Does Red-cockaded Woodpecker excavation of resin wells increase risk of Bark Beetle infestation of cavity trees?

Auk, The, Jan 2001 by Conner, Richard N, Saenz, Daniel, Rudolph, D Craig, Ross, William G, Et al

The Red-cockaded Woodpecker (Picoides borealis) is unique among North American woodpeckers in that it nests and roosts nearly exclusively in living pines (Pinus spp.). Red-cockaded Woodpeckers make daily excavations at small wounds, termed "resin wells," around their cavity entrance and on the bole of their cavity tree, from which resin flows down the tree (Ligon 1970). The woodpeckers also flake off loose bark which results in a smoother surface on the pine trees bole. Those behaviors result in a resin barrier that serves as an effective defense against rat snakes (Elaphe spp.; Jackson 1974, Rudolph et al. 1990). Rat snakes regularly attempt to climb active Red-cockaded Woodpecker cavity trees (cavity trees currently in use for nesting and roosting) and are known to prey on Red-cockaded Woodpeckers when the resin barrier is inadequate (Jackson 1978b, Neal et al. 1993). The resin barrier is believed to increase the probability of a breeding pair's nest success and survival of roosting woodpeckers (Conner et al. 1998).

Red-cockaded Woodpecker cavity trees in eastern Texas, especially active cavity trees, are regularly attacked and killed by southern pine beetles (Dendroctonus frontalis) and occasionally by various species of engraver beetles (Ips spp.; Conner et al. 1991, Conner and Rudolph 1995, Rudolph and Conner 1995). The pine tree's resin, which woodpeckers use to create a barrier against rat snakes, serves also as the pine trees primary defense against bark beetle infestation (Wahlenberg 1946, Hodges et al. 1977, Conner et al. 1998). The resins flow rate and total production (yield) influence the pine tree's ability to physically repel a bark beetle attack. However, daily maintenance of resin wells by woodpeckers may decrease the pine trees resin yield, and thus, reduce its ability to repel attacks by bark beetles.

We examined resin yield and bark beetle infestation rates in Red-cockaded Woodpecker cavity trees in longleaf (Pinus palustris), loblolly (R taeda), and shortleaf (P echinata) pines. Longleaf pine is widely known to produce greater yields of resin than loblolly and shortleaf pines and, as a result, is much more resistant to bark-beetle infestation (Hodges et al. 1977). Thus, if Red-cockaded Woodpeckers affect the ability of cavity trees to produce resin, the effect would most likely occur in loblolly and shortleaf pines. Also, if woodpecker activity at resin wells does increase susceptibility to bark beetles, the increase in bark-beetle-induced mortality should be greater in loblolly and shortleaf pines than in longleaf pines.

Methods.-We determined causes of mortality of Red-cockaded Woodpecker cavity trees on the Angelina National Forest (62,423 ha; 31 deg N15'N, 94 deg N15'W) in eastern Texas. The northern portion of the forest is predominantly covered by a mixture of loblolly and shortleaf pines on shrink-swell soils, whereas, longleaf pine is the dominant tree species in the deep sandy soils in the southern portion of the forest. Only a few remnant longleaf pines still occur on the northern portion of the Angelina National Forest. Small subpopulations of Red-cockaded Woodpeckers occur on both portions of the national forest (Conner and Rudolph 1989).

We visited all active and inactive (cavity trees previously used but currently not being used by woodpeckers) Red-cockaded Woodpecker cavity-tree clusters (a cluster is the aggregation of cavity trees used by a group of woodpeckers) during March through June from 1983 through 1998 to evaluate cavity tree status and condition. We used woodpecker activity at resin wells, amount of bark scaling, and condition of the cavity entrance as indicators of tree status (see Jackson 1977, 1978a). Active cavity tree clusters were visited several times per year. The age of many cavities within particular trees was determined by the year (and month if possible) they were completed, not the year that excavation began (see Conner et al. 1998). During each visit, we determined occurrence and causes of cavity tree mortality, such as wind throw, wind snap, fire, bark beetles, and lightning (see Conner et al. 1991). Cavity trees infested by bark beetles typically had numerous white "popcorn-- like" pitch tubes of crystallized pine resin around wounds where individual attacking beetles had chewed through the bark and into the cambium of the pine trees bole, or many small "shotgun-pellet-- like" holes from which brood beetles had emerged. Dead cavity trees with signs of bark beetle infestation were examined closely to determine whether a lightning strike had contributed to the tree's death. Here we report observations for cavity trees that were infested and killed singly by bark beetles and not those killed during the growth of a beetle spot where multiple trees die in an expanding infestation. During such large infestations and epidemics, any pine tree in close proximity can be overwhelmed by the sheer numbers of bark beetles, regardless of the pine tree's ability to produce pine resin (Billings and Varner 1986). As a measure of beetle population levels, we obtained records of annual number of southern pine beetle infestations (beetle spots) and number of pines infested on both northern and southern portions of the Angelina National Forest in forest compartments where Red-cockaded Woodpeckers occur from the United States Forest Service Pest Management Office in Pineville, Louisiana (SPBIS, Southern Pine Beetle Information System data base).