Thinning Practices in Southern Pines - With Pest Management Recommendations
United States Department of Agriculture, Forest Service, Technical Bulletin 1703, December 1985.
Management Recommendations to Reduce Losses
Any thinning strategy must consider the potential hazards associated with intensive silvicultural practices. The following management practices are recommended to minimize the impact of damaging organisms and environmental factors on pine stands.
Southern pine beetle. Southern pine beetle infestations are often associated with poor tree vigor. Because tree vigor is basically related to site, tree, stand, and environmental conditions, the development of southern pine beetle outbreaks is strongly influenced by these conditions. Though vigor is difficult to quantify, radial growth rate can serve as a strong indicator of tree condition on vigor. Other factors that affect vigor include age, stand density, species composition, soil texture, and type, drainage patterns, and stand disturbances associated with cultural practices.
pine beetle hazard is recommended when basal area approaches 120 square feet/acre or when live crown ratios drop to about 40 percent. A carefully carried out thinning will stimulate radial growth, reduce evapotranspiration, and increase rain throughfall. The reduction in evapotranspiration slows down the exhaustion of groundwater supply and favors continued diameter growth. The prevention of severe water stress results in lower concentrations of monoterpenes and higher levels of resin acids, which could be involved in making the stand less attractive to beetles (Hodges and Lorio 1975).
Pine stands in low-lying areas are frequently subjected to flooding and become attractive to southern pine beetle. In these areas, thinning alone may not correct the problem. Additional management actions such as drainage to divert excess water may be needed.
Any thinning strategy to reduce the risk of southern pine beetle attack should be compatible with management goals and consider such things as site and stand factors, equipment, seasonality, and product objective. Management of other potential hazard (e.g., annosus root rot, Ips spp., and black turpentine beetle) that might conflict with recommendations for southern pine beetle must also enter into the decisonmaking process.
Annosus root rot. Thinning is the single most important factor contributing to annosus root rot in pine stands, since cutting exposes stump surfaces to infection. Damage due to the fungus increases with time after thinning up to about 8 years, after which the damage level stabilizes. Because annosus spore production is at its highest level in January and February, thinning during the winter increases the likelihood of infection. In addition to thinning, species susceptibility, virulence of the disease, deep sandy soils, low soil organic matter, air temperatures below 70ºF, duration of stump susceptibility, and pruning contribute to and/or facilitate infection.
A comprehensive survey of annosus root rot damage in planted and natural stands throughout the South revealed that 2.8 and 0.07 percent, respectively, were infected. In scattered high-hazard areas, the 5-year loss in volume following thinning was estimated to be 20 percent of the stand (9 of 46 cords/acre). In general, volume loss following thinning ranges from 0.1 to 0.5 cord/acre/year (Alexander et al. 1981).
For high-harzard sites, the following measures are recommended for minimizing losses to annosus root rot (Kuhlman et al. 1976):
There is some evidence that prescribed burning will reduce the severity of annosus root rot in thinned plantations (Froelich et al. 1978). On low-hazard sites, chemical treatment of the stumps is of doubtful value (Hodges 1974). It is generally believed that on sites rated low hazard for annosus root rot, no restrictions on thinning are necessary. Although the best strategy for reducing the disease on high-hazard sites may be to delay or do no thinning, stands on low-hazard sites may be thinned based on normal silvicultural prescriptions dictated by product objectives, biological constraints, and desired capital recovery. As recommended above, stumps on high-hazard sites should be treated with borax and, in stands with confirmed root rot, Peniophora gigantea (Fr.) Massee, a saprophytic fungus, and, when possible, thinning should be done during the hottest months of the year (May-August) to take advantage of high temperatures and low spore production and survival conditions. Prescribed burning may be done before and after thinning to further insure the protection of residual stands from infection.
If spacing is wider than 8 by 8 feet, and the product objective is pulpwood, thinning may be foregone, particularly on high-hazard sites. Chemical thinning should be done for precommercial thinning on high-hazard sites.
Because thinning to reduce the hazard of southern pine beetle incidence conflicts with management recommendations for annosus root rot, foresters should be aware of the tradeoffs in areas where both pests are likely to occur. Benefits must be weighed against potential losses for any chosen thinning strategy. In most cases, thinning should be done in the winter to reduce the hazard of southern pine beetle infestation, and the stumps should be treated with borax to prevent annosus infection.
Fusiform rust. Losses due to fusiform rust have been estimated to exceed $25 million annually in value, making it the most economically damaging disease of southern pines. Slash and loblolly pines are the preferred hosts, slash pine being the more seriously affected. The disease is more severe in plantations than in natural stands, with mortality occurring primarily in the seedling stage.
Interestingly enough, cultural practices that favor fast growth of stands increase the incidence of fusiform rust. However, to prescribe against cultural practices that improve growth is not silvicultually and economically sound. It has been claimed that even a 50-percent rust infection rate in a stand can be offset by an increase in volume resulting from such intensive cultural practices as site preparation and fertilization.
Thinning has little or no practical value of reducing the incidence of fusiform rust because infection occurs at the early stages of stand development. It must, therefore, be practiced for a different purpose – to minimize losses due to rust, i.e., salvage. The first 5 years after planting are the critial period. Precommercial thinning may not be justified and may aggravate the problem by increasing the surface area for infection and by preventing natural pruning. This implication is supported by the finding that close spacing reduces fusiform rust incidence. Heavy thinning may also have an adverse effect by favoring the growth of alternate hosts (oaks), thereby enhancing rust incidence. Thinning of heavily infected stands, on the other hand, can profoundly affect total wood production if heavily infected trees certain to die before final harvest are removed. If rust incidence is less than 25 percent, the first thinning should remove the majority of the diseased trees. Opening up the stand too much can have unfavorable consequences on the residual stand in terms of growth and damage from ice and wind.
Wind/windthrow. Wind and windthrow are natural phenomena that cause extensive damage to southern pine stands. The severity of damage depends on geographic location, wind gustiness, and other factors.
In a number of studies, thinning influenced the amount of damage due to wind and windthrow. The heavier the thinning, the greater the wind damage. More crown damage (limbs and small branches broken off, needles and bark whipped off) occurred in heavily thinned stands. Although thinning in general increases wind and windthrow damage, it can potentially reduce such damage by removing diseased, high-risk trees. Because bigger trees are more prone to windthrow, an early thinning will improve the stability of stands after the remaining trees have adapted to greater exposure.
The formulation of a thinning strategy within the Coastal Plain hurricane belt should take into account the possibility of windthrow damage. The following considerations could help in developing an optimum thinning strategy:
Ice/glaze. Slash, longleaf, and loblolly pines are generally more susceptible than shortleaf pine to glaze damage. Glaze damage can be very serious, depending on species, geographic location, age of trees, amount of ice formed on trees, stand density, presence of disease, crown characteristics, and diameter/height ratio. Thinning has very profound effects in modifying the degree of glaze damage. Studies have shown that increasing thinning intensity causes increasing amounts of glaze damage.
Glaze damage can be minimized by early manipulation of the growing space (precommercial thinning) to develop trees with sturdy, compact crowns. Adequate stocking must be maintained to provide mutual support among trees (Brender and Romancier 1965; Lemon 1961). In ice storm belts, loblolly should be thinned lightly (no more than one-third of basal area at a time) and more frequently (from below or selectively). If selective thinning is not feasible nor practical, row thinning at wider intervals (say every eighth or tenth row) with selective thinning within leave rows would be a desirable alternative (Shepard 1975). Selective thinning should remove the smaller, weaker trees. Bent trees should be pruned (Williston 1974).
The following practices are suggested to limit damage to pines resulting from felling:
Skidding-related damage can be reduced by adhering to the following practices:
It has been suggested that 40 years are required for natural forces to bring soil conditions in loblolly pine stands of the southeastern United States back to normal (Hatchell and Ralston 1971). With regard to ameliorative conditions and restoration, the presence of logging residues after delimbing contributes to the prevention of significant compaction (King and Haines 1979). Disking, ripping, and subsoiling also ameliorate compacted soil conditions (Hatchell et al. 1970; Moehring 1970; Peters 1977).
Bedding (or a moderate amount of fertilizer) has been shown to improve growth of loblolly pine on compacted soils with a greater growth response obtained than on uncompacted soil. This has been attributed to elimination of competing vegetation by skidding (Hatchell 1981). Other biotic factors important in natural recovery of compacted soil include increased percolation rates and noncapillary pore space attributable to deep-rooted species like kudzu and alfalfa (Uhland 1950).
Dense layers of herbage aid in preventing soil compaction, just as does slash placed on the skid roads and over areas of heavier traffic. Increased herbage also reduces the amount of erosion, rain impact, vehicle impact, and high soil temperatures resulting from direct solar radiation.
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