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.

Georgia Mountains

R. P. Belanger and G. E. Hatchell

Study Area

Stand, site, and soil conditions were studied from 1975 through 1977 in the mountains of northeast Georgia.  The survey included all or part of Fannin, Lumpkin, Rabun, Towns, and White Counties (Fig. 13).  All attacked plots occurred in natural stands, and most were in National Forests.  SPB populations during 1975 were in decline from epidemic conditions; populations were generally endemic during 1976 and 1977. Additional Procedures Forest Service inventory statistics were used as baseline data to characterize representative forest types and conditions (Information supplied by Renewable Resources Evaluation, USDA For. Serv., Asheville, N.C. Data collected July-September 1972.).  These baseline data came from the same counties as attacked plots.

Figure 13. - Study area in the Mountains of northeast Georgia.

Results and Discussion

Plots attacked by SPB in the mountains of Georgia (table 47) were older and more heavily stocked than stands representative of the area (table 48).  Baseline stands averaged 33 years of age and contained 103 ft² of pine and hardwood BA per acre.  Attacked plots averaged 62 years of age and supported 129 ft² of stocking per acre.  More than 80 percent of the attacked plots had BA > 100 ft²/acre (table 49).  Woody understory vegetation covered an additional 67 percent of the area in the lower canopy.  This is extremely high density for site index 66 lands to support.

The high BA associated with attacked plots is reflected in the slow growth rate of the pine overstory.  Radial growth on dominant and codominant trees averaged 16.7 mm during the last 10 years and 7.7 mm during the last 5 years.  These mature stands are in an obvious state of decline.

Comparisons between attacked plot data and baseline data indicate that SPB more frequently attacked shortleaf pine and pitch pine  ( Pinus rigida Mill. ).  Shortleaf pine areas accounted for 59 percent of the infestations; 23 percent occurred in the pitch pine type (table 50).  Yet these two species represent only 29 and 6 percent of the total pine component in the area.  Virginia pine is the dominant pine species in the mountains of Georgia.  A small percentage of the infestations occurred in this type.  No SPB infestations were located in eastern white pine  ( Pinus strobus L. ) stands.  These levels of susceptibility between species were also evident from examination of individual tree characteristics (Belanger et al. 1979).

More than 90 percent of the infestations were < 1 acre in size; 54 percent were < 1/4 acre in size (table 51).  The small sizes could be related to the decline in number of beetles during 1976 and 1977, and/or the heterogeneous patterns of forest stands and sites in the mountains.  The largest infestation studied -20 acres-occurred in a shortleaf pine and pitch pine mixture during a peak period of beetle activity.

Table 47. - Site and stand characteristics of attacked plots in natural, undisturbed stands-Georgia mountains

Variable	Units	Attacked plots (22)
		Mean	SD
Slope	%	24.9	10.3
Aspect	degree	161.6	92.7
Surface sand	%	53.6	11.9
Surface silt	%	53.6	7.2
Surface clay	%	27.0	9.9
Surface pH	log 1/ [H+]	4.7	0.3
Surface soil depth	cm	13.5	6.4
Subsoil sand	%	44.0	14.7
Subsoil silt	%	18.5	6.0
Subsoil clay	%	37.5	12.2
Subsoil pH	log 1/ [H+]	5.0	0.2
Pine BA	ft²/acre	92	31.1
Hdwd.BA	ft²/acre	37	20.3
Total BA	ft²/acre	129	34.5
Stand understory	%	67	21.5
Age	years	62	19.0
Density	trees/acre	658	597.6
Site index at 50 yrs¹	ft	66	5.8
SPB-killed trees	number	42.8	68.1
Avg. bark-fissure	in	0.5	1.0
Avg. bark-ridge	in	0.8	0.2
Avg. radial growth 0-5 yrs ago	mm	7.7	3.3
Avg. radial growth 6-10 yrs ago	mm	9.0	3.5

¹Based on height of shortleaf pine.

Table 48. - Basal areas and age of baseline plots-Georgia Mountains

Variable	Units	Baseline
		Mean
Pine BA	ft²/acre	76
Hdwd. BA	ft²/acre	27
Total BA	ft²/acre	103
Age	years	33

Table 49. - Basal area class of attacked plots in natural, undisturbed stands-Georgia Mountains

Basal area class - ft²/acre	Attacked (86)
	Percent
50-70	13.6
80-100	4.6
110-130	31.8
140-160	40.9
170-190	9.1
	100.0
Average BA in ft²/acre	128.6
Average percent pine	71.4

Table 50. -  Pine types of attacked and baseline plots-Georgia Mountains

Pine type*	Attacked	Baseline
	Percent
Shortleaf	59	29
Virginia	9	55
Pitch	23	6
Eastern white		10
Loblolly	9
	100	100

*Stands in which one species represents 70 percent or more of the crowns in the dominant and codominant positions.

Table 51. - Size of frequency distribution of infestations in natural, undisturbed stands-Georgia Mountains

Acres (nearest 0.25 acre)		Percent of total
Lower limit	Upper limit
0.00	0.25	54.4
0.50	1.00	36.5
1.25	2.00	4.5
10.00	20.00	4.5
		100.0

Table 52. - Landform classification of attacked plots in natural, undisturbed stands-Georgia Mountains

Landform	Attacked (22)
	Percent
Bench	4.6
Cove	22.7
Northerly aspect slopes - <2500 ft	13.6
Southerly aspect slopes - <2500 ft	18.2
Southerly aspect slopes - 2500-4500 ft	22.7
Ridge and upper slope - < 2500 ft	13.6
Ridge and upper slope - 2500-4500 ft	4.5
	100.0

Soils on the study plots were formed from predominantly mica and mafic parent materials.  Laboratory analysis indicate that the average texture of the surface horizon from 0 to 6 inches is sandy clay loam and the B horizon is sandy clay.  Average depth of the A horizon was 13.5 cm.  Attacks were noted across a wide range of landforms (table 52).  More infestations occurred on southerly aspects than northerly aspects.  South-facing slopes are where most of the pines occur in the Southern Appalachians.

Stands attacked and killed by the southern pine beetle in the Mountains of Georgia were characterized by dense stocking, slow radial growth, and overmature, predominantly pine species.  Although silviculture techniques offer means of reducing pine mortality from beetle attacks (Belanger et al. 1979), accessibility of stands, market conditions, and stumpage values often prohibit the intensive management of these problem locations.  Primary emphasis should be given to highrisk stands that contribute significantly to management objectives.

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