because of its positive charge, NH^ can also be adsorbed on negatively charged soil particles. As a result, NH^ losses in ground and stream water are relatively unimportant. However, through the microbial process of_ nitrification, NH^ is converted into the_ highly mobile nitrate (NCL ) anion. Because of its high mobility, NCL is believed to be more available for seedling uptake than NH^ , but NCL is also subject to leaching losses. Forest removal can increase Tiitrification rates, probably through a combination of changes in microenvironment and increased NH^ availability (Robertson 1982). For example at Coweeta, nitrification increased 3~fold to 15-fold following clearcutting (Waide et al. 1988). This response was paralleled by a 10-fold to 20-fold increase in nitrifying bacteria. Similar results have been shown in a clearcut forest at Hubbard Brook (Likens et al. 1968). Nitrogen may be increased in clearcut forests by a variety of plant species an< ^ N fixation bacteria existing together. Symbiotic associations (Frankia, Rhizobium) in roots of early successional tree species such as black locust can result in increased N inputs of 9 pounds N per acre per year or more (Boring and Swank 1984) . Clearcutting can also increase bacterial N fixation. For example, at Coweeta, clearcutting increased bacterial N fixation by 8-fold in the 0- to