Decision Making for Natural Regeneration (Marquis and Twery)

Decision Criteria

When using the decision charts, one must answer each decision question either "Yes" or "No." Although the questions and answers appear subjective, it is the intent of the system that these determinations be made as objectively as possible, on the basis of specific parameters that are measured in the stand under question. This BROKEN-LINK BROKEN-LINK table illustrates these parameters, and the levels that qualify for either a "Yes" or "No" answer (from Marquis 1991b). The decision questions are listed in the approximate sequence they are encountered in the decision charts. The example table is from the Allegheny region. Similar tables are being developed for other geographic regions, and for situations where success requires the regeneration of specific target species (such as oaks).

The question "Is sunlight limiting to seedling establishment?" illustrates how specific stand parameters are used to arrive at an objective answer. The stand parameter used to evaluate the density of the overstory and thus the amount of sunlight reaching the forest floor is relative stand density or stocking percent. This measure works well in stands of any species composition, is easily calculated from standard cruise data, and requires no subjective judgment. In stands with more than 75 percent relative density, the overstory is dense enough that sunlight may limit seedling establishment. At lower densities, seedlings can usually become established, although shade-intolerant species may not grow much there. So, 75 percent relative density is the breaking point; at densities above 75, sunlight is considered limiting and shelterwood cutting becomes a possible treatment. At densities below 75, light is probably adequate for establishment and any lack of advance seedlings is probably due to other limiting factors.

Some of the decision questions require much more complex criteria to evaluate adequately. The amount of advance regeneration that is adequate to ensure successful reproduction following harvest is a good example. In the Allegheny region, adequate advance regeneration requires that at least minimum numbers of seedlings be present over at least 70 percent of the stand area. The minimum numbers of seedlings depends on the species, their size, and the amount of deer browsing expected. The table, below (from Marquis and others 1992), shows the weighted minimum numbers of seedlings required on each plot, as a function of species and deer impact index. Weighted numbers are derived by counting (or estimating) seedling numbers in each of three size classes, and applying weights to each size class to arrive at a weighted total number. Larger seedlings have a better chance of survival, and thus carry a higher weight than small seedlings.(Table:Number of Seedlings required for regenration plots to be stocked)

The deer impact index is a value determined from a chart showing the estimated deer population (in deer per square mile) for various habitat conditions. Deer population estimates may be obtained from the state game agency, or from a sample census conducted for this purpose. Habitat condition is determined from the proportions of the 1-square-mile area surrounding the stand in question (the deer home range roughly) in such conditions as: agricultural fields, open land, recent harvest cuts, thinned and unthinned forest, etc. All of these vegetative conditions affect the amount of deer food available in the vicinity to reduce browsing pressure on regeneration in any new harvest cuts.

Thus, the evaluation of advance regeneration adequacy is based on the percentage of understory that meets certain required minimum conditions. These conditions vary tremendously depending on such factors as tree species, seedling size, and deer pressure. To illustrate: in the Allegheny region, the minimum number of maple or ash seedlings per 6-ft. radius plot is 100, if deer pressure is high (level 4) and all seedlings are 2 in. to 1 ft. tall; but, the minimum number is only 7 or 8 if deer pressure is low and all seedlings exceed 1 ft. in height. Since the decision question in the chart considers both advance regeneration and sprouts, the sprouting potential of all trees to be cut must also be evaluated. Then, estimates of the proportion of the area that will be regenerated by sprouts must be added to the advance regeneration estimates.

Thus, a simple "Yes" or "No" answer in this case involves a whole host of factors that must be carefully evaluated to make effective use of the decision charts. Other decision criteria, such as the density of interfering plants and seed supply adequacy, are evaluated in a similar manner. Several factors must be included in determining whether or not these factors are limiting to or are adequate for regeneration. Details on these and other factors for the Allegheny region are presented in the SILVAH literature (Marquis and others 1992), and are not repeated here.

Developing suitable decision criteria for a specific forest type and geographic region is a major task. In the Allegheny region, it took nearly 15 years of research aimed specifically at that goal to complete the SILVAH system. Information needed to develop decision criteria for other regions and forest types that have received less focused research effort will not always be available. Nevertheless, many of the general principles and procedures from the Allegheny region can be used elsewhere, and experienced silviculturists can probably make informed estimates of the critical parameters in their areas. Such estimates provide a starting point that is certainly better than no system at all. At the very least, it forces the decision maker to systematically consider all of the many factors that affect regeneration success, and to make a judgment on each based upon the best information currently available.

Furthermore, the system itself serves as a model or framework on which to evaluate research needs. Those factors or criteria on which knowledge is inadequate become quickly apparent as one attempts to define the decision questions and criteria. The relative importance of each research need also can be assessed in terms of its relative importance and frequency of occurrence in the overall oak regeneration process.

The decision charts presented here are intended for general use over a wide range of forest types and geographic areas. As a result, some of the decision points and decision criteria will be of little importance in some types or regions. Rather than creating a whole series of specialized charts that eliminate these factors entirely for the charts for that region, it is best to simply set the criteria at a level that will effectively ignore the factor. For example, deer browsing may not be a factor at all where deer populations are low across a wide geographic area. In these cases, the criteria table can simply suggest that deer impact index be set to 1 (very low) or 2 (low) anywhere in that region.

Additional decision criteria are being formulated to extend this system of decision making to a wide variety of resource values. For example, visual qualities within a forest stand are highly dependent on numbers and sizes of woody stems close to the ground, as is habitat quality for many wildlife species. Criteria that ensure the desired stems densities for these objectives fit very comfortably in this system.