The Southern Pine Beetle
Chapter 7: Impacts of the Southern Pine Beetle
William A. Leuschner — Associate Professor of Forest Economics, School of Forestry and Wildlife Resources, Virginia Polytechnic Institute and State University, Blacksburg, Va.
In the most general terms, the impact of the southern pine beetle is that it kills trees. But this phenomenon may be just the first in a series of events. SPB-related tree deaths cause openings in the forest canopy, and these openings affect the amount of sunlight reaching the understory below. Changes in sunlight alter both the overstory and understory species that grow back after a beetle infestation. Canopy reduction also changes water yields. This chain of cause-and-effect relationships can go on and on, until an economic impact is reached. The purpose of this chapter is to examine economic impacts of the SPB, as they relate to forest products (timber, recreation, wildlife, etc.). Statistical procedures exist for estimating some economic impacts in quantifiable units. By using these procedures, foresters can make pest management decisions on a carefully thought out, rather than intuitive, basis.
"Impact" is a word with a variety of meanings (see also Stark 1979 and Johnson 1973). For our purposes, an impact is simply any change brought about in the forest by an insect population. It may be positive or negative, affecting either flora or fauna.
A physical impact is any impact measurable in physical units, such as a change in numbers of woodpeckers or cords of pulpwood. A physical impact may or may not be of value. An economic impact is any change in (1) a socially useful forest product, (2) socially useful items needed to produce a fixed level of forest products or, (3) the distribution of forest products, the income derived from them, or their cost of production. Thus economic impact has three elements: production level, inputs for production, and the distribution of production and costs. This chapter will focus mainly on the first element — the beetle’s impact on forest products.
If a result of beetle activity can be measured in physical units and affects at least one of the three economic impact elements, then the activity is said to result in a physical economic impact. In addition to measuring the result of beetle activity in physical units, its value to society must be determined. Social value is often estimated by the market price of the product that is affected. The product of a physical economic impact and social value is the impact value. To illustrate, consider this hypothetical example of the impact value of SPB in a campground area:
|Physical economic impact||X||Social value per unit||=||Impact value|
|2,000 fewer visitor days at campground||X||$5.00 per visitor per day||=||$10,000|
But determining the impact value is not as straightforward as the campground example suggests. Difficulties may arise. It may be hard to quantify the physical impact in readily understood units. For instance, with the campground example, one impact of SPB may be the loss of enjoyment experienced by campers using an SPB-attacked campsite. Scientists in human behavior may be able to quantify changes in enjoyment by using various indices, but foresters and pest managers would have a hard time using these. And it may be difficult, or impossible, to place a monetary value on some impacts. For example, even though we could measure the increased number of birds found in outbreak areas, we could not estimate their dollar value.
Sometimes, then, economic impact can be measured in dollars or physical units. In other cases, we may be able to state only the direction of a change, not its amount. Economic impact is finally determined not by our ability to measure but by the usefulness of the changed element to society.
We can also distinguish between primary and secondary impacts. A primary impact is caused by the direct action of the insect (killing a tree). A secondary impact flows from the primary impact (e.g., changes in water yields caused by the canopy reduction from the dead tree). The chain of secondary impacts can continue almost indefinitely ("for want of a nail…the war was lost"), until finally an economic impact occurs.
Forest managers are interested in southern pine beetle impacts precisely because they are economic — they affect products, inputs, or distributions desired by society. The economic impact must be assessed so society (governmental agencies or the private sector) can decide whether control efforts are worthwhile.
To assess beetle impact for a forest or an outbreak, we must aggregate the impacts of individual spots. The impact on one or several spots may be economic but not large enough to make control actions cost-effective. Aggregate impact must be assessed to make that decision. The aggregate attack configuration also affects impact and must be determined. The impact of a single 50-acre spot will be different from that of 100 half-acre spots. These differences are reflected in the secondary impacts on several diverse forest products.
When southern pine beetles kill trees, harvested volume may be sharply reduced. This reduced volume is due to either unsalvaged merchantable trees, harvesting trees ahead of schedule, or both. The value of salvaged timber may also be reduced by decay, stain or insect holes, or increased logging and handling costs. There may also be an impact on the stand replacing the one killed by SPB. The tree species in the subsequent stand may be more or less commercially desirable, so the stumpage price may change. Stocking can be either increased or decreased, resulting in a yield change. And regeneration may be delayed, resulting in increased management costs when the time value of money is considered. In the subsequent stand, other impacts unique to the individual attack and stand conditions may occur.
Physical Timber Impacts
Timber impacts determination is simple to outline but complex to apply. The physical impact on the original stand is the difference between the volume of timber that would have been harvested had the stand not been attacked and the volume of timber that was salvaged after the attack. Differences in product quantities (e.g., decreased sawtimber volume) should be taken into account. The volume and species in the post-SPB stand, plus those stands that would continue into perpetuity, should also be estimated. These are subtracted from the stands that would have replaced the original stands in perpetuity if it were unattacked and grown to full rotation. Thus, the difference in timber volumes harvested, by species and in perpetuity, is estimated for the stand with and without SPB attack. This difference is the physical timber impact.
Estimates of further timber volumes can be obtained with growth simulators. PTAEDA is one such model. It stimulates stand growth for loblolly pine plantations using individual trees as the basic growth units (Daniels and Burkhart 1975). It includes a stochastic element to provide for probability in the prediction function. Randomly chosen probabilistic factors are used to generate mortality and to represent microsite and/or genetic variability when projecting growth. Work is currently underway to include SPB-specific mortality. The response of stands to site preparation, thinning, and fertilization may be simulated and outputs are basal area per acre, number of trees per acre by diameter class, total stem cubic foot volume, total above-ground biomass, and frequency of tree mortality by diameter class. PTAEDA was developed for simulating tree growth and stand development in managed loblolly pine plantations, but a second version — seed PTAEDA — is under development for naturally regenerated loblolly pine stands (Daniels et al. 1979).
Present Net Worth Model
Next, the management costs and stumpage prices for the timber volumes are estimated and the present net worths (PNW) are calculated for the stand in perpetuity with and without the SPB attack. PNW calculations are not discussed here but may be found in forest or financial management texts. The difference in PNW with and without attack is the value of the economic timber impact.
This present net worth model, without the perpetual rotation or subsequent stand impacts, is
Impact = PNWwa – PNWwoa
= DD1PQ + PDQ + DPQ + DPDQ – D2DC - DD2(C + DC) (1)
|PNWwa||=||present net worth per acre with attack|
|PNWwoa||=||present net worth per acre without attack|
|P||=||stumpage price per unit of volume harvested|
|Q||=||volume harvested per acre at rotation|
|D||=||algebraic change in the variable caused by attack|
|D1||=||discount factor for the present value of a single payment|
|D2||=||discount factor for the present value of a terminating series of payments|
|C||=||management cost per acre per year|
This model was demonstrated by Leuschner et al. (1978) using data from the Trinity District of the Davy Crockett National Forest in east Texas (Leuschner et al. 1976). The demonstration used the Timber Benefits Analysis Program (TBAP), which included the subsequent stand and perpetual rotations. The estimated value of the economic timber impact determined after the damages were done was $5,764 for 44.31 acres infested from July 1, 1974 to June 30, 1975.
These results were compared to the traditional timber impact model, which is the sum of (1) the volume salvaged multiplied by the difference between the salvaged and unattacked stumpage price, plus (2) the unsalvaged volume multiplied by the unattacked stumpage price. The traditional model estimate was $17,877 — over $12,000 more than the PNW model. The traditional model usually (1) underestimates damages by the value of timber lost due to premature harvest, (2) overestimates damages by the change in the present value of total revenue due to earlier harvest, and (3) overestimates damages by the change in the present value of the management costs no longer incurred. The volume lost due to premature harvest — the element traditionally underestimated — is generally small because SPB prefers to attack more mature stands. Hence the traditional model tends to overestimate SPB damages.
Both the PNW and traditional models assume that the observed stumpage prices for unattacked and salvaged timber reflect their social value. The salvaged stumpage price is usually less than the unattacked pine because, it is assumed, the salvaged timber cannot be converted into the same products as the unattacked timber due to rot and decay. Also, the logging and milling conversion costs may be higher. Studies were performed to verify these assumptions. The economic timber impact could be decreased if conventional beliefs about rate and amount of deterioration are wrong and if the potential purchasers are informed of the facts. Impact could be decreased by increased salvage stumpage prices, increased amounts of attacked timber being salvaged, or both.
Deterioration of beetle-killed sawtimber and pulpwood trees was studied in Virginia, North Carolina, and east Texas. Trees were harvested from SPB spots at different times after being killed by beetles and then sawn to determine the grade and yield of lumber. Lumber strength and pulping characteristics were determined with laboratory tests and were compared to results from green, unattacked trees from the same area.
Figure 7-1 – No. 1 Structural Grade recovery from control and SPB-killed trees in Virginia.
Both lumber grade and yield were lower from the beetle-killed logs. For example, in Virginia (Sinclair and Ifju 1979) the green logs yielded 71 percent high-grade lumber (No. 1 Structural, 8/4) from butt logs. Logs that had been dead for 20 months produced only 17 percent high-grade lumber (fig. 7-1). Yield, as measured by the lumber recovery factor, was only slightly reduced (fig 7-2). Decreased yield from beetle-killed logs was mainly from increased slabbing and more cull boards.
Figure 7-2 – Lumber yield from control and SPB-killed trees in Virginia.
Logically, the rate of sawtimber deterioration differed across the South. Recovery in the warm, humid east Texas region had dropped to about 75 percent of the control value at 90 days after the trees were killed (Walters, Weldon, and Rutherford 1979 unpublished). Similar recovery loss took 360 days in the cooler, drier climate of Virginia.
The strength of beetle-killed timber from Virginia was measured using standard toughness tests (Sinclair, McLain, and Ifju 1979) for trees dead 2, 12, and 20 months (fig. 7-3). About half to two-thirds of the strength was lost after the dead tree had gone through the first warm season, and there was no statistical difference between strength losses and time since death beyond that period. Similar results were obtained for radially loaded toughness tests.
Figure 7-3 – Strength retained by SPB-killed trees in
Virginia, measured by tangentially loaded toughness tests.
Thus, the yield and grade recovery and strength of SPB-killed timber, although lowered, are still acceptable in many circumstances. An economic guide for purchasers of beetle-killed sawlogs (Sinclair 1979) takes these factors into account and illustrates how to calculate a break-even sawlog purchase price.
Gross Kraft pulp yield from SPB-killed timber was not significantly altered for up to 2 years after death in Virginia (Ifju et al. 1979) and 1 year after death in east Texas (Walters et al. 1979 unpublished). Again, differences in deterioration time are probably due to climatic differences. Paper properties (Canadian Standard Freeness, tear strength, and tensile strength) were somewhat less from beetle-killed trees. But wood scientists concluded that these trees could be pulped up to 24 months after death, depending on the climate, with only a slight effect on paper properties.
Beetle-killed timber may also be used for plywood, but the trees must be harvested quickly. Veneer grade and wide sheet recovery were evaluated in an operational plant in east Texas. They were unchanged up to 45 days after death but then decreased until the logs were unusable for plywood after 360 days (Walters et al. 1979 unpublished). Changes in moisture content of the beetle-killed trees caused some problems when normal production processes and schedules are used. Oven drying of plywood panels must be avoided, and gluing procedures may require alteration in order to produce plywood that consistently meets commercial standards.
When southern pine beetles attack a high-density recreation site, recreation impacts occur. Tree death results in reduced shade and screening, leaves unsightly dead snags, and creates a safety hazard to recreationists. Three kinds of impacts can accrue: the cost of removing the attacked trees, the decreased satisfaction of those recreationists who no longer use the site because of the attack, and the decreased satisfaction of those recreationists who continue to use the site. The value of the economic impact can be estimated for the first two impacts but not for the last.
The cost of removing the dead trees may be estimated by a straightforward accounting procedure that accumulates the labor, equipment, and materials costs for the removal. Total cost will vary depending on the removal technique, the distance traveled to the recreation area, and other variables unique to the specific infestation. In one study, Leuschner and Young (1978) estimated the impact at $3.96 per tree.
Estimating the value of the economic impact for recreationists who stop using the site is more complex. Outdoor recreation market values are seldom observable; therefore, a substitute measure is used. The Hotelling-Clawson-Knetsch (HCK) method (Clawson and Knetsch 1966), which uses travel costs as a price substitute in constructing a demand curve, has gained wide acceptance. The value of the recreation is the area under the demand curve, which is a measure of the recreationists’ willingness to pay for the recreation and hence a measure of social value. SPB impact is estimated by including the proportion of the recreation site covered by pine crowns as an independent variable in the demand function. An SPB attack reduces the pine crown cover, causing a shift in the demand curve. The area under the shifted demand curve is the value of recreation on the site after the attack. The difference between the area under the curve before and after the attack is the onsite value of the economic impact on recreationists no longer using the site (fig. 7-4).
Figure 7-4 – Technique for estimating recreation impact.
However, recreationists no longer using an attacked site may either stop recreating or may substitute another recreation site for the attacked one. The social value of the substituted recreation site should be added back to reduce the onsite impact and provide an aggregate estimate of impact for the recreation system.
Leuschner and Young (1978) examined campground recreation during 1973 at the Rayburn and Steinhagen Reservoirs in east Texas. Demand functions were fitted for both U.S. Forest Service (USFS) and U.S. Corps of Engineers (COE) campgrounds because of their different characteristics. The general forms of the fitted equations were:
|LFSVij||=||Lb0 + b1LCj + b2LTPj +b3LTCij + b4LAOIij + b5LPi (2)|
|LCOEVij||=||Lb0 + b1LCj + b2LTPj + b3LTCij + b4LAOIij + b5LPi + b6LTHj + b7LEHj (3)|
|L||=||the natural logarithm of the variable or coefficient|
|FSVij||=||the annual number of visits from origin i to USFS recreation site j|
|COEVij||=||the annual number of visits from origin i to COE recreation site j|
|Cj||=||the number of designated camping units without electrical hookups at site j|
|TPj||=||the percentage of site j’s area covered by pine crowns|
|TCij||=||the total cost per visit from origin i to site j|
|AOIij||=||an alternative opportunity index, the sum of the number of reservoirs closer to origin i than the reservoir at which site j is located|
|Pi||=||thousands of households at origin i|
|THj||=||the percentage of site j’s area covered by hardwood crowns|
|EHj||=||the number of electrical hookups for trailers at site j|
The functions were fitted with estimates of travel costs and with travel costs plus an allowance for the time spent traveling. The estimated recreation value in 1973 was $7.7 million excluding travel time costs (but including travel costs) and $12.4 million with travel time costs. Onsite impact depends on the particular site, the amount of crown reduction caused by an attack, and whether travel time costs are included. For example, the impacts on the Cassells Boykin and Ebenezer sites were $22,000 and $35,000, respectively, at 10 percent crown reduction (table 7-1). The impact at the Ebenezer site increased from $35,000 to $110,000 to $415,000 as crown reduction increased from 10 to 30 to 90 percent. And at the Ebenezer impact at 10 percent crown reduction without travel time cost was $35,000 and with travel time cost, $59,000. Adjustments for substituting unattacked sites within the recreation system were made for two campsites (Twin Dikes and East End) that are near the high and low end of the impact value distribution. These adjustments indicated that the recreation system impact is only 10 to 15 percent of the onsite impact (table 7-2).
The HCK methodology is useful for assessing SPB recreation impact, but system-wide substitution must be included and aggregate rather than onsite impacts must be used to guide management decisions. The methodology is limited to high-density use sites, however, and may be so complex as to be used only where obviously large recreation values exist.
Recreation impact can be relatively important, as indicated by aggregate impacts as high as $76,000 on the Twin Dikes site (compared to the $6,000 timber impact on the 85,000-acre Trinity Ranger District). This high potential impact may well justify intensive beetle prevention and/or suppression programs on these sites. However, recreation impacts should not be averaged with other forestwide impacts to justify larger control programs because the recreation impact is separable and site-specific. Certainly, the proportion of pines in the overstory stand will influence potential impacts on individual sites.
|Percentage of pine crown coverage reduction|
|Without time cost||With time cost|
Source: Leuschner and Young 1978.
|% TPj Reduction||East End||Twin Dikes|
|Number of Visits||Dollar Value||Number of Visits||Dollar Value|
Source: Leuschner and Young 1978.
The deterioration of attacked stands also causes esthetic impact. Esthetic impact on heavily used sites is at least partially measured in recreation impacts. But esthetic impacts can also occur with dispersed recreation. This impact can be divided into that occurring when people view SPB damage from within the forest and when they view it from outside the forest. For example, hunters and hikers view damage from within a forest and pleasure drivers or casual passersby view it from the outside. One might speculate that damage viewed from within has less total impact because those viewing it are concentrating on another activity (such as hunting) or may see less of it per person because they cover less ground by foot, and because they are usually fewer in number.
The impact of SPB damage on those viewing it from outside the forest has been estimated by showing people photographic slides of forests with varying amounts and stages of beetle damage. The slides are controlled for season, physiography, sky composition, vegetation patterns, and several other variables. Two slides are shown simultaneously, each on a separate screen, in all possible combinations. Respondents thus see 45 different pairs of slides put together from 10 original slides. The respondent has 5 seconds to indicate whether the right- or left-hand slide is preferred before the next pair of slides is shown. Thurstone’s (1927) Law of Comparative Judgment is then used to compute an interval preference scale for each slide.
Buhyoff and Leuschner (1978) reported the results when this technique was applied to 277 persons with backgrounds representing different levels of knowledge about forestry. Each study group was randomly divided into two subgroups, one informed that they were viewing SPB damage and one not informed. Industrial and Federal foresters were not divided because it was assumed that they would know that they were viewing damage. The proportion of vegetation damaged in each slide was calculated with a ¼-inch grid overlay on an 8 X 10-inch print of each slide. The logarithmic function Y = b lnX, where Y = landscape preference value and X = proportion of landscape damage, was fitted for each subgroup. The functions were not statistically different between the two observer groups, so data were pooled by informed-uninformed categories and the regressions reestimated to obtain
|YI = 1.03 – 0.28 lnX||R2 = 0.84||(4)|
|YU = 1.46 – 0.14 lnX||R2 = 0.33||(5)|
|YI||=||the landscape preference value for the informed group|
|YU||=||the landscape preference value for the uninformed group|
|lnX||=||the natural logarithm for the proportion of the vegetation damaged|
Buhyoff and Leuschner drew several conclusions. (1) Preference apparently was unaffected by forestry background. (2) People apparently evaluated damage differently based on whether or not they know it is damage. And, as shown more clearly by a plot the functions, (3a) the informed group had a stronger preference for undamaged stands but lost it more rapidly once damage occurred; (3b) the informed group lost more preference than the uninformed groups as damage increased; and (3c) preference loss was very small in both groups when damage exceeded 10 percent.
These results have several management implications. First, the increased impact on those knowing they are viewing SPB damage should be weighed against the benefits of publicity campaigns. Second, professional foresters may have faith that their reactions to SPB esthetic impact are similar to other people’s. Finally, to reduce esthetic impact, it is probably more important to prevent or control initial SPB damage than deal with extensive loss.
Hydrologic impacts are usually measured by quantity of water obtained from watershed (yield), the timing and duration of the high and low flows (regimen), and water quality. Generally, water yield increases as vegetation decreases. SPB may temporarily increase yield by killing vegetation and reducing transpiration and the amount of precipitation intercepted by healthy pines. The regimen is determined by precipitation timing and intensity, soil permeability, soil water deficits, and soil depth. SPB impact on regimen depends on the distance of the spot from the stream: the more distant the spot, the less likely it will affect stream flow. Quality has several dimensions, of which only sediment, nutrient content, and water temperature are considered. SPB can have a quality impact if infestations cause increased erosion (and hence sedimentation) or increased nutrient leaching. Water temperature impacts occur only if SPB removes sufficient shade from streams.
Hydrologic impacts can be examined by using existing hydrological models that contain a measure of crown cover or stand density as an independent variable. We can then examine the beetle’s impact on yield by changing these variables to reflect different levels of attack, much the same as in the recreation impact technique. Regimen and quality impacts can then be examined by synthesizing water yield changes as shown by the hydrologic model, the established hydrologic relationships, and the characteristics of the SPB outbreak. This technique may not be as accurate as direct observation, but it requires less time and money.
Such a study was performed by Leuschner, Shore, and Smith (1979) using the Rogerson model (1976). They selected sites within the SPB’s range, representing high, average, and low water yields (Corinth, Miss.; Dalton, Ga.; and Blackstone, Va., respectively) and examined changes in these yields as stand basal area changed. Yield changes were examined for original stand BA of 150 and 90 ft2/acre, although any original BA can be used.
Results showed that yield increased between 9.0 and 0.3 acre-inches/year for an acre of SPB spot depending on the site, original BA, and the amount of BA reduction (table 7-3). A rough indication of forestwide physical impact can be obtained by using the Trinity District data (Leuschner et al. 1976), where about 44 acres of SPB spots occurred on about 85,000 acres of host type in 1 year. The largest spot was 2.17 acres; assuming maximum yield, the annual increase would have been 19.53 (9.0 X 2.17) acre-inches from that spot. Similarly, the increase from all spots would be 396 acre-inches (9.0 X 44). But the average is only 0.0047 acre-inches (396/85,000) per acre of host type. These increases are small, considering the total area affected. The estimates are conservative because (1) increases will decrease to zero as vegetative cover returns, (2) they are based on maximum yield changes, and (3) the yield change for a spot will be less the further the spot is from a stream.
|Stand basal area before attack|
|150 ft2/acre||90 ft2/acre|
Source: Leuschner, Shore, and Smith 1979.
Water quality is also unlikely to be affected by SPB attacks. Studies show that erosion, and hence sedimentation, is not adversely affected by overstory removal (e.g., Aubertin and Patric 1974, Dickerson 1975, and Hornbeck 1967). Nor is nutrient loss significantly accelerated by usual management practices (e.g., Swank and Douglass 1977 and Pierce et al. 1970). Further, the relatively small and dispersed spots make a large impact on water temperature via shade removal improbable.
The economic impact is most likely to occur in water yield, and that should be valued as close as possible to the watershed, not at higher levels of production, after the water has become "more valuable." But throughout the beetle’s range, water at the watershed is generally zero valued because it is usually replaceable from alternative sources. Economically, it a "free good" (Young and Gray 1972 and Gregory 1972).
In summary, the southern pine beetle’s physical economic impact on water yield is small and its impact on water quality is zero. The economic value of these impacts is also zero. The reader is cautioned, however, that these conclusions are based on the Trinity District infestation configuration, which had small and dispersed spots. The conclusions could change if large contiguous areas are attacked, particularly in rougher terrain, or if water is in short supply and not a free good. Therefore, a separate analysis might be desirable if these conditions are likely to hold.
The southern pine beetle’s direct, or primary, wildlife impact occurs when it is a food for some species, most notably woodpeckers. The indirect, or secondary, impacts occur either by increasing associated insect populations that can act as food or by killing trees and decreasing crown cover. Reduction in crown cover can result in increasing stream temperature and sedimentation, increasing edge, changing the availability of nesting sites, and changing the understory vegetation. The latter causes changes in shelter and cover as well as the amount and kind of food (fig. 7-5). The impacts on individual wildlife species will differ because each species can have unique requirements. For example, the SPB is a food for woodpeckers but not for squirrels.
Figure 7-5 – Qualitative model of SPB impacts on wildlife populations. (Source: Maine, Leuschner, and Tipton 1980).
Direct observation and measurement of SPB wildlife impacts is difficult, if not impossible, due to the state of population censusing technology and the costs of such work. Further, exact production relationships between amounts of food, shelter, etc., and wildlife population numbers are still being developed. Therefore, the analyst is dependent on synthesizing published data with known biological relationships and qualitatively analyzing results to examine suspected impacts. Maine, Leuschner, and Tipto (1980) performed this kind of study. Impact on amount and kind of food due to changing understory vegetation was assessed by inferring changes in crown cover from Leuschner et al. (1976) and Ovington (1957), translating these into changes in the amount and kind of browse and herbage using Schuster’s (1967) model and results, and obtaining an average change by weighting by the spot size distribution found on the Trinity District (Leuschner et al. 1976). SPB attacks also provide edge by causing forest openings. Linear feet of edge were calculated by assuming circular, triangular, and rectangular spot shapes and weighting by the Trinity District (Leuschner et al. 1976) spot size distribution. About 2,000 ft of edge per acre of SPB spot was found. This amounts to about 660 ft of edge per square mile of forest [(2,000 X 44 X 640)/85,000].
These techniques apply to several species and hence were discussed in one place. Other techniques are species-specific, and the interested reader should refer to Maine, Leuschner, and Tipton (1980) for a discussion of impacts on ten species or species groups.
Downy, hairy, and pileated woodpeckers are major SPB predators (Coulson et al. 1972b and Kroll, Conner, and Fleet 1980), and SPB can provide winter food for them. In fact, the availability of SPB often limits woodpecker populations (Kroll and Fleet 1979). SPB could also increase woodpecker populations by providing dead, standing trees for nesting sites (fig. 7-6). The size of these impacts is unknown but is probably limited because of the small proportion of the forested area with spots.
The southern pine beetle’s impact on turkey habitat in pure stands is likely to be negligible because turkeys usually frequent hardwood or pine-hardwood stands. There may be a slight positive effect in mixed stands when SPB serves as a direct food, increases edge, or favors increases in grasses and legumes that are part of the turkey diet. However, this increase may be offset by the large decrease in dogwood species, a preferred turkey food. Thus, the net effect of SPB on turkey is probably negligible.
Figure 7-6 – Snags containing a pileated woodpecker’s
nest cavity. (From Kroll, Conner, and Fleet 1980).
Quail is an "edge species" requiring five types of cover plus food, all within a limited cruising radius. SPB spots increase edge and understory vegetation, thereby providing additional cover, particularly in the purer pine stands. Legumes, an important quail food, also increase slightly with SPB spots. Hence the food impact may be slightly positive. There is, therefore, a positive net impact from increased edge and slightly increased food.
This is a catchall category including predators, such as owls and hawks, and nongame species. Edwards (1978) found high populations of small mammals and members of the finch family in newly cut stands. These animals are food for predatory birds. Hence SPB can have a positive impact on their food, which might be partially offset by increased cover. Meyers and Johnson (1978) state that nongame bird population diversity and density are high in early stages of loblolly-shortleaf succession but decrease with stand age. Thus SPB can have a positive impact on nongame birds by returning pine stands to early succession stages. We conclude that the net effect of SPB on other birds is positive because food and other habitat requirements are increased.
Rabbits are prey for nearly every carnivorous bird and animal (Madson 1959). Escape cover is, therefore, critical and can be limiting in the winter. The small home range of rabbits also requires interspersion of cover and food like that found in edge. SPB creates openings that promote increased cover, understory growth, and edge. Rabbits’ favored food plants also develop in these openings. Thus, SPB has a positive impact on rabbits.
Grey and fox squirrels, the two major species within the beetle’s range, inhabit hardwood forests where mast, other food, and den trees are found. They rarely inhabit pine monocultures, brush, or cutover land. And their small cruising radius keeps them close to the hardwood types. Thus impacts in the purer pine types are unlikely. A slight positive impact in pine-hardwood types might occur if SPB removed pine competition, thereby increasing mast production. But this positive impact might be offset by destruction of old pines that could serve as leaf nest trees. Therefore, the net SPB impact on squirrels is likely to be negligible.
White-tailed deer inhabit almost any wooded or brushy area that provides thick cover from predators during the day, while the deer sleep. Pine and pine-hardwood forests can provide this cover, but winter food is often the limiting factor. SPB attacks can increase honeysuckle and grasses in pine stands and mast production via decreased competition in pine-hardwood stands. Harlow and Hooper (1971) found honeysuckle, acorns, and grasses made up over 70 percent of winter food in the Coastal Plain. Hence SPB can have a positive impact on deer. Maine, Leuschner, and Tipton (1980) estimated that 1acre of SPB spot provided 14.5 deer days’ increase in food. Thus 25 acres of spots are needed to increase carrying capacity by one deer. This is the equivalent of one deer per 50,000 acres of host type if the infestation has the characteristics of the Trinity District (Leuschner et al. 1976). Deer browse open areas at night and retreat to thick cover during the day. The interspersion caused by edge is, therefore, another positive impact. The net impact of SPB on deer is positive through increased food and edge.
Small Mammals, Fish, and Other Animals
The small mammal category includes mice, shrews, moles, voles, rats, and other small Insectivoria and Rodentia. Lack of published information makes this analysis even shallower than others. The major impact might be through bringing vegetation closer to the ground, thereby increasing food and shelter. But the importance of this effect is undocumented. Murray (1957) reported that edge effect is unimportant for these animals. The net SPB impact is assumed negligible based on the lack of published associations rather than firm evidence indicating no impact.
Southern pine beetle impact on fish would occur through increased sedimentation and water temperature. In the Hydrologic Impacts section we concluded that these would be negligible; thus the beetle’s impact on fish is negligible.
"Other Animals" includes opossums, skunks, and other fur bearers. Again, lack of published information results in a particularly shallow analysis. However, we do know that many fur bearers are carnivorous, so SPB could have a positive effect by increasing rabbit and other prey populations. Other fur bearers, found mostly in and around water, would be unaffected by SPB-induced changes in food, edge, or cover. A negligible net impact is assumed, based, again, on lack of evidence rather than evidence of no effect.
The preceding analyses indicated a positive SPB impact on woodpecker, quail, rabbit, deer, small mammal, and other bird populations — mostly through increases in edge and food. Therefore, SPB control is a cost to wildlife because it reduces positive impacts of the beetle. The impacts’ magnitude is difficult to determine but is likely to be quite small if outbreak characteristics are similar to those used in the analyses. Special analyses may be desirable if different outbreak characteristics are suspected or if unique local conditions prevail.
Southern pine beetle attacks could increase grazing capacity by opening the overstory and thereby causing increased production of grazing herbage. But the usual SPB outbreaks appear to be so small and dispersed that the increase in herbage would not be enough to justify investing in grazing unless it were already present. Therefore, it is hypothesized that grazing impacts are likely to occur only on those 30.3 million acres reported by the Forest Range Task Force (U.S. Department of Agriculture Forest Service 1972b) as grazed loblolly-shortleaf type.
A rudimentary grazing impact estimate can be made following techniques described previously. More specifically, (1) the impact on crown cover or density is estimated, (2) this change is related to herbage using published models, (3) herbage changes are then translated to grazing capacity changes, and (4) capacity changes can be expanded to a region- or Southwide basis assuming the distributions and intensity found on the Trinity District (Leuschner et al. 1976). The vale of the economic impact can be estimated by using either the market value of grazing leases for similar range or the cost of developing range to replace that which would have been generated by SPB attacks.
This technique was applied by Maine (1979), who wrote a computer program to make the calculations. The program used both the Halls and Schuster (1965) and the Wolters (1973) models to translate changes in basal area to changes in herbage. Herbage changes were translated to animal unit months, assuming that 100 lb of herbage are needed each day for year-long grazing (based on Pearson 1975, Duvall and Whitaker 1964, and other studies) and 75 lb/day are needed for seasonal grazing (based on Duvall and Linnartz 1967). An animal unit month was valued at $4.03, the average cost of production in 1970 (U.S. Department of Agriculture Forest Service 1972b).
Grazing impact of SPB varies depending on the distribution of spot sizes, the width of the shaded area within the spot, the hardwood BA present, the herbage model, and whether year-long or seasonal grazing is assumed. Maine (1979) estimated that maximum impact on the Trinity District would have been between 4.4 and 29.3 animal unit months (or a loss of between $18 and $118) if the entire District were grazed and if SPB were completely controlled. This dollar estimate compares to the $6,000 timber impact that would have been saved if there was complete control. Maine (1979) also estimated that the Southwide impact in 1 year would lie between zero and $42,000, again depending on the preceding assumptions. He concluded that although the impact is positive and hence a cost of SPB control, grazing impact is negligible Southwide or over large areas and hence not generally important for management considerations. He cautioned, however, that these results were based on crude estimates and broad averages that might increase if severe or concentrated outbreaks occurred.
Many foresters believe that insect outbreaks can cause increased fire losses either by providing snags or by increasing fuels and thereby increasing incidence or fire intensity and subsequent losses. Direct observation and field measurement are expensive and time consuming and hence impractical for most analyses. So we are once again dependent on synthesizing published information on insect outbreaks, fire studies in general, and those few studies relating insects and fire. Two general impact areas may be examined: losses due to increased fire control expenditures (an increased cost of production), and increased timer losses due to insect-induced fires (decreased production). Timber losses can increase if more fires start, the rate of spread increases, or more damage is caused.
Maine (1979) performed this type of study for southern pine beetles. He concluded that fire control expenditures were unlikely to increase because (1) Gobeil (1941) found they did not on the Gaspé peninsula for the spruce bark beetle (D. piceaperda Hopk.), and (2) the dispersed nature of the SPB infestations supports extending this conclusion to SPB. The conclusion is also supported by his rough estimate that only 64 acres of SPB spots burn annually Southwide. (About 0.25892 percent of commercial forest acreage in the South is burned annually (USDA — Forest Service 1976), about 0.05 percent of the commercial forest has SPB spots (Leuschner et al. 1976), and there are about 49.4 million acres of loblolly-shortleaf pine type (USDA — Forest Service 1973). Then 0.0025892 X 0.0005 X 49.4 million = 63.95 acres. This assumes that the Trinity District outbreak characteristics hold Southwide and that fire incidence is independent of SPB attack).
Fire starts were considered negligible because only 3 percent of southern fires are naturally caused (U.S. Department of Agriculture Forest Service 1976). Increased spotting due to burning standing, dead trees could increase incidence. But small, dispersed spots and the low estimated SPB acreage burned imply this impact is also likely to be small. Rate of spread could be increased if ground-level fuel with a lower moisture content were increased. However, ground-level fuel increase is likely to be small at any one time due to the slow decay of standing trees and, again, the small, dispersed spots and low SPB acreage burned.
Maine (1979) found that the ability of fire to do damage was influenced by stand value, stand susceptibility, and fire intensity. Attacked stands have diminished or no value when burned because the pine trees are already dead, although there may be some value in hardwoods. Stand susceptibility and intensity might be slightly increased. But the usual low value of the dead pines and residual hardwoods makes it unlikely that SPB-related wildfires will do much damage.
Fire impacts may generally be ignored in management decisions. Rate of spread and intensity are probably increased by some small, unknown amount. But the Southwide impact is believed negligible because of the small, dispersed spots, because only an estimated 64 acres a year are burned, and because pines are already dead and the residual hardwoods tend to have a lower commercial value. The reader is again cautioned that these conclusions are based on crude estimates, broad averages, and the assumption that Trinity District (Leuschner et al. 1976) outbreak characteristics generally apply Southwide. The conclusions should be reassessed if large, contiguous areas of SPB damage occur.
Economic impacts of the southern pine beetle are those causing changes in the production, inputs needed for production, or the production distribution useful to society. Impacts can be measured in either qualitative, physical, or value terms. Timber, recreation, hydrologic, and grazing impacts can be estimated in dollars; esthetic impacts with an interval preference scale; and wildlife and wildfire impacts, only qualitatively (table 7-4). SPB usually decreases timber, recreation, and esthetic products; has slight positive effects on wildlife, wildfire, and grazing; and also slightly increases water yield, although the economic value of this increase Southwide is usually zero. Timber and recreation impacts have the highest dollar impacts and hence should be considered in making management decisions. Hydrologic, grazing, and wildfire impacts are generally too small to consider in making management decisions.
The reader is cautioned again about the weaknesses in specific analyses and the general nature of the results. Perhaps no one is more cognizant than a forester of the diversity of natural conditions over wide geographic areas and the ever-present possibility that unique local conditions will result in a different answer than the general case. The practitioner is therefore urged to make local analyses where the size of possible expenditures or losses justifies the expense.
The reader is also cautioned that many general conclusions are based on the Trinity District infestation characteristics and that they may not represent the entire South. Unfortunately, these were the only summarized data available at the time research was executed. They remain, to our knowledge, the only published data characterizing an infestation with frequency distributions. Parallel analyses can be made using the techniques reported herein when other data become available if an analyst believes the general conclusions would be substantially different. Similarly, the techniques and assumptions can be refined as more of SPB’s interrelationships become known and quantified. These results are offered as guidelines for decisions that must be made between now and the time when new and better information is available.
|Timber||Dollars||Present net worth||Negative||Traditional model usually over estimates impact.|
|Recreation||Dollars||HCK method||Negative||Relatively high impact for high-density use areas.|
|Esthetic||Interval preference||Psychological disutility||Negative||Attack prevention more important than spread.|
|Hydrologic||Dollars||Rogerson||Zero||Small yield increase but water is free good; hence zero dollars.|
|Wildlife||Qualitative||Synthesize pub. studies||Positive
|Positive impact on woodpecker, quail, rabbit, deer, small mammal, and other bird populations.|
|Grazing||Dollars||Synthesize pub. studies||Positive||Total usually too small for consideration.|
|Wildfire||Qualitative||Synthesize pub. studies||Positive||Total usually too small for consideration.|
Developed by the University of Georgia Bugwood Network in cooperation with USDA Forest Service - Forest Health Protection, USDA APHIS PPQ, Georgia Forestry Commission, Texas Forest Service
and the Pests and Diseases Image Library - Australia
Last updated August 2018
www.barkbeetles.org version 2.0