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Linking the ecology of natural oak regeneration to silviculture
Forest Ecology and Management 106 Ž1998. 1–7
Linking the ecology of natural oak regeneration to silviculture David R. Larsen a
a,)
, Paul S. Johnson
b
The School of Natural Resources, UniÕersity of Missouri-Columbia, 1-30 Agriculture Building, Columbia, MO 65211, USA b USDA Forest SerÕice, North Central Forest Experiment Station, 1-26 Agriculture Building, Columbia, MO 65211, USA Received 21 May 1996
Abstract The regeneration requirements of oaks Ž Quercus spp.. differ among species. Oaks differ in their ability to produce seed, germinate and, as for reproduction, to endure shade, drought, and other stresses. Under the low to moderate shade that characterizes the understories of their natural habitats, the xerophytic oaks depend heavily on their drought tolerance and capacity to die back and resprout repeatedly. Successful regeneration of xerophytic oaks depends largely on the long-term survival and accumulation of oak reproduction, which may span 2 or more decades. Some of this reproduction develops large roots in advance of final harvest, which thereby enhances long-term survival and the potential for rapid shoot growth after overstory removal. In the more nutrient- and moisture-rich ecosystems, seedling populations of indigenous oaks arising from a single cohort typically fall to near extinction within a few years. High mortality rates are related to seedling intolerance to shade and the prevailing low light intensities under the high stand densities and stratified canopies typical of those ecosystems. In the absence of natural disturbance or silvicultural intervention, oak reproduction beneath dense stands in rich ecosystems typically fails to accumulate over time and thus fails to form the large root systems necessary for competitive success after overstory removal. The required timing and intensity of silvicultural operations for regenerating oaks therefore depend on the ecosystem-specific population dynamics of each species. Knowledge of birth, death, and other population processes of oak reproduction within defined classes of ecosystems, as well as knowledge of periodicity in seed production are prerequisites to the development of ecologically sound silvicultural prescriptions and realistic predictive models for regenerating oaks. q 1998 Elsevier Science B.V. Keywords: Oak regeneration; Oak silviculture; Regeneration models
1. Introduction In North America there are between 200 and 250 species of oaks Ž Quercus spp.., 58 of which are considered commercial in the United States and Canada. Artificial regeneration in North America has dominated much of commercial forestry because of )
Corresponding author. Tel.: q1-573-882-4775; fax: q1-573882-1977; e-mail [email protected].
its perceived reliability. Although planting oaks is an option, it is seldom used in the United States ŽJohnson, 1984; Johnson and Zimmer, 1985; Johnson et al., 1986.. Natural regeneration of most oak forests in North America depends on the accumulation of advance reproduction beneath the parent stand over successive acorn crops and the creation and maintenance of the conditions that favor such accumulation. New oak seedlings usually grow more slowly in height than do older seedling sprouts of the same
0378-1127r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 1 1 2 7 Ž 9 7 . 0 0 2 3 3 - 8
2
D.R. Larsen, P.S. Johnsonr Forest Ecology and Management 106 (1998) 1–7
species. Although oaks are relatively intolerant to shade and grow slowly under heavy shade, survival rates are relatively high under moderate shading ŽBeck, 1970; Loftis, 1988; Johnson, 1993.. Under those conditions, oaks often develop a large root system while minimizing investment in aboveground organs. Large root systems allow oaks to grow rapidly in height once favorable conditions occur ŽSander, 1971.. Large roots also facilitate survival after aboveground parts are killed or injured. Stand disturbance thus confers a competitive advantage to the oaks provided that the requisite advance reproduction has accumulated before the disturbance occurs. Understanding the factors associated with such competitive advantages therefore is essential to oak silviculture. Factors that encourage the development of large root systems are especially important in facilitating high survival and growth rates of oak reproduction ŽFig. 1.. The physiology, adaptive strategies, and regeneration niches of oaks vary considerably among oak species. In the oak-dominated forests of eastern North America, species differences are often most conspicuously defined by soil moisture relations, which strongly influence the dynamics of the regeneration process. Accordingly, it is convenient to classify oaks as xerophytes, mesophytes, or hydrophytes. Xerophytic oaks strongly depend on drought tolerance and their ability to die back and resprout to regenerate successfully. Competition from other species is often minimal due to dry conditions. Xerophytic oaks commonly grow in fire-prone regions. The large root systems that characterize accumulated
Fig. 1. Schematic diagram illustrating relationships that are encouraged by large root systems in oak seedlings. This diagram also illustrates that large root systems lead to high survival rates.
Fig. 2. Diagram illustrating the relative dependence on seeding versus the relative dependence on sprouting of several oak species. The size of the circle reflects the amount of flexibility in regeneration method, and the order reflects the relative ranking of these species. The diagram is adapted after Johnson, 1993.
populations of oak reproduction on such sites facilitate both fire and drought survival. Growth of the mesophytic and hydrophytic oaks is less dependent on root mass than that of the xerophytic oaks. The former are also less dependent on sprouting and more dependent on seeding as a regeneration strategy ŽFig. 2.. Although these species may have low survival probabilities beneath the dense overstories that characterize their habitats, overstory destruction facilitates their regeneration. The enormous numbers of oak seedlings that can become established on mesic and hydric sites, coupled with their rapid height growth, confers a probabilistic advantage to the oaks even when most oak seedlings are not competitively successful. In these environments, mesic and hydric oak species can regenerate from seed directly ŽJohnson et al., 1989; Loewenstein and Golden, 1995.. The physiographic and climatic conditions of a particular site strongly influence the species present and the nature of their interaction. With this in mind, ecological classification based on these factors can be used to determine which species and types of interactions to expect. Silviculturists who manage oak-dominated forests using natural regeneration need to know how target species will respond. Integral to this knowledge is
D.R. Larsen, P.S. Johnsonr Forest Ecology and Management 106 (1998) 1–7
the understanding that stands are not infinitely flexible. Stand structure can be changed with varying success depending on the stage of stand development. At times of rapid change in the stand Žrapid height growth, crown closure, understory reinitiation., it is much easier to effect change in the stand structure. Silviculturists can use these ‘windows of opportunity’ for silvicultural operations to facilitate natural oak regeneration ŽSander et al., 1984; Oliver and Larson, 1996.. Such ‘windows of opportunity’ are critical to defining effective silvicultural prescriptions. This paper will develop several points about oak ecology and their implications in silviculture in the context of oak-dominated forests of eastern North America.
2. Oak regeneration ecology Oaks regenerate by seeding and sprouting; however, the relative importance of these two modes of regeneration differs among species ŽFig. 2.. The periodicity of seed production, survival, and the height growth rates of the reproduction all influence successful oak regeneration. It is well documented that oaks are periodic and abundant seed producers ŽSchopmeyer, 1974; Burns and Honkala, 1990.. Periodic seed producers are dependent on either advantageous disturbance or accumulation of advance reproduction. To take advantage of a disturbance, a seedling must be able to grow in height as fast or faster than its competitors. However, in some cases, competitive growth rates of oaks may not be obvious in the reproduction ŽClatterbuck and Hodges, 1988.. First-year seedlings tend to grow quite slowly in height, unless they are hydrophytic species. First-year seedlings in most cases are poor competitors in terms of height growth. For advance reproduction to accumulate in the understory, the species must be able to survive in at least a moderately shaded understory. Most oaks range from moderately to very shade intolerant. All of these factors would seem to deter accumulation of advance reproduction. Nevertheless, oaks have adapted to these apparent impediments to regeneration partly because of their capacity to die back and resprout. In this process,
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seedlings establish and grow for a few years in the understory. The aboveground part of the plant will typically die back to the root collar every 3–10 years, depending on environmental conditions. The root collar has a large number of dormant buds, some of which activate upon loss of the old stem. This cycle, which can be repeated several times during the lifespan of a seedling, leads to the development of a large root system. The belowground portions of these seedling sprouts have been observed to live up to 50 years ŽMerz and Boyce, 1956; Tryon and Powell, 1984 and Unpublished oak root data on file at North Central Forest Experiment Station, Columbia, MO.. Large seedling root systems confer many advantages in environments common to oak forests. One advantage is enhanced drought tolerance because of the periodic reduction of transpirational surface area ŽParker, 1949.. Fires typically remove or kill the aboveground parts of plants but often leave the root systems intact. The dieback and resprouting process allows the accumulation of oak advance reproduction in the understory for long periods of time in spite of the intolerance of most oaks to shade. Factors that facilitate or discourage this process become powerful tools for silviculturists managing oak forests ŽRogers et al., 1993.. Sprouting of stumps is closely related to sprouting of seedlings. All oaks have the potential to resprout after removal of the stem ŽJohnson, 1975.. This capacity declines with increasing stem size ŽFig. 3. and differs greatly among species and in relation to other factors including tree age and site quality ŽJohnson, 1977.. In effective natural regeneration of xeric oakdominated forests, a mixture of seedling sprouts and stump sprouts is the main source of reproduction. In terms of the genetic inheritance of the forest, this is a very conservative approach to regeneration. Only a portion of the new forest is really new genetic stock, and this new stock has accumulated from many seed years over several decades. In xeric forests, group selection can be used because of the minimal non-oak competition and the ability of moderately tolerant oak species such as Q. alba L. Žwhite oak. to maintain a presence in the understory ŽLoewenstein, 1996..
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D.R. Larsen, P.S. Johnsonr Forest Ecology and Management 106 (1998) 1–7
Fig. 3. Diagram describing the range of probability of stump sprouting for different-sized parent trees. This diagram summarizes the information of Johnson, 1977. The shaded area indicates the range of probabilities predicted by Johnson’s equations. The range is large for smaller diameter trees because of both unexplained variation and variation from species and site quality. Note that no large diameter trees have high probabilities of stump sprouting.
As oak forests become more mesic, this regeneration process becomes less effective because of the increased competition of mesic non-oak species. In these forests, the disturbance and treatment history becomes the dominant factor determining whether oak will again dominate the successive stands. For example, on mesic sites, frequent light fires can create a situation where non-oak, shade tolerant, fire intolerant species will be at a disadvantage and accumulating oak seedling sprouts will be at an advantage ŽMaslen, 1989.. Oak recruitment, i.e., the growth of reproduction from the understory into the overstory, is a separate and relatively independent issue in oak silviculture. Oak advance reproduction under a closed canopy rarely grows into the overstory because of its relative intolerance to shade. In most instances, the density of the canopy must be reduced to open growing space to allow recruitment of oak seedlings into the overstory. The recruitment rate varies by the size, intensity, and nature of the canopy density reduction. Because oaks frequently sprout, a tree with less branching and a more vigorous height growth rate is often produced from a sprout originating near the
root collar. Because such trees are free to grow in the newly opened environment, they will often dominate the new stand. The response of oak regeneration to disturbance or treatment varies by how the disturbance affects the competing species and the overstory. The structure of the new forest is defined by the nature of the interaction of the disturbance and the previous stand. For example, frequent light fires may favor oak recruitment by reducing overstory density, stimulating shoot growth in sprouts, and killing fire intolerant species. If overstory density is sufficiently reduced, oak reproduction will be recruited into the overstory. Fires also produce negative effects, including increased overstory mortality, increased bole damage, and increased soil erosion. Each disturbance thus can be viewed as producing both advantages and disadvantages from a silvicultural perspective. Linking silviculture to natural oak ecology, therefore requires understanding how different species react to various disturbances and treatments, and how expected compositional–structural outcomes can be translated into silvicultural prescriptions that fit management objectives.
3. Oak silviculture The term silviculture usually implies maintaining a high degree of control over natural processes. In this paper, we discuss silvicultural treatments and natural disturbances in a similar light. In one sense, both treatments and disturbances remove portions or all of the plants in a stand. In both cases, the concern is the response of the residual or newly established plants. Philosophically and in practice, traditional silviculture is based on the assumption that silvicultural methods effectively control most ecological processes. This is a very intensive approach. In contrast, silviculture in the United States seems to be evolving toward the creation and maintenance of ecologically ‘natural’ forests ŽSedjo, 1996., a very extensive approach. This evolution is even being institutionalized by the establishment of a policy of ecosystem management for the USDA Forest Service. Hardwood silviculture in the United States has traditionally been rather natural—but for economic
D.R. Larsen, P.S. Johnsonr Forest Ecology and Management 106 (1998) 1–7
rather than socio–ecological reasons. In any event, the ‘ecological’ approach forces us to live with the stochastic nature of ecosystems, which is strongly expressed in the oak regeneration process. Almost all the traditional silvicultural methods of regeneration are being used to regenerate oakdominated forests in North America. Clearcutting is a common, successful and reliable method to regenerate oak forests. However, with the interest in ecosystem management, which includes objectives other than timber management Že.g., aesthetics, wildlife, biodiversity, recreational., many silviculturists have been experimenting with alternative methods of regenerating oak forests. A cutting treatment similar to shelterwood has successfully been used to establish oak reproduction, mainly through sprouting, but the overstory must be removed within a decade or the reproduction will not be recruited into the overstory ŽSchlesinger et al., 1993.. Regeneration of oak forests by various single-tree selection cutting techniques has been widely debated ŽSander, 1971; Sander et al., 1983; Sander and Graney, 1993.. Single-tree openings are not big enough to allow reproduction to grow at a rate that permits overstory recruitment. Single-tree selection that reduces the overstory stocking by over half is possible in the xerophytic oak forests of Missouri ŽLoewenstein, 1996.. But this method may not succeed elsewhere, especially where it encourages shade tolerant non-oak species. Oaks require a more complicated level of preharvest treatment than other eastern hardwood species. For example, the need to develop and maintain a component of oak reproduction in the understory often requires understanding the interactions among overstory density, site productivity, life histories of interacting tree species, and other factors. This knowledge then can be used to identify opportunities for influencing stand structural development, implementing treatments, and monitoring treatment effectiveness ŽOliver et al., 1991.. In the structural development of a stand, a given treatment may greatly change the course of stand development, while at other times the same treatment may have minimal or no effect. These events when treatments are most effective can be called ‘windows of opportunity.’ A window of opportunity to change the size structure commonly occurs just after crown
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closure in even-aged stands when stem density begins to reduce the growth rate of trees. Thinning at this time consequently may strongly determine future stand composition and structure. Holding stands beyond these windows assuming options are being maintained may actually eliminate many options. Delaying also may make it more difficult to realize an objective that might otherwise have been easily accomplished during a window of opportunity. In xeric forests, where regeneration of oaks depends on the accumulation of advanced reproduction, treatments that encourage establishment and survival of seedlings in the understory can influence the structure of the post-harvest stand. Because oaks can survive in the understory for a long time, many of these treatments can be done 10–20 years before a harvest treatment. The window of opportunity for establishment of advance reproduction is shorter and closer to the harvest time in mesophytic as opposed to xerophytic oak stands. The natural regeneration of oak stands is also highly stochastic. Consequently, the average trends many be very stable and repeatable while the outcome of any given treatment on a particular stand is highly variable. 4. Use of models One way to understand the process of natural oak regeneration is through modeling. Predictive regeneration models can play an important role in managing oak stands. Considering the stochastic nature of reproduction population dynamics and the low probability that any two stands will behave in exactly the same way, models are often useful for predicting potential outcomes. Three types of models have been used to model oak reproduction: probabilistic, deterministic, and stochastic. Two examples of probabilistic models of advance reproduction both predict the contribution of the advanced reproduction to future stocking ŽSander et al., 1984; Johnson and Sander, 1988.. In this approach, silviculturists establish a future stocking goal for the stand near the time of overstory harvest. Using logistic equations, they can predict the probability of an individual oak stem attaining a given size. Stem basal diameter can be used to infer root size, the single best predictor of a sprout’s ability to compete. The probability of individual success can
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D.R. Larsen, P.S. Johnsonr Forest Ecology and Management 106 (1998) 1–7
be translated into the contribution of the current seedling population to future stocking. Additionally, both Sander et al. Ž1984. and Johnson and Sander Ž1988. estimate the contribution of stump sprouting to the stocking of the future stand. These two estimates are used to predict the future stocking of the stand. A Comprehensive Ozark Regenerator ŽACORn. ŽDey, 1991. is a probabilistic model for predicting the size distribution of the sapling population 21 years after clearcut harvest, given a seedling inventory. This model is designed to use an inventory of advanced reproduction and overstory trees to be removed to predict a single most-likely diameter distribution 21 years after clearcutting based on empirical probabilities. Output from this model would be suitable as input to the growth and yield model used in the Ozark highlands. ACORn assumes that height growth of advanced reproduction and stump sprouts is a function of the size or maturity of the root system. Many of the ideas discussed in this paper are the basis for the equations in this model. Deterministic models are useful for predicting the average outcome when the variance in the observed outcomes is small. However, when the process being modeled has a large variance, the average outcome becomes much less valuable to know. For a process that produces highly variable outcomes, a stochastic model can be used to determine a typical response of the variable of interest. Differences between deterministic and stochastic models are explained by Renshaw Ž1991.. A stochastic model of particular interest is SIMSEED, a conceptual mathematical model to describe the oak regeneration process ŽRogers et al., 1993.. This model ŽSIMSEED. produces a probabilistic simulation of advance reproduction density of northern red oak Ž Q. rubra L... It uses a seedling establishment rate and a seedling survival rate to predict the size of the annual seedling population. The model output describes the annual variation in an oak seedling population as well as the potential impacts of silvicultural treatments on this population. An example of output from this model is given in Fig. 4. This figure contains three lines. The center bold line is a single realization produced by the model using the appropriate coefficients K and I. In
Fig. 4. Model output from stochastic model SIMSEED with parameters K s 0.7 and I s1000. The center bold line represents a typical outcome, a single realization of the model and given parameters. The lighter lines show the range of potential outcomes, the approximate 95% confidence interval for the model output.
the model, K is the survival rate from one year to the next, and I is the scale parameter setting the level of the average population. In the figure, there are two light lines indicating the approximate 95% confidence interval for the model and given parameters. The confidence intervals were determined empirically from a number of simulations. This figure illustrates the range of variation that can be expected given the assumptions of the model.
5. Conclusion Effective silviculture of naturally regenerated oak systems depends on the silviculturist’s ability to apply time treatments to produce effective control over stand composition and structure. This ability greatly depends on the knowledge and understanding of the stand dynamics of oak forests. These dynamics can vary considerably among and within different classes of oak-dominated ecosystems. This potential variation must accordingly be incorporated into any decision making process. Effective silviculture therefore depends on developing methods of assessing current stand condition and predicting how stands will change with time in relation to various treatments. Existing tools and predictive models can be used to develop an understanding of the dynamics of the process as well as to predict future conditions.
D.R. Larsen, P.S. Johnsonr Forest Ecology and Management 106 (1998) 1–7
Several interesting models are being produced to help silviculturists understand the accumulation and development of oak reproduction. While some of the models are more practical and others are more academic, they all help to relate field observations to expectations. As we develop our knowledge of natural regeneration of oaks and build diagnostic tools, we become more adept at linking oak ecology to silviculture.
References Beck, D.E., 1970. Effect of competition on survival and height growth of red oak seedlings. Res. Pap. SE-56, USDA Forest Service, 7 pp. Burns, R.M., Honkala, B.H., 1990. Silvics of North America, Hardwoods. Agr. Handb. 654, USDA Forest Service, Washington, DC, 877 pp. Clatterbuck, W.K., Hodges, J.D., 1988. Development of cherrybark oak and sweetgum in mixed, even-aged bottomland stands in central Mississippi USA. Can. J. For. Res. 18 Ž1., 12–18. Dey, D.C., 1991. A comprehensive Ozark regenerator. Phd dissertation, University of Missouri-Columbia, Columbia, MO, USA, 283 pp. Johnson, P.S., 1975. Growth and structural development of red oak sprout clumps. For. Sci. 21 Ž4., 413–418. Johnson, P.S., 1977. Predicting oak stump sprouting and sprout development in the Missouri Ozarks. Res. Pap. NC-149, USDA Forest Service, 11 pp. Johnson, P.S., 1984. Responses of planted northern red oak to three overstory treatments. Can. J. For. Res. 14, 536–542. Johnson, P.S., 1993. Sources of oak reproduction. In: Loftis, D.L., McGee, C.E., ŽEds.., Oak Regeneration: Serious Problems, Practical Recommendations. Gen. Tech. Rep. SE-84, USDA Forest Service, 112–131 pp. Johnson, P.S., Dale, C.D., Davison, K.R., 1986. Planting northern red oak in the Missouri Ozarks: A prescription. North J. Appl. For. 3, 66–68. Johnson, P.S., Jacobs, R.D., Martin, A.J., Godel, E.D., 1989. Regenerating northern red oak: Three successful case histories. North J. Appl. For. 6, 174–178. Johnson, P.S., Sander, I.L., 1988. Quantifying regeneration potentials of Quercus forests in the Missouri Ozarks. In: Ek, A.R., Shifley, S.R., Burk, T.E. ŽEds.., Forest Growth Modelling and Prediction. Gen. Tech. Rep. NC-120, USDA Forest Service, 377-385 pp. International Union of Forestry Research Organizations ŽIUFRO., Society of American Foresters ŽSAF. and University of Minnesota. Johnson, R.B., Zimmer, W.J., 1985. A more powerful test for dispersion using distance measurements. Ecology 66, 1669– 1675.
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Loewenstein, E.F., 1996. An analysis of the size and age structure of a managed uneven-aged forest. Phd dissertation, University of Missouri-Columbia, Columbia, MO, USA, 168 pp. Loewenstein, E.F., Golden, M.S., 1995. Establishment of water oak is not dependent on advanced reproduction. In: 8th Biennial Southern Silvicultural Research Conference. Gen. Tech. Rep. SRS-1, Aulburn University, AL. USDA Forest Service, Southern Research Station, 443–446 pp. Loftis, D.L., 1988. Regenerating oaks on high-quality sites, an update. In: Smith, H.C., Perkey, A.W., Williams, J.W.E., ŽEds.., Proceedings of the Guidelines for Regenerating Appalachian Hardwood Stands, 199–209 pp. Soc. Am. Foresters. Maslen, P., 1989. Response of immature oaks to prescribed fire in the North Carolina Piedmont. Gen. Tech. Rep. SO-74, USDA Forest Service, 259–266 pp. Merz, R.W., Boyce, S.G., 1956. Age of oak seedlings. J. For. 540 Ž11., 774–775. Oliver, C.D., Berg, D.R., Larsen, D.R., O’Hara, K.L., 1991. Integrating management tools, ecological knowledge, and silviculture. In: Naiman, R., ŽEd.., New Perspectives for Watershed Management: Balancing Long-Term Sustainability with Cumulative Environmental Change, 542 pp. Center for Streamside Studies, College of Forest Resources, AR-10, University of Washington, Springer-Verlag. Oliver, C.D., Larson, B.C., 1996. Forest Stand Dynamics, Update edn. Wiley, New York, 520 pp. Parker, J., 1949. Effects of variation in the root–leaf ratio on transpiration rate. Plant Physiol. 24, 739–743. Renshaw, E., 1991. Modelling Biological Populations in Space and Time. Cambridge Univ. Press, Cambridge, London, 403 pp. Rogers, R., Johnson, P.S., Loftis, D.L., 1993. An overview of oak silviculture in the United States: the past, present, and future. Ann. Sci. For. 50, 535–542. Sander, I.L., 1971. Height growth of new oak sprouts depend on size of advance reproduction. J. For. 69, 809–811. Sander, I.L., Graney, D.L., 1993. Regenerating oaks in the central states. In: Loftis, D.L., McGee, C.E., ŽEds.., Oak Regeneration: Serious Problems, Practical Recommendations. SE-84, USDA Forest Service, 174–183 pp. Sander, I.L., Johnson, P.S., Rogers, R., 1984. Evaluating oak advanced reproduction in the Missouri Ozarks. Res. Pap. NC-251, USDA Forest Service. Sander, I.L., McGee, C.E., Day, K.G., Willard, R.E., 1983. Oak-Hickory. In: Burns, R.M., ŽEd.., Silvicultural Systems for the Major Forest Types of the United States, pp. 116–120. Agr. handb., 191 pp., Washington, DC. Schlesinger, R.C., Sander, I.L., Davidson, K.R., 1993. Oak regeneration potential increased by shelterwood treatments. North J. Appl. For. 100 Ž4., 149–153. Schopmeyer, C.S., 1974. Seeds of woody plants in the United States. Agr. Handb. 450, USDA Forest Service, Washington, DC, 887 pp. Sedjo, R.A., 1996. Toward an operational approach to public forest management. J. For. 940 Ž8., 24–27. Tryon, E.H., Powell, D.S., 1984. Root ages of advanced hardwood reproduction. For. Ecol. Manage. 8, 293–298.
Linking the ecology of natural oak regeneration to silviculture David R. Larsen a
a,)
, Paul S. Johnson
b
The School of Natural Resources, UniÕersity of Missouri-Columbia, 1-30 Agriculture Building, Columbia, MO 65211, USA b USDA Forest SerÕice, North Central Forest Experiment Station, 1-26 Agriculture Building, Columbia, MO 65211, USA Received 21 May 1996
Abstract The regeneration requirements of oaks Ž Quercus spp.. differ among species. Oaks differ in their ability to produce seed, germinate and, as for reproduction, to endure shade, drought, and other stresses. Under the low to moderate shade that characterizes the understories of their natural habitats, the xerophytic oaks depend heavily on their drought tolerance and capacity to die back and resprout repeatedly. Successful regeneration of xerophytic oaks depends largely on the long-term survival and accumulation of oak reproduction, which may span 2 or more decades. Some of this reproduction develops large roots in advance of final harvest, which thereby enhances long-term survival and the potential for rapid shoot growth after overstory removal. In the more nutrient- and moisture-rich ecosystems, seedling populations of indigenous oaks arising from a single cohort typically fall to near extinction within a few years. High mortality rates are related to seedling intolerance to shade and the prevailing low light intensities under the high stand densities and stratified canopies typical of those ecosystems. In the absence of natural disturbance or silvicultural intervention, oak reproduction beneath dense stands in rich ecosystems typically fails to accumulate over time and thus fails to form the large root systems necessary for competitive success after overstory removal. The required timing and intensity of silvicultural operations for regenerating oaks therefore depend on the ecosystem-specific population dynamics of each species. Knowledge of birth, death, and other population processes of oak reproduction within defined classes of ecosystems, as well as knowledge of periodicity in seed production are prerequisites to the development of ecologically sound silvicultural prescriptions and realistic predictive models for regenerating oaks. q 1998 Elsevier Science B.V. Keywords: Oak regeneration; Oak silviculture; Regeneration models
1. Introduction In North America there are between 200 and 250 species of oaks Ž Quercus spp.., 58 of which are considered commercial in the United States and Canada. Artificial regeneration in North America has dominated much of commercial forestry because of )
Corresponding author. Tel.: q1-573-882-4775; fax: q1-573882-1977; e-mail [email protected].
its perceived reliability. Although planting oaks is an option, it is seldom used in the United States ŽJohnson, 1984; Johnson and Zimmer, 1985; Johnson et al., 1986.. Natural regeneration of most oak forests in North America depends on the accumulation of advance reproduction beneath the parent stand over successive acorn crops and the creation and maintenance of the conditions that favor such accumulation. New oak seedlings usually grow more slowly in height than do older seedling sprouts of the same
0378-1127r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 1 1 2 7 Ž 9 7 . 0 0 2 3 3 - 8
2
D.R. Larsen, P.S. Johnsonr Forest Ecology and Management 106 (1998) 1–7
species. Although oaks are relatively intolerant to shade and grow slowly under heavy shade, survival rates are relatively high under moderate shading ŽBeck, 1970; Loftis, 1988; Johnson, 1993.. Under those conditions, oaks often develop a large root system while minimizing investment in aboveground organs. Large root systems allow oaks to grow rapidly in height once favorable conditions occur ŽSander, 1971.. Large roots also facilitate survival after aboveground parts are killed or injured. Stand disturbance thus confers a competitive advantage to the oaks provided that the requisite advance reproduction has accumulated before the disturbance occurs. Understanding the factors associated with such competitive advantages therefore is essential to oak silviculture. Factors that encourage the development of large root systems are especially important in facilitating high survival and growth rates of oak reproduction ŽFig. 1.. The physiology, adaptive strategies, and regeneration niches of oaks vary considerably among oak species. In the oak-dominated forests of eastern North America, species differences are often most conspicuously defined by soil moisture relations, which strongly influence the dynamics of the regeneration process. Accordingly, it is convenient to classify oaks as xerophytes, mesophytes, or hydrophytes. Xerophytic oaks strongly depend on drought tolerance and their ability to die back and resprout to regenerate successfully. Competition from other species is often minimal due to dry conditions. Xerophytic oaks commonly grow in fire-prone regions. The large root systems that characterize accumulated
Fig. 1. Schematic diagram illustrating relationships that are encouraged by large root systems in oak seedlings. This diagram also illustrates that large root systems lead to high survival rates.
Fig. 2. Diagram illustrating the relative dependence on seeding versus the relative dependence on sprouting of several oak species. The size of the circle reflects the amount of flexibility in regeneration method, and the order reflects the relative ranking of these species. The diagram is adapted after Johnson, 1993.
populations of oak reproduction on such sites facilitate both fire and drought survival. Growth of the mesophytic and hydrophytic oaks is less dependent on root mass than that of the xerophytic oaks. The former are also less dependent on sprouting and more dependent on seeding as a regeneration strategy ŽFig. 2.. Although these species may have low survival probabilities beneath the dense overstories that characterize their habitats, overstory destruction facilitates their regeneration. The enormous numbers of oak seedlings that can become established on mesic and hydric sites, coupled with their rapid height growth, confers a probabilistic advantage to the oaks even when most oak seedlings are not competitively successful. In these environments, mesic and hydric oak species can regenerate from seed directly ŽJohnson et al., 1989; Loewenstein and Golden, 1995.. The physiographic and climatic conditions of a particular site strongly influence the species present and the nature of their interaction. With this in mind, ecological classification based on these factors can be used to determine which species and types of interactions to expect. Silviculturists who manage oak-dominated forests using natural regeneration need to know how target species will respond. Integral to this knowledge is
D.R. Larsen, P.S. Johnsonr Forest Ecology and Management 106 (1998) 1–7
the understanding that stands are not infinitely flexible. Stand structure can be changed with varying success depending on the stage of stand development. At times of rapid change in the stand Žrapid height growth, crown closure, understory reinitiation., it is much easier to effect change in the stand structure. Silviculturists can use these ‘windows of opportunity’ for silvicultural operations to facilitate natural oak regeneration ŽSander et al., 1984; Oliver and Larson, 1996.. Such ‘windows of opportunity’ are critical to defining effective silvicultural prescriptions. This paper will develop several points about oak ecology and their implications in silviculture in the context of oak-dominated forests of eastern North America.
2. Oak regeneration ecology Oaks regenerate by seeding and sprouting; however, the relative importance of these two modes of regeneration differs among species ŽFig. 2.. The periodicity of seed production, survival, and the height growth rates of the reproduction all influence successful oak regeneration. It is well documented that oaks are periodic and abundant seed producers ŽSchopmeyer, 1974; Burns and Honkala, 1990.. Periodic seed producers are dependent on either advantageous disturbance or accumulation of advance reproduction. To take advantage of a disturbance, a seedling must be able to grow in height as fast or faster than its competitors. However, in some cases, competitive growth rates of oaks may not be obvious in the reproduction ŽClatterbuck and Hodges, 1988.. First-year seedlings tend to grow quite slowly in height, unless they are hydrophytic species. First-year seedlings in most cases are poor competitors in terms of height growth. For advance reproduction to accumulate in the understory, the species must be able to survive in at least a moderately shaded understory. Most oaks range from moderately to very shade intolerant. All of these factors would seem to deter accumulation of advance reproduction. Nevertheless, oaks have adapted to these apparent impediments to regeneration partly because of their capacity to die back and resprout. In this process,
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seedlings establish and grow for a few years in the understory. The aboveground part of the plant will typically die back to the root collar every 3–10 years, depending on environmental conditions. The root collar has a large number of dormant buds, some of which activate upon loss of the old stem. This cycle, which can be repeated several times during the lifespan of a seedling, leads to the development of a large root system. The belowground portions of these seedling sprouts have been observed to live up to 50 years ŽMerz and Boyce, 1956; Tryon and Powell, 1984 and Unpublished oak root data on file at North Central Forest Experiment Station, Columbia, MO.. Large seedling root systems confer many advantages in environments common to oak forests. One advantage is enhanced drought tolerance because of the periodic reduction of transpirational surface area ŽParker, 1949.. Fires typically remove or kill the aboveground parts of plants but often leave the root systems intact. The dieback and resprouting process allows the accumulation of oak advance reproduction in the understory for long periods of time in spite of the intolerance of most oaks to shade. Factors that facilitate or discourage this process become powerful tools for silviculturists managing oak forests ŽRogers et al., 1993.. Sprouting of stumps is closely related to sprouting of seedlings. All oaks have the potential to resprout after removal of the stem ŽJohnson, 1975.. This capacity declines with increasing stem size ŽFig. 3. and differs greatly among species and in relation to other factors including tree age and site quality ŽJohnson, 1977.. In effective natural regeneration of xeric oakdominated forests, a mixture of seedling sprouts and stump sprouts is the main source of reproduction. In terms of the genetic inheritance of the forest, this is a very conservative approach to regeneration. Only a portion of the new forest is really new genetic stock, and this new stock has accumulated from many seed years over several decades. In xeric forests, group selection can be used because of the minimal non-oak competition and the ability of moderately tolerant oak species such as Q. alba L. Žwhite oak. to maintain a presence in the understory ŽLoewenstein, 1996..
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Fig. 3. Diagram describing the range of probability of stump sprouting for different-sized parent trees. This diagram summarizes the information of Johnson, 1977. The shaded area indicates the range of probabilities predicted by Johnson’s equations. The range is large for smaller diameter trees because of both unexplained variation and variation from species and site quality. Note that no large diameter trees have high probabilities of stump sprouting.
As oak forests become more mesic, this regeneration process becomes less effective because of the increased competition of mesic non-oak species. In these forests, the disturbance and treatment history becomes the dominant factor determining whether oak will again dominate the successive stands. For example, on mesic sites, frequent light fires can create a situation where non-oak, shade tolerant, fire intolerant species will be at a disadvantage and accumulating oak seedling sprouts will be at an advantage ŽMaslen, 1989.. Oak recruitment, i.e., the growth of reproduction from the understory into the overstory, is a separate and relatively independent issue in oak silviculture. Oak advance reproduction under a closed canopy rarely grows into the overstory because of its relative intolerance to shade. In most instances, the density of the canopy must be reduced to open growing space to allow recruitment of oak seedlings into the overstory. The recruitment rate varies by the size, intensity, and nature of the canopy density reduction. Because oaks frequently sprout, a tree with less branching and a more vigorous height growth rate is often produced from a sprout originating near the
root collar. Because such trees are free to grow in the newly opened environment, they will often dominate the new stand. The response of oak regeneration to disturbance or treatment varies by how the disturbance affects the competing species and the overstory. The structure of the new forest is defined by the nature of the interaction of the disturbance and the previous stand. For example, frequent light fires may favor oak recruitment by reducing overstory density, stimulating shoot growth in sprouts, and killing fire intolerant species. If overstory density is sufficiently reduced, oak reproduction will be recruited into the overstory. Fires also produce negative effects, including increased overstory mortality, increased bole damage, and increased soil erosion. Each disturbance thus can be viewed as producing both advantages and disadvantages from a silvicultural perspective. Linking silviculture to natural oak ecology, therefore requires understanding how different species react to various disturbances and treatments, and how expected compositional–structural outcomes can be translated into silvicultural prescriptions that fit management objectives.
3. Oak silviculture The term silviculture usually implies maintaining a high degree of control over natural processes. In this paper, we discuss silvicultural treatments and natural disturbances in a similar light. In one sense, both treatments and disturbances remove portions or all of the plants in a stand. In both cases, the concern is the response of the residual or newly established plants. Philosophically and in practice, traditional silviculture is based on the assumption that silvicultural methods effectively control most ecological processes. This is a very intensive approach. In contrast, silviculture in the United States seems to be evolving toward the creation and maintenance of ecologically ‘natural’ forests ŽSedjo, 1996., a very extensive approach. This evolution is even being institutionalized by the establishment of a policy of ecosystem management for the USDA Forest Service. Hardwood silviculture in the United States has traditionally been rather natural—but for economic
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rather than socio–ecological reasons. In any event, the ‘ecological’ approach forces us to live with the stochastic nature of ecosystems, which is strongly expressed in the oak regeneration process. Almost all the traditional silvicultural methods of regeneration are being used to regenerate oakdominated forests in North America. Clearcutting is a common, successful and reliable method to regenerate oak forests. However, with the interest in ecosystem management, which includes objectives other than timber management Že.g., aesthetics, wildlife, biodiversity, recreational., many silviculturists have been experimenting with alternative methods of regenerating oak forests. A cutting treatment similar to shelterwood has successfully been used to establish oak reproduction, mainly through sprouting, but the overstory must be removed within a decade or the reproduction will not be recruited into the overstory ŽSchlesinger et al., 1993.. Regeneration of oak forests by various single-tree selection cutting techniques has been widely debated ŽSander, 1971; Sander et al., 1983; Sander and Graney, 1993.. Single-tree openings are not big enough to allow reproduction to grow at a rate that permits overstory recruitment. Single-tree selection that reduces the overstory stocking by over half is possible in the xerophytic oak forests of Missouri ŽLoewenstein, 1996.. But this method may not succeed elsewhere, especially where it encourages shade tolerant non-oak species. Oaks require a more complicated level of preharvest treatment than other eastern hardwood species. For example, the need to develop and maintain a component of oak reproduction in the understory often requires understanding the interactions among overstory density, site productivity, life histories of interacting tree species, and other factors. This knowledge then can be used to identify opportunities for influencing stand structural development, implementing treatments, and monitoring treatment effectiveness ŽOliver et al., 1991.. In the structural development of a stand, a given treatment may greatly change the course of stand development, while at other times the same treatment may have minimal or no effect. These events when treatments are most effective can be called ‘windows of opportunity.’ A window of opportunity to change the size structure commonly occurs just after crown
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closure in even-aged stands when stem density begins to reduce the growth rate of trees. Thinning at this time consequently may strongly determine future stand composition and structure. Holding stands beyond these windows assuming options are being maintained may actually eliminate many options. Delaying also may make it more difficult to realize an objective that might otherwise have been easily accomplished during a window of opportunity. In xeric forests, where regeneration of oaks depends on the accumulation of advanced reproduction, treatments that encourage establishment and survival of seedlings in the understory can influence the structure of the post-harvest stand. Because oaks can survive in the understory for a long time, many of these treatments can be done 10–20 years before a harvest treatment. The window of opportunity for establishment of advance reproduction is shorter and closer to the harvest time in mesophytic as opposed to xerophytic oak stands. The natural regeneration of oak stands is also highly stochastic. Consequently, the average trends many be very stable and repeatable while the outcome of any given treatment on a particular stand is highly variable. 4. Use of models One way to understand the process of natural oak regeneration is through modeling. Predictive regeneration models can play an important role in managing oak stands. Considering the stochastic nature of reproduction population dynamics and the low probability that any two stands will behave in exactly the same way, models are often useful for predicting potential outcomes. Three types of models have been used to model oak reproduction: probabilistic, deterministic, and stochastic. Two examples of probabilistic models of advance reproduction both predict the contribution of the advanced reproduction to future stocking ŽSander et al., 1984; Johnson and Sander, 1988.. In this approach, silviculturists establish a future stocking goal for the stand near the time of overstory harvest. Using logistic equations, they can predict the probability of an individual oak stem attaining a given size. Stem basal diameter can be used to infer root size, the single best predictor of a sprout’s ability to compete. The probability of individual success can
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be translated into the contribution of the current seedling population to future stocking. Additionally, both Sander et al. Ž1984. and Johnson and Sander Ž1988. estimate the contribution of stump sprouting to the stocking of the future stand. These two estimates are used to predict the future stocking of the stand. A Comprehensive Ozark Regenerator ŽACORn. ŽDey, 1991. is a probabilistic model for predicting the size distribution of the sapling population 21 years after clearcut harvest, given a seedling inventory. This model is designed to use an inventory of advanced reproduction and overstory trees to be removed to predict a single most-likely diameter distribution 21 years after clearcutting based on empirical probabilities. Output from this model would be suitable as input to the growth and yield model used in the Ozark highlands. ACORn assumes that height growth of advanced reproduction and stump sprouts is a function of the size or maturity of the root system. Many of the ideas discussed in this paper are the basis for the equations in this model. Deterministic models are useful for predicting the average outcome when the variance in the observed outcomes is small. However, when the process being modeled has a large variance, the average outcome becomes much less valuable to know. For a process that produces highly variable outcomes, a stochastic model can be used to determine a typical response of the variable of interest. Differences between deterministic and stochastic models are explained by Renshaw Ž1991.. A stochastic model of particular interest is SIMSEED, a conceptual mathematical model to describe the oak regeneration process ŽRogers et al., 1993.. This model ŽSIMSEED. produces a probabilistic simulation of advance reproduction density of northern red oak Ž Q. rubra L... It uses a seedling establishment rate and a seedling survival rate to predict the size of the annual seedling population. The model output describes the annual variation in an oak seedling population as well as the potential impacts of silvicultural treatments on this population. An example of output from this model is given in Fig. 4. This figure contains three lines. The center bold line is a single realization produced by the model using the appropriate coefficients K and I. In
Fig. 4. Model output from stochastic model SIMSEED with parameters K s 0.7 and I s1000. The center bold line represents a typical outcome, a single realization of the model and given parameters. The lighter lines show the range of potential outcomes, the approximate 95% confidence interval for the model output.
the model, K is the survival rate from one year to the next, and I is the scale parameter setting the level of the average population. In the figure, there are two light lines indicating the approximate 95% confidence interval for the model and given parameters. The confidence intervals were determined empirically from a number of simulations. This figure illustrates the range of variation that can be expected given the assumptions of the model.
5. Conclusion Effective silviculture of naturally regenerated oak systems depends on the silviculturist’s ability to apply time treatments to produce effective control over stand composition and structure. This ability greatly depends on the knowledge and understanding of the stand dynamics of oak forests. These dynamics can vary considerably among and within different classes of oak-dominated ecosystems. This potential variation must accordingly be incorporated into any decision making process. Effective silviculture therefore depends on developing methods of assessing current stand condition and predicting how stands will change with time in relation to various treatments. Existing tools and predictive models can be used to develop an understanding of the dynamics of the process as well as to predict future conditions.
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Several interesting models are being produced to help silviculturists understand the accumulation and development of oak reproduction. While some of the models are more practical and others are more academic, they all help to relate field observations to expectations. As we develop our knowledge of natural regeneration of oaks and build diagnostic tools, we become more adept at linking oak ecology to silviculture.
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Loewenstein, E.F., 1996. An analysis of the size and age structure of a managed uneven-aged forest. Phd dissertation, University of Missouri-Columbia, Columbia, MO, USA, 168 pp. Loewenstein, E.F., Golden, M.S., 1995. Establishment of water oak is not dependent on advanced reproduction. In: 8th Biennial Southern Silvicultural Research Conference. Gen. Tech. Rep. SRS-1, Aulburn University, AL. USDA Forest Service, Southern Research Station, 443–446 pp. Loftis, D.L., 1988. Regenerating oaks on high-quality sites, an update. In: Smith, H.C., Perkey, A.W., Williams, J.W.E., ŽEds.., Proceedings of the Guidelines for Regenerating Appalachian Hardwood Stands, 199–209 pp. Soc. Am. Foresters. Maslen, P., 1989. Response of immature oaks to prescribed fire in the North Carolina Piedmont. Gen. Tech. Rep. SO-74, USDA Forest Service, 259–266 pp. Merz, R.W., Boyce, S.G., 1956. Age of oak seedlings. J. For. 540 Ž11., 774–775. Oliver, C.D., Berg, D.R., Larsen, D.R., O’Hara, K.L., 1991. Integrating management tools, ecological knowledge, and silviculture. In: Naiman, R., ŽEd.., New Perspectives for Watershed Management: Balancing Long-Term Sustainability with Cumulative Environmental Change, 542 pp. Center for Streamside Studies, College of Forest Resources, AR-10, University of Washington, Springer-Verlag. Oliver, C.D., Larson, B.C., 1996. Forest Stand Dynamics, Update edn. Wiley, New York, 520 pp. Parker, J., 1949. Effects of variation in the root–leaf ratio on transpiration rate. Plant Physiol. 24, 739–743. Renshaw, E., 1991. Modelling Biological Populations in Space and Time. Cambridge Univ. Press, Cambridge, London, 403 pp. Rogers, R., Johnson, P.S., Loftis, D.L., 1993. An overview of oak silviculture in the United States: the past, present, and future. Ann. Sci. For. 50, 535–542. Sander, I.L., 1971. Height growth of new oak sprouts depend on size of advance reproduction. J. For. 69, 809–811. Sander, I.L., Graney, D.L., 1993. Regenerating oaks in the central states. In: Loftis, D.L., McGee, C.E., ŽEds.., Oak Regeneration: Serious Problems, Practical Recommendations. SE-84, USDA Forest Service, 174–183 pp. Sander, I.L., Johnson, P.S., Rogers, R., 1984. Evaluating oak advanced reproduction in the Missouri Ozarks. Res. Pap. NC-251, USDA Forest Service. Sander, I.L., McGee, C.E., Day, K.G., Willard, R.E., 1983. Oak-Hickory. In: Burns, R.M., ŽEd.., Silvicultural Systems for the Major Forest Types of the United States, pp. 116–120. Agr. handb., 191 pp., Washington, DC. Schlesinger, R.C., Sander, I.L., Davidson, K.R., 1993. Oak regeneration potential increased by shelterwood treatments. North J. Appl. For. 100 Ž4., 149–153. Schopmeyer, C.S., 1974. Seeds of woody plants in the United States. Agr. Handb. 450, USDA Forest Service, Washington, DC, 887 pp. Sedjo, R.A., 1996. Toward an operational approach to public forest management. J. For. 940 Ž8., 24–27. Tryon, E.H., Powell, D.S., 1984. Root ages of advanced hardwood reproduction. For. Ecol. Manage. 8, 293–298.