Mortality Caused by Southern Pine Beetle

In the southern Appalachians, most pine species can be considered hosts for the southern pine beetle (SPB; Dendroctonus frontalis), although historically the SPB has been associated primarily with yellow pine species (subgenus Diploxylon). Yellow pine species native to the southern Appalachians include pitch pine (Pinus rigida), shortleaf pine (P. echinata), Virginia pine (P. virginiana), and table mountain pine (P. pungens), as well as small populations of planted and naturalized loblolly pine (P. taeda). More recently, eastern white pine (P. strobus), which occurs throughout the southern Appalachians, has also been under attack by the SPB, resulting in widespread mortality. There have also been reports of isolated SPB attacks on eastern hemlock (Tsuga canadensis).

SPB populations, as with most insect species, are cyclic. The coincidence of SPB outbreaks at epidemic levels with severe and sustained drought can result in significant impacts on pine populations, particularly the yellow pines. The primary mechanism used to combat bark beetle attack is the production of large quantities of oleoresins at the point of attack. Drought stress reduces oleoresin flow and pressure, disabling the trees' primary defense system (Lorio and Hodges 1977). During the period 1971-1988, the land area occupied by yellow pine species in the Coweeta Basin was reduced by 97% as a result of SPB attack (Smith 1991). More recently, watersheds planted in eastern white pine within the Coweeta Basin sustained heavy mortality caused by SPB attack. There are two monocultures of eastern white pine at Coweeta: one in the north-facing watershed 17 and another in the

Table 3.1 Eastern white pine density (stems ha-1) and basal area (m2 ha-1) (mean + SE) characteristics for the south-facing watershed 1 and north-facing watershed 17 at Coweeta Hydrologic Laboratory near Otto, North Carolina, USA.

Watershed

Variable

Total

Living

Dead

Beetle-Killed Deada

1

Density

1064 ± 56

695 ± 36

369 ± 39

33 ± 22 (4.7)

Basal area

66.2 ± 1.9

55.0 ± 2.4

11.2 ±1.9

2.1 ± 1.4 (3.8)

17

Density

899 ± 34

600 ± 23

299 ± 19

7 ±6 (1.2)

Basal area

68.3 ± 2.0

59.3 ± 1.7

9.0 ± 0.8

0.6 ± 0.5 (1.0)

aValues in parentheses are the percentage of mortality caused by southern-pine beetle attack.

aValues in parentheses are the percentage of mortality caused by southern-pine beetle attack.

south-facing watershed 1. On the more exposed watershed 1, mortality from the SPB was estimated to be 5% of the density and 4% of basal area (table 3.1). By contrast, in north-facing watershed 17, only 1% of the density and 1% of the basal area was lost (table 3.1). The apparent difference in susceptibility to SPB attack between the two watersheds suggests the linkage between moisture availability (higher on the north-facing watershed) and susceptibility to attack.

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