Source: https://www.barkbeetles.org/spb/SSHSPB/SSHSPBCh.html
Timestamp: 2019-04-23 21:09:16+00:00

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Substantial areas within the range of SPB are not represented by the seven preceding projects. If there are factors in common among infestations throughout a subregion, or even in common between subregions, then there is justification for extending recommendations based on these data to wider areas. What follows is an analysis if the ESPBRAP/Coordinated Regional Project data on a geographic subregion basis in order to provide a Southwide characterization of factors associated with SPB infestations.
The data are pooled across projects for this analysis; and, as might be expected, project-to-project differences are important for a number of site/stand variables. However, these differences are significantly reduced and in some cases disappear altogether when all projects are compared on a uniform basis. For example, project-to-project differences are greatly reduced when a particular landform, such as upland flats in the Coastal Plain, is examined across projects. The reliability of measurements of the standard data set among various project crews is expected to be good (Hicks et al. 1978).
Most of the SPB data were collected in the Coastal Plain subregion. Investigators established some 2,021 plots at SPB infestations and another 1,092 baseline plots in uninfested stands to provide a representation of general forest conditions. Coastal Plain plots were established, almost exclusively, in the mid-South region: Texas, Louisiana, Arkansas, and Mississippi. Fewer than 60 plots were available form the North Carolina and Virginia Coastal Plains; however, these plots do resemble those from the mid-South.
In all, 92 percent of the SPB plots and 95 percent of the baseline plots were established in naturally occurring stands (table 53). If present, various disturbances such as ice damage or recent harvesting were recorded at time of plot establishment; this breakdown is also provided in table 53. Table 54 shows the distribution of plots by specific type of disturbance for natural stands. Although most disturbances seem unrelated to SPB attacks, it is quite evident that lightning strikes do predispose stands to beetle activity. In the Coastal Plain SPB plots, 23 percent evidenced recent lightning strikes, while such strikes occurred only 1 percent of the time in the baseline sample.
Another factor apparent in table 54 and consistent across individual projects is the impact of logging activity. Recent logging (within the year) has a tendency to increase SPB attack, but stands harvested earlier (1 to 5 years ago) are significantly less attacked. It appears that recent logging damage makes conditions more favorable for SPB, but once the residual trees begin to respond to growing conditions in the thinned stand, the trees may become less susceptible to attack. This relationship points out the merits of careful logging and hints at the longer-term benefits of reduced BA. Reducing understory competition through the use of fire may also increase resistance once the trees respond (table 54). The plantation data are not adequate for detailed analysis; however, 54 percent of the infestations in plantations were undisturbed and 23 percent of all disturbances recorded were lightning strikes.
Table 55 documents the characteristics of Coastal Plain infections. These statistics cover only natural, undisturbed stands in order to eliminate the possibility that disturbances may predispose the stand to attack regardless of existing site/stand conditions. The average value for slope reflects the occurrence of extensive, nearly level areas in the Coastal Plain. Where some slope does occur, attacked stands most often have a southerly exposure; the standard deviation for aspect indicates this trait is quite varied, however.
The A horizon (surface soil depth) of sites with SPB attacks is adequate for good root mass, and both the surface soil and subsoil have significant sand components. Soil pH is low. Of the total BA on plots established at SPB infestations, 80 percent is pine (table 55). Average total BA is high, given the average age of 40 years.
The 50 percent understory occupation by woody stems (table 55) suggests that the average infestation is a pine stand with a small-diameter hardwood understory. This deduction is supported by other facts also: the average age is 40 years, and though there are some 764 stems per acre, only 263 of these are over 4.6 inches at breast height.
Measurements of pine volume were derived using a single southern pine volume equation developed for a wide range of d.b.h. and height combinations (MacKinney and Chaiken 1939). Tree volumes for "in" trees on the BA plot were converted to a per-acre basis and summed (table 55). These volume figures, both total and sawtimber size only, should probably be treated as indices of actual volume, because the single equation was applied across the South. Obviously, there is substantial variation in total stand volume of pine and in the sawtimber component. The coefficient of variation for sawtimber volume is 70 percent. Using 4.0 fbm/ft³ (Williams and Hopkins 1978), the Doyle volume is approximately 7,900 fbm/acre in these infested stands.
¹Volume in pine trees with d.b.h. >= 4.6 in.
²Volume in pine trees with d.b.h .= 9.6 in.
The average size of SPB infestations is small (Fig. 14); 63 percent were estimated to cover <1/4 acre, and not all the trees within this area were killed by SPB. SPB-killed trees were often scattered within the spot. Nearly all (95 percent) of the infestations were considered inactive at time of measurement. Some 32 percent of them contained 10 or fewer SPB-killed trees, and 53 percent consisted of 25 SPB-killed trees or fewer. Number of SPB-killed trees per infestation was significantly positively correlated with stand BA and stand cubic-foot volume.
undisturbed Coastal Plan SPB infestations.
In addition to the variables listed in table 55, the investigators recorded several descriptive characteristics associated with each infestation. These included landform, foil texture, source of soil material, water regime, modifiers, and accessory characteristics. These variables were coded according to the Soil Resource Guide, Southern Region (USDA Forest Service 1972). Foremost in importance with regard to these variables is landform.
Table 56 shows the distribution of SPB-attacked and baseline plots, according to landform classification. The landform titles provided are descriptive, but a full definition can be found in the Soil Resource Guide. Assuming that the baseline plots compose a representative sample of general forest conditions, there are significant differences by ( X² test at P = 0.05) in number of SPB infestations by landform (table 56). Side slopes had fewer SPB attacks. Flood plains and upland flats are subject to flooding; stands on these landforms are more likely to experience temporary moisture stress due to lack of normally available rooting area (Lorio and Hodges 1971). Ridges-the upper part or crests of broad interstream divides-tend to dry out quickly in periods of low rainfall, thus increasing water stress on the pines. Ridges are also often associated with lightening strikes.
¹See Table 55 measurement units.
Table 57 records site/stands variables that differed significantly between attacked and baseline plots. Some of these variables may be useful from the resource manager's perspective; others do not appear to be. Attacked plots typically occurred on sites that were flatter, had higher BA and correspondingly greater volume, and were slower growing. These traits are dependable discriminating characteristics in nearly every situation. The differences in average age and site index are not adequate to explain the higher BA and volumes in the attacked plots (Fig. 15).
Fifty percent of the infestations had BA over 130 ft²/acre, while almost 80 percent of the baseline plots had BA of 130 ft²/acre or less. Total stand volume, pulpwood and sawtimber-sized material,. on infested sites was 185 percent of the comparable baseline figure (2,759 v. 1,497, table 57). In addition, SPB-attacked stands were 80 percent pine on the basis of BA, whereas the baseline stands were only 66 percent pine. These statistics reflect a stand composition difference as well as a total BA difference between attacked and baseline stands.
Thinning could, perhaps, reduce the susceptibility of dense stands. Assuming 75 ft³/cord, removal of one-third of the total volume of SPB-attacked stands as pine pulpwood would yield over 12 cords/acre, some of which could be sawtimber. And this figure ignores the hardwood volume that could be removed. Such a thinning would still leave an operable pine BA of between 75 and 80 ft²/acre plus whatever hardwood was not removed.
Such a thinning would reduce per-acre volume and BA (especially if hardwood were removed) and, with time, increase growth rate and improve live crown ratios. All of these changes would tend to reduce the incidence of SPB attack (table 57). Since stand characteristics are more closely associated with SPB attacks in the Coastal Plain, the resource manager can reduce the incidence of SPB attacks through modification of stand conditions and basic stand management systems. Even the use of controlled burning to reduce small-hardwood competition would be useful.
The little data available for SPB-attacked and baseline plantations in the Coastal Plain indicate that attacked and unattacked plantations are quite similar. Slope and radial growth are the only variables of explanatory importance, and they behave in a manner similar to natural stands. Baseline plantations are on sites with an average slope of 4.8 percent ( v. 2.4 for SPB-attacked plots ) and average 47.9 mm of radial growth in the last 10 years ( v. 40.3 for SPB-attacked plots ). Pine volume, BA, stand density, and site index are very similar for attacked and baseline plots for the Coastal Plain plantation data.
Infested stands were sampled in Georgia, Virginia, and a small region of intense SPB activity surrounding the Kerr Reservoir in the north central part of North Carolina. Fewer plots were established in the Piedmont subregion (table 58) than in the Coastal Plain.
Eighty-five percent of the SPB-attacked data came from natural stands (table 58). While 58 percent of attacked natural stands in the Coastal Plain were disturbed, a disturbance was recorded on only 9 percent of attacked natural stands in the Piedmont (table 59). The impact of lightening strikes in still evident but appears less important in the Piedmont.
The average size of Piedmont infestations is small, just as in the Coastal Plain. Forty-seven percent of all spots are <1/4 acre in size and the average spot is < 1.0 acre. Furthermore, the average size of Piedmont spots is greatly influenced by several large spots in North Carolina. Ninety-four percent of the 30 infestations in natural undisturbed Piedmont stands > 2.0 acres in size were found in North Carolina (Fig. 16). These large infestations also influenced the average number of SPB-killed trees per infestation. Using all Piedmont infestations, the average number of SPB-killed trees per infestations was 28, while the number, excluding the North Carolina data, was 9 trees per infestation. Excluding the North Carolina data, some 61 percent of the infestations contained fewer than 25 trees.
Piedmont SPB infestations by spot size.
Once again, to eliminate possible bias in the data, plots with disturbances that may predispose stands to SPB attack were deleted form consideration; only natural, undisturbed infestations are characterized in table 60.
The geographically limited area from which the North Carolina data were collected, along with the fact that about 40 percent of the attacked plots in the Piedmont were from the Kerr Reservoir area, suggest that the total Piedmont data were weighted towards conditions in this area. Inspection of the data in tables 41 and 60 reveals obvious differences in the values of certain major variables between attacked plots in North Carolina and the total Piedmont; North Carolina infestations were in stands of higher BA, somewhat higher site index, and less radial growth. Total stand volume of pine in North Carolina was computed to be 2,387 ft³/acre while it was 2,036 ft³/acre for the total Piedmont plots (table 60). Accordingly, the values for all variables were recomputed without the North Carolina plots. These values (table 60) may be more representative of attacked plot conditions in the Piedmont that the "total Piedmont" data.
²Volume in pine trees with d.b.h. >= 9.6 in.
Piedmont infestations occurred on sites with greater slope, higher soil clay content, and shallower surface soils than Coastal Plain infestations. Several Piedmont plots were found to have only 1.0 cm of surface soil. The pine BA in Piedmont infestations is similar to that of Coastal Plain infestations (table 60 v. table 55), and for Piedmont infestations the stands also appear to be at least two-tiered, as indicated by stand density. However, average tree size and volume per acre is significantly higher for the Coastal Plain infestations, as is radial growth. Using 4.0 fbm/ft³, Piedmont infestations average between 3,400 and 3,800 Doyle fbm/acre.
Piedmont stands on stream terraces and ridges are less susceptible terraces and ridges are less susceptible to attack, white stands on steep side slopes and narrow flood plains are attacked more often than baseline stands (table 61; all Piedmont baseline plots were in Georgia). Site characteristics are most helpful in explaining SPB-attacked v. baseline differences in the Georgia Piedmont (table 62). Specifically, soil pH and surface soil depth are important. Unlike the Coastal Plain, variables such as slope, BA, and pine volume are not significantly different on attacked plots in the Piedmont. The higher pH values associated with baseline plots, even though the differences appear small, may mean that nutrients are more available in baseline stands.
Given these results, the resource manager may not be able to reduce the incidence of SPB in natural Piedmont stands as much as in Coastal Plain stands through silvicultural treatments. Actions to promote growth rate would be somewhat helpful and loblolly pine could be favored in natural stand management, but susceptibility is linked more closely to site. It is also possible that the susceptibility of Piedmont stands could be altered through use of fertilizers to promote growth and nutrient levels, but this is largely a matter of conjecture. An extensive survey of fertilized stands conducted by Haines and others (1976) did not substantiate that fertilization decreased susceptibility of forest stands to SPB attack.
The limited amount of undisturbed plantation data in the Piedmont suggest a single principle. Baseline plots in plantations have a significantly greater (P <= 0.05) average total BA (153.5 ft²/acre). Volume is correspondingly higher for the baseline plantations. Average plantation age is 17 years for both baseline and attacked stands. These facts, plus (1) the knowledge that these loblolly plantations frequently occur on former shortleaf sites, and (2) the significant difference in site indices (70 ft attacked v. 80 ft baseline), suggest that SPB attacks may be associated with loblolly pine planted "off-site" and therefore in a potentially high stress situation. This possibility certainly deserves more investigation since it may directly affect plantation establishment recommendations.
Slope % 11.6 10.6 N.S.
Pine BA ft²/acre 100 97 N.S.
Total BA ft²/acre 118 118 N.S.
Avg. live crown ratio % 41.3 41.5 N.S.
Total stand volume-pine¹ ft³/acre 1774.3 1660.2 N.S.
Total sawtimber volume-pine² ft³/acre 841.3 889.7 N.S.
Age years 33.8 31.6 N.S.
Very few plots are available to represent the Mountain subregion (table 63). These plots come form the Ouachita mountains in west central Arkansas and the Appalachians, mainly in Georgia. No comparative baseline data are available, and no plots were established in plantations. Seventy-nine percent of Mountain plots had no disturbance associated with them, and once again lightning strikes were common (table 64).
Table 65 describes Mountain infestations. The slope is steep and the exposure is generally southeast but has substantial variance. As with Piedmont infestations, the soil clay content is high. Surface soil depth averaged a respectable 18.3 cm and ranged from 3 to 56 cm.
Pine BA averaged < 100 ft²/acre, but there is a substantial hardwood component that brings the average total BA to a level comparable with infested plots in the Coastal Plain and Piedmont (table 65 and fig.17). Given the average site index of 73 feet, the Mountain infestations may well be overstocked. Their average age was much greater than either Coastal Plain or Piedmont infestations, even though the average d.b.h. of the pine component was similar. Smaller diameter for age reflects the much slower growth rate on these Mountain plots. Total volume was high; the stands average 60 percent of their cubic volume in material >9.6 inches d.b.h.-about 5,200 Doyle fbm/acre.
Mountain stands by basal area classification.
Table 66 shows the distribution of Mountain infestations by landform classification. Since there is no comparative baseline data, conclusions as to landform susceptibility are speculative; but 72 percent of the infestations were recorded on southerly aspects and those northerly aspects above 4,500 ft in elevation. Such sites would likely be under more stress during temporary drought conditions. This fact would increase the susceptibility of these landforms above others. In fact, 72 percent of the infested Mountain plots had a recorded "water regime" of dry to very droughty (see the Soil Resources Guide for water regime definitions [USDA Forest Service 1972]).
Average infestation size was comparable to that in the Coastal Plain and Piedmont; 89 percent were < 1.0 acre in size and 63 percent contained 25 or fewer SPB-killed trees. Only 5 percent of Mountain infestations are >2.25 acres. However, 11 percent of them involved over 100 killed trees, so the severity of attack (as measured by proportion of trees killed) may be greater in the Mountains.

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