Site, Stand, and Host Characteristics of Southern Pine Beetle Infestations

Jack E. Coster and Janet L. Searcy - editors

Southern Pine Beetle Handbook
U.S.D.A. Combined Forest Pest Research and Development Program
Technical Bulletin No. 1612

In 1974 the U.S. Department of Agriculture initiated the Combined Forest Pest Research and Development Program, an interagency effort that concentrated on the Douglas-fir tussock moth in the West, on the gypsy moth in the Northeast, and on the southern pine beetle in the South. The work reported in this publication was funded in whole or in part by the Program. This technical bulletin is one in a series on the southern pine beetle.

Piedmont

Georgia
Roger P. Belanger

Study Area

The Piedmont of Georgia can be divided into two distinct physiographic regions.  The lower Piedmont has broad interstream areas of gently rolling hills.  The upper Piedmont is characterized by hilly topography with pronounced ridges.  At one time most of this land was in clean-cultivated crop production.  Depletion of the soil by continuous cropping and severe erosion resulted in widespread land abandonment.  The more sever sites have reverted back to forests.  The better soils are still in agriculture.

SPB outbreaks, spread, and impact have historically been most severe in the upper Piedmont.  Study plots were located in Banks, Clarke, Elbert, Forsyth, Franklin, Green, Hall Hart, Jackson, Madison, Oconee, Oglethorpe, Stephens, and Wilkes Counties (Fig. 11). Loblolly pine and shortleaf pine represent approximately 97 percent of the natural pine component in this area.  The remaining 3 percent is mostly Virginia pine (Pinus virginiana Mill).  Predominant soil types are slick red subsoil formed from micaceous rocks, and heterogeneous soils formed from a mixture of acid and basic crystalline rocks. Study plots were installed in 1975 through 1977.  SPB populations during 1975 were declining from epidemic conditions and the populations were generally endemic during 1976 and 1977.  Most of the study plots were located on nonindustrial private lands.  One field crew did all installations and soil analyses (Edward J. Porterfield and Robert J. Pittard are gratefully acknowledged for their exceptional work in the field and laboratory.).

Figure 11. - Study area in the upper Piedmont of Georgia.

Results and Discussion

Stands attacked by SPB are characterized by slower radial growth, a higher percentage of clay in the surface and subsurface horizons, and a deeper surface soil depth than baseline plots (table 37).  The two study populations differed only slightly in amount of pine BA, age, site index, or bark characteristics.  The hardwood component and amount of understory vegetation were significantly less for attacked plots than baseline plots.  Collectively, these two components could alter stand conditions or beetle behavior in a manner that reduces the susceptibility of stands to SPB attack.  Belanger and others (1979) found that infestations in the mountains of Georgia were more prevalent in pure pine stands than in stands with a mixture of pine and hardwood.

Table 37. - Site and stand characteristics of attacked and baseline plots in natural, undisturbed stands-Georgia Piedmont

Variable	Units	Attacked plots (122)		Baseline plots (140)
		Mean	SD	Mean	SD
Slope*	%	11.6	6.5	11.3	4.6
Aspect*	degree	196.1	98.9	190.8	107.3
Surface sand	%	56.4	10.5	60.6	12.2
Surface silt	%	18.9	5.5	20.6	7.5
Surface clay	%	24.7	9.2	18.8	9.4
Surface pH	log 1/ [H+]	5.1	0.3	5.1	0.3
Surface soil depth	cm	10.7	6.3	7.0	5.4
Subsoil sand	%	40.7	12.3	42.0	13.5
Subsoil silt	%	17.1	5.4	19.0	7.7
Subsoil clay	%	42.1	12.1	39.0	13.5
Subsoil pH	log 1/ [H+]	5.2	0.2	5.3	0.3
Pine BA	ft²/acre	100	34.3	101	3.9
Hdwd.BA	ft²/acre	17	20.0	24	25.4
Total BA	ft²/acre	118	32.3	124	40.9
Stand understory	%	50	20.7	57	19.1
Age	years	34	12.8	35	14.2
Density	trees/acre	675	508.5	773	940.1
Site index at 50 yrs	ft	75.7	114.1	72.2	11.9
SPB-killed trees	number	45.6	79.5
Avg. bark-fissure	in	0.5	0.1	0.6	0.1
Avg. bark-ridge	in	0.9	0.2	0.9	0.2
Avg. radial growth 0-5 yrs ago	mm	11.5	4.4	14.1	5.6
Avg. radial growth 6-10 yrs ago	mm	15.1	6.0	18.0	9.0

*Mean and standard deviation for those plots with nonzero slope and aspect.  Eighty-three percent of the attacked stands had no slope.

Table 38. - Landform classification of attacked and baseline plots in natural, undisturbed stands-Georgia Piedmont

Landform	Attacked (122)	Baseline (140)
	Percent
Flood plain		0.7
Stream terrace	4.0	5.7
Upland flat	3.3	5.7
Ridge	32.0	49.3
Steep side slope	60.7	38.6
	100.0	100.0

More than 60 percent of infested stands were located on steep side slopes (slopes >= 10 percent).  Ridge tops and gentle slopes are characteristic of the baseline plots (table 38).  Soil types are closely associated with landforms in the Piedmont.  Soils with high proportions of sand are generally found on flat areas where erosion has been minimal.  Clay loams are characteristic of slopes from which much of the topsoil has been removed by erosion.  These micaceous clay soils have a high erosion potential and require careful tending.

Heavy stocking was common on both attacked and baseline plots (table 39).  Basal areas >= 110 ft²/acre occurred on 64 percent of the attacked plots and 66.5 percent of the baseline plots.

The most striking difference between attacked and baseline plots is the proportion of the pine component in shortleaf and loblolly pine.  Attacked stands were predominantly shortleaf pine (68.6 percent), whereas baseline plots contained mostly loblolly pine (55.5 percent).

Table 39. - Basal area class of attacked and baseline plots in natural, undisturbed stands-Georgia Piedmont

Basal area class	Attacked (122)	Baseline (140)
	ft²/acre	Percent
<50	0.8	1.4
50-70	8.2	10.7
80-100	27.0	21.4
110-130	37.0	27.9
140-160	23.8	22.9
170-190	1.6	12.1
200-220	1.6	3.6
	100.0	100.0
Average BA in ft²/acre	117.6	124.3
Average percent pine	85.5	81.0
Percent of pine component in:
Shortleaf	68.6	44.5
Loblolly	31.4	55.5

Table 40. - Size and frequency distribution of infestations in natural, undisturbed stands-Georgia Piedmont (n = 122)

Acres (nearest 0.25 acre)		Percent of total
Lower limit	Upper limit
0.00	0.25	54.1
0.50	1.00	36.9
1.25	2.00	7.4
2.25	10.00	1.6
		100.0

More than 90 percent of the attacked spots were <= 1.0 acre in size; 54 percent were <= 1/4 acre in size (table 40).  The small spot spread could be related to the decline in number of beetles during the study  period and/or the heterogeneous patterns of forest stands, soils, and sites in the Piedmont.

Stands infested by SPB in the upper Piedmont of Georgia usually have a large percentage of the host component in shortleaf pine, a high clay content in the surface and subsurface horizons, and slow radial growth during the last 10 years.  These variables must be considered collectively in ascertaining a possible cause-and-effect relationship.

Inherently, shortleaf pine has a shallow root system.  Poor percolation and inadequate aeration in clay soil further restrict expansive root development.  These conditions can also lead to the onset of root disease and the killing of fine roots.  Deteriorating root systems cause a sustained reduction in radial growth.

Physiological stress on trees and stands can be severe during periods of drought or excess moisture.  "Locus" trees-those first attacked and preferred by SPB-appear to be dominant and codominant trees  with large live crown ratios and root systems in incipient stages of decline.  Trees in advanced stages of decline are seldom killed by SPB.  Moisture and nutrient supply in these trees may not be adequate for successful attack or brood production.

Many of the site, stand, and host tree characteristics associated with SPB infestations in the Piedmont of Georgia can be recognized or measured readily in the field.  Cultural treatments should be scheduled for high-hazard stands to reduce or eliminate timber losses from SPB. 

North Carolina
T. E. Maki, D. W. Hazel, and J. R. Hall

Study Area Plots were located in forests adjacent to Kerr Reservoir in Vance County, North Carolina (Fig. 12).  The physiographic character of the area is typical of the Southern Piedmont, with long, rolling hills and well-developed valleys and stream channels. Forest occupy roughly two-thirds of the land in counties around Kerr Reservoir.  About 70 or 80 percent of the commercial forest land is in farm and miscellaneous private individual ownerships.  The levels of forest management vary tremendously, from extensive to none.

Figure 12. - Study area in the Piedmont of North Carolina.

A narrow zone of forest land around Kerr Reservoir has been flooded several times during recent years.  The most severe floods (June-July 1972, April 1975, and May 1978) inundated as much as 20,000 acres of upland forests, causing heavy damage to most hardwood species.

Results and Discussion

Site/Stand Variables. - A total of 163 plots were established.  Unattacked plot data came from nearby control plots.  Comparing the means, we found that the SPB plots and control plots were not greatly different, at least for many factors thought to be associated with stand susceptibility (table 41).  Values for pine BA, hardwood BA, slope, stands age, stand understory, stand density, site index, average d.b.h., and bark thickness at ridge and fissure were similar in both attacked and unattacked stands.  There is a slight trend, however, for higher radial growth rates in unattacked stands.

Comparisons of soil characteristics between attacked and control plots revealed few major differences in the subsurface soils (table 41).  However, percentage of sand and silt in the surface soils differed significantly (P<=.05) between control and attacked plots.  Control plots were on soils with a higher proportion of sand in the top 15 cm.  This prevalence of sand was further seen in the surface soil textural classes for attacked and unattacked plots; 88 percent of the control plots were on loamy sand and sandy loams, while only 59 percent of the attacked plots were the same two textural  classes.

Table 41. - Site and stand characteristics of attacked and baseline plots in natural, undisturbed stands-North Carolina Piedmont 

Variable	Units	Attacked plots (122)		Baseline plots (140)
		Mean	SD	Mean	SD
Slope*	%	7.5	6.3	8.0	7.7
Aspect*	degree	197.8	101.4	198.0	100.8
Surface sand	%	54.5	18.7	58.6	15.1
Surface silt	%	32.2	14.9	27.8	10.6
Surface clay	%	14.3	9.8	13.5	6.4
Surface pH	log 1/ [H+]	5.0	0.4	5.2	0.4
Surface soil depth	cm	17.7	7.7	17.7	6.1
Subsoil sand	%	35.5	15.6	36.6	15.2
Subsoil silt	%	30.6	9.7	28.2	8.9
Subsoil clay	%	33.9	11.7	35.2	11.7
Subsoil pH	log 1/ [H+]	5.2	0.4	5.4	0.4
Pine BA	ft²/acre	128	34.3	134	32.5
Hdwd.BA	ft²/acre	25	26.9	23	23.3
Total BA	ft²/acre	154	39.1	157	36.4
Stand understory	%	62	30.0	61	27.2
Age	years	37	14.0	37	17.0
Density	trees/acre	894	735.3	858	601.6
Site index at 50 yrs	ft	79.7	9.1	81.3	11.8
SPB-killed trees	number	860	1454.8
Avg. bark-fissure	in	0.3	0.7	0.3	0.5
Avg. bark-ridge	in	0.6	0.2	0.7	0.1
Avg. radial growth 0-5 yrs ago	mm	8.6	3.7	9.1	3.3
Avg. radial growth 6-10 yrs ago	mm	10.2	5.0	10.4	3.6

*Mean and standard deviation for those plots with nonzero slope and aspect.  Eighty-three percent of the attacked stands had no slope.

Table 42. - Landform classification of attacked and control plots in natural, undisturbed stands-North Carolina Piedmont

Landform	Attacked (86)	Control (36)
	Percent
Flood plain
Stream terrace
Upland flat	28.0	29.0
Side slope
Steep side slope	5.8	14.2
Ridge	66.2	56.8
Other
	100.0	100.0

An examination of attacked and control plots reveals a higher percentage of attacked plots occurring on ridges while a lower percentage of attacked plots occurred on steep side slopes (table 42).  The absence of plots on side slopes probably represents the field crews' failure to differentiate between side slope and steep side slope.  Similar inconsistencies in field-crew performance in estimating landform were reported by Hicks et al. (1978).

Table 43. - Disturbance categories of attacked and control plots-North Carolina Piedmont

Disturbance	Attacked (109)	Control (54)
	Percent
No known disturbance	87.2	77.8
Logging activity within previous yr
Logging activity more than 1 yr ago	10.0	22.2
Ice and/or hail damage, severe-over one-half of stems affected
Ice and/or hail damage, light-less than one-half of stems affected
Fire within previous yr
Fire more than 1 yr ago
Lightening strike evident	2.8
Chem. brush control within previous yr
Chem. brush control more than 1 yr ago
Wind damage
Other
	100.0	100.0

About 13 percent of the attacked plots were associated with stand disturbances, whereas 22 percent of the control plots were disturbed (table 43).  Old logging activity (>= 1 year ago) occurred in control plots at about twice the frequency it occurred in attacked plots.  Lightening strikes were found only in attacked plots.

Distribution of plots by BA class appears similar for attacked and control plots (table 44).  In both cases, however, mean stand density was high, although the distribution is more closely grouped about the mean for unattacked stands.  The mode is about the same for both (140-160 ft²/acre class).

Table 44. - Basal area class of attacked and control plots in natural, undisturbed stands-North Carolina Piedmont

Basal area class - ft²/acre	Attacked (86)	Baseline (36)
	Percent
<50
50-70
80-100	9.3	2.8
110-130	25.6	25.0
140-160	29.0	38.9
170-190	24.4	22.1
200-220	5.8	5.6
230-250	4.7	2.8
>=260	1.2	2.8
	100.0	100.0
Average BA in ft²/acre	153.6	156.9
Average percent pine	83.3	85.5

Table 45 indicates that 38.5 percent of infestations were < 1/4 acre in size, yet more than 30 percent of infestations covered >2 acres.  The mean size of infestations was 4.9 acres.

Table 45. - Size and frequency distribution of infestations in natural, undisturbed stands-North Carolina Piedmont (n = 109)

Acres (nearest 0.25 acre)		Percent of total
Lower limit	Upper limit
0.00	0.25	38.5
0.5	1.0	20.2
1.25	2.0	8.3
2.25	10.0	24.8
10.25	20.0	1.8
20.25	50.0	5.5
50.25	+	0.9
		100.0

Table 46. - Pine species by tree status on attacked plots-North Carolina Piedmont

	Pine Species	Tree Status
		Dead	Live	Survived attack	Dead- unknown	Total
Frequency row %	Loblolly	211	84	2	0	297
		71.0	28.3	0.7	0.0
	Shortleaf	832	153	14	2	1001
		83.1	15.3	1.4	0.2
	Virginia	36	16	23	0	75
		48.0	21.3	30.7	0.0
	Totals	1079	253	39	2	1373

A contingency table of pine species by tree status (table 46) demonstrates the relative susceptibility of loblolly, shortleaf, and Virginia pine to SPB.  The X² test was significant, indicating interaction of tree species and tree status.  Of 297 loblolly pines on attacked plots, 71 percent were attacked.  Of the shortleaf pines, 83 percent were attacked while only 48 percent of the Virginia pines were attacked.  The survived attacked category is also of interest.  Trees in this category had evidence of previous attack by SPB (pitch tubes, etc.) but were apparently living when the plots were established.  About 31 percent of the Virginia pines had survived attacks, whereas only about 1 percent of loblolly and shortleaf pines did so.

The evidence suggests that, in the Piedmont, shortleaf pine is more prone to attack by SPB and to death when attacked.  One reason for shortleaf's apparent susceptibility may be the possibility of association with incipient presence of littleleaf disease  ( Phytophthora cinnamoni ).  Many shortleaf pines show signs of littleleaf, especially those close to water.

[ Contents ]   [ Previous ]   [ Next ]   [ Home ]