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Contents of Heikkenen

Water to Wood

Herman J. Heikkenen and Robert H. Giles, Jr., July, 2001

CONTENTS Abstract Overview Beetle Distribution and Control Efforts Dynamic Stand Density Occupied Volumes Standing Volume Per Acre Soil Volume Soil Moisture for the Trees Soil Water Volume Site Index Temperatures of the Tree Bole Root Fungi and Tree Death Tree Mortality So What: The Management Implications

Abstract: This document describes the hypothesis that advanced-age loblolly pine tree (Pinus taeda) stands undergo temperature stress, overheating when they are unable to sustain water "flow" within their system. If they are are unable to achieve satisfactory water budgets of individual trees, then trees over-heat and die. When they die, they emit substances equivalent to food- and sex-phermone-signals that attract the southern pine beetle, Dendroctonus frontalis. Similarly, stressed roots develop fungal disease symptoms.

Insects and fungi are decomposition agents in the forest. Insects, such as the southern pine beetle, are not the cause of tree death but the result of pine trees being planted on the wrong sites (improper soil moisture and insolation), too densely, or allowed to grow to a size for which they cannot achieve a satisfactory water budget.

Investigations reported herein are combinations of literature that support, suggest alternatives, or refute the hypothesis, simplistic computer-aided computations of reasonable field conditions, and of preliminary studies conducted by the senior author. A major consequence, if the hypothesis is supported, might be to prompt re-evaluating management and control strategies for the southern pine bark beetle. "Beetle-control" now seems meaningless in the context of what is needed, i.e., superior management of forests having pine stands or land with pine stand sites.

Overview

This paper reports on investigations of an hypothesis of the senior author that he developed over many years of observations and study. The complex hypothesis is that tree mortality occurs when there is a major imbalance in the energy budget of the tree, a budget dependent on heat-exchange by water. That amount of water is related to available soil moisture and evapotranspiration. As trees grow, the cell water they require (simply because of increased tree size) increases, and evapotranspiration from the stand of trees increases. Sufficient water to sustain such a "flow" of water is needed. Rarely discussed, a sufficient volume of water moving in trees in the growing season (related to evapotranspiration) is needed for cell cooling.

If excessive soil water occurs, root suffocation occurs. Also, if insufficient water exists for continuous water movement, the tree dies. This "multi-pathway" situation with its various probabilities is difficult to analyze with classical statistics.

Sketch of natural juvenile insect hormones and mimics suggesting the nature of the substances implied herein (from Bowers et al.1976:543)

The hypothesis, if accepted, might lead to a series of conclusions about the beetle, its perceived damage, needs for control, and the need for modern sophisticated forest management. The pine stand management activities would include preventative tactics such as properly selecting sites (pines planted or encouraged only on the right soils), properly placing well-spaced trees, and harvesting pines at the right time just before they become stressed, all in lieu of costly and largely ineffective direct insect control efforts.

Evidence for death in pines may be delayed for 2 years. As commonly observed in "Christmas trees", trees may be severed and dead, but may remain green for many days. The primary visual evidence for forest tree death is browning of needles and then leaf abscission. Unseen, Alpha-pinines and other volatiles emerge from the tree (well-known since 1964 Vité et al. 1964). Beetles are attracted to these substances (Gara and Coster 1968), exudates of dead trees. Overhulser et al. 1980:163-165) showed that hollow fibers with a synthetic sex attractant can control effects of the western pine shoot borer, suggesting a similar strategy might be effective in "confusing" the mating behavior of southern pine bark beetles. On warm winter days following cold day, clear pheromone signals emanating from point sources will typically be borne by air which has been cleansed by rain. Barras and Hodges (1974) sampled all stages of beetles from trees in April " when the beetle is considered to be in the most vigorous condition." Activity of beetles at spots does not usually persist over winter (Doggett 1971). The rate of gallery growth varies with the condition of the cambium (Dixon and Osgood 1961). Berryman (1974:579) said migratory behavior was characterized by an initial search for suitable hosts by "pioneer" beetles and then by colonization or mass attack of selected trees by large numbers of beetles. Heikkenen, however, suggests the alternative that dead trees emit a sex-pheromone or prize-food-attractant that is equivalent to the sex pheromone and that a "search" is not involved. Once the attractant is present, then large numbers of insects fly to the tree. There, they are evident.

Contrary statements about the beetle and its effects were being made as late as 1996.

The southern pine beetle attacks all southern yellow pine species and kill entire stands very quickly. These insects favor slow-growing, over-stocked pine stands. Pine beetle risks can be reduced by thinning pine stands before their growth stagnates. However, once a large population of pine beetles is established in an area, all pines stands are at risk, including healthy ones.

Thomas and Hodkinson (1991), though working with lepidoptera, added support to White's hypothesis that when plants are subjected to stress, the tissues become more palatable to insect herbivores, and that the effect will be greatest for insects feeding on plants growing in soils of low nitrogen status which are subjected to drought stress. When food is available, insects eat. Maybe not, ...maybe they just congregate ...there is little innerbark energy to eat! ...But perhaps it is of such high quality (for protein building and reproduction) that it must be sought.

Distribution of the loblolly pine (Pinus taeda). Each dot represents an average of 5 million cubic feet (Janssen and Weiland, USDA Southern Forest Exp. Sta. 1960).

Beetle Distribution and Control Efforts

Rarely are insecticides used on bark beetles but there is associated with these substances the left-over concept of rapid treatment when evidence is found.

"Often the rationale for treating such areas (as those in which spruce budworm is evident) is the need to destroy foci of infestations or to halt the spread of the pest insect into commercial areas. This is a concept that has not be adequately studied, and evidence is contradictory as to whether such movements really occur (Blais 1973). Unfortunately, since many of these areas are are inaccessible except by air, there is little in the way of evaluation except for aerial interpretation of defoliation, which is almost always an inadequate means for evaluating actual population movement." (Natl. Acad of Sci. 1975:68).

The "foci of infection" may be small groups of trees within stands that are moisture stressed. Small spots within stands, the so-called "foci" can be very different and may be flooded (by seeps and springs) or very dry from delayed evident combinations of upland topography, soil dryness, and adjacent tree competition.

Although this document is about the southern pine beetle and its relations to trees, observations of the mountain pine beetle ( Dendroctonus ponderosae Hopkins) seem to us to be supportive of the concepts presented here. Mitchell et al. (1983) showed that if they thinned lodgepole pine stands (Oregon) they improved tree vigor and reduced "beetle attack. " We think their well-made observations were biased by a theory that "beetles kill trees." They found that dead trees (with beetles present) were numerous in inthinned and lightly thinned stands where current growth was low. They identified a vigor rating of 100 when beetles were first observed, "were beginning to suffer beetle attack." They observed no beetle-related activity or tree death in vigorous stands. As claimed herein as part of the hypothesis being developed, thinning to reduce stresses of moisture competition and removal at an appropriate large size can avoid or reduce beetle problems ... but not the "beetle attack."

As long ago as 1975, after committee and publishing delays, the National Academy of Science(1975:16) noted that "Organized timber products harvesting has permanently eliminated overmature timber on most commercial forests, with consequent reduction in susceptibility to the pests of over-maturity. That process has changed and continues to change the nature of the forest pest problem, but this in no sense reduces its importance." Part of Heikkenen's concept is that loblolly pine maturity occurs earlier than in many other trees and unless the trees are harvested, they become susceptible to the "forest pest problem." Few people comprehend that on the 750 million acres of forests of the US, less than two thirds of the area is subject to timber harvests (Natl. Acad. Sci 1975:15). Bennett (1971) discussed some of the ideas related to the silvicultural control of the pest bark beetle. Lorio and Hodges(1968) long ago presented data on moisture stress in southern pines that directly relate to the ecological factors influencing the beetle. Lorio and Bennett (1974) reported recurring infestations associated with high stand density. Why these concepts have not been assembled and have not been included in solutions to the bark beetle problem remains a mystery.

Table 1. Metrics, coefficients, and transformations used herein.

1 acre = 0.4047 hectares 1 acre = 43560 square feet 1 cubic foot per acre = 0.06997 cubic meters per hectare 1 square foot per acre = 0.2296 square meters per hectare 1 cubic foot = 0.02832 cubic meters loge /2.302585 = log 10 1 bar = 0.987 atmospheres -15 bars = point of permanent wilting death of many plants.

Dynamic Stand Density

Heikkenen has described for many audiences the situation of a stand of planted loblolly pines . The tightly-planted (or well-seeded) seedlings grow on various soils to a height of about 5 feet. They reach that height having been subjects of an extremely variable tree and root environment beyond reasonable modeling (requiring data on soils, planting depth and root form, vertebrate animals, insects, competition, shade, cultural practice, and others). A straight-line growth from zero to that height is assumed.

Lorio and Bennett (1974) said control might be facilitated if beetle attacks can be linked to specific forest conditions and environmental factors. As for many biologists, they did not realize that several different conditions can produce the same apparent results (equifinality) as in the case of drought or excessive moisture that produces the death responses of mature pines that attract beetles.

Pines seeded or planted are numerous and the count per acre varies with the selected spacing. See Table 1 and 2.

Table 2. Trees typically expected within an acre when planted at the named spacing
Spacing (in feet)	Trees per acre
3 x 3	4,840
5 x 5	1,742
7 x 7	889
10 x 10	436
13 x 13	258
17 x 17	150

Soil Moisture for the Trees

The high moisture content of beetles corresponds with inner-bark moisture, often over 200% (p.3). (172-268%; mean being 214) near parent beetle galleries. Outer-bark moisture of course is lower, (27-45%).

Merkel (1956) concluded that mild winters and soil moisture deficits are important factors contributing to outbreaks in the southern Appalachians.

Dixon and Osgood (1961:14) called for intense study of moisture content of bark and phloem, which they thought to affect survival of broods of the beetle. The role of moisture stress has been known. Dixon and Osgood (1961: 11) said

" Many factors may predispose trees and stands to attack ... and contribute to development of epidemics. Such factors as drought, rainfall deficiencies, rainfall excess ... are complex in themselves and in their interrelationships ..."

Schultz and Hewlett (1977) observed that forests grow on immense supplies of soil moisture. The ability of trees to use this soil moisture depends upon the physical state of the water in the zone of aeration. The soil moisture reservoir yields water upward to tree roots in such as way as to make serious forest drought a rarity and the concept of soil moisture availability relatively meaningless (Schultz and Hewlett 1977:15). These authors thought "field capacity" was now a useless concept and "permanent wilting percentage" was not a constant for all vegetation. "However, the wilting percentage is a good index of the lower limit of available soil moisture because the relation of soil moisture to soil water potential is very flat in the region of -15 bars." (Schultz and Hewlett 1977:8). Soil water is typically always on the move. When conditions are dry, it moves upward. Rain moves downward. Thus, no limit to "available water" is possible. Deep-rooted trees may draw on moisture below 3m. Trees may experience diurnal internal moisture stress that will affect photosynthesis and the growth process. The trees may be able to re-hydrate at night.. Available moisture in the Southeast is dynamic, variable and "impossible to estimate by usual means "Schultz and Hewlett 1977:11).

Similar moisture stress related phenomena seem to be evident in ponderosa pine stands (Yazvenko, S.B. and D. J. Rapport. 1997.) and even in long-lived yellow or Alaska cedar (Chamaecyparis nootkatensis)(Hennon and Shaw 1997). In the latter description, complex interactions are suggested leading to a natural decline in the species ... a phenomenon likely to be played out rapidly in relatively short-lived loblolly pine stands

Spurr (1952:289) and other notable forest observers have concluded that "competition for soil moisture is more often than not the factor which limits growth." Growth will not vary much for widely different stem densities if soil moisture is fully utilized. Use of soil moisture is highly dependent on the extent of ectomycorrhizal infection and development, the ancillary "root hairs." Such developmenbt is a function of soil fertility. Marx et al. 1977 showed that it was the sugar in the soluable carbohydrate pool in short roots of loblolly pine that influence to this highly desireable ectomycorrhizal infection by Pisolithus. The sugar in the root is not a reducing-sugar as once believed but the nonreducing sugar, sucrose, and sucrose levels are highest in low N and P concenetrations. If high stem density reduces photosynthetic rates, growth can be reduced even though moisture is fully utilized. "...the poorer the site, the more space must be given to a tree in order that it may obtain enough soil moisture and nutrients for growth"(Spurr 1952:294). Studies of forest stand composition and growth have found them "...not clearly related to individual soil types but...related to a broad grouping based on soil moisture" (Spurr 1952:303). Later (1952:303) he observed "that soil moisture is in many cases the one soil characteristic most closely related to site." Campbell (1975) found volume growth of loblolly pines of direct-seeded pines (tending to be dense) was lower than those in similar planted stands in Louisiana.

Dynamic Stand Density

Trees-per-acre declines over time, a response typically said to be due to mortality from light and moisture competition. In some forests with row planting, thinning is done with the removal of every other row, a removal or mortality of about half (depending on area configuration and odd-even row removal).

Spurr (1952:278) described the number of loblolly trees per acre from permanent plots as

logN = 4.075 -1.603 logD

where N is the number of trees and D the diameter of the trees of the average basal area. Intuitively, as the trees age and grow (seen in diameter and height change) some die and eventually the site, whatever its characteristics, is fully utilized by trees of advanced age. [We may encounter the logarithmic decline rate of 1.6 later.] As spatial limits (which are limits to physical space that is present, nutrients, moisture, and solar radiation) are reached, trees die. The results for a series of diameters is shown in Table 3.

A curve developed by us from trees per acre in a site-90 loblolly pine stand (age 15 to 60 is

lnN = 6.9753 - 0.0312 X (2)

where lnN is the logarithm (base e) of the number of trees per acre and A is age of the stand. The R is 0.98 and standard error 0.055. When values are inserted for A as 15, the estimate is 670 trees (640 was expected) and when A is 60, then the estimate was 164 (175 was expected). The curve is only approximate and over estimates numbers in the younger ages, underestimates those in older stands.

The number of trees per acre can be estimated by

N = (183.3465 B) / D²

(Spurr 1952:280). Stand basal area is better correlated with growth (the value of interest) than is stem density so interest in density has been limited. It needs to be restudied in relationship to available water per tree within a unit area.

The geometry of trees per acre is probably best described for natural stands as a mosaic of hexagonal spaces. Trees in nature are clumped. Trees, however, are planted in rows and thus in rectangles and the roots over time intertwine in approximately equal rectangular areas. Where there are 175 trees at age 60 (past currently used harvest age) then there are 249 square feet per tree and on average. They are about 16 feet apart. On superior sites there are reported to be 105 trees per acre (thus about 20 feet apart). We use the declining numbers estimated by the above equation (2) aware that the early-year numbers are over estimated and the number in later years under estimated.

Standing Volume Per Acre

Heikkenen retained interest in the volume of "the outer cone", the annual growing and transpiring volume.

Williston and Dell (1974) showed 2.1 cords per acre with basal area (sq..ft/acre) site index 90.

Tree and Stand Volume

"The size of the canopy available for transpiration may be closely coupled with the amount of stem tissue available for water flow." In many tree species, the relationship between leaf area and sapwood area appears to be linear (though the slope may differ among species).

Depending on soil and nutrients and shade they grow in volume according to long-studied relationships such as that from Spurr (1952:90) for loblolly pine:

logV = 1.94 logD + 1.12 logH - 2.77

where V is volume, D is diameter at dbh, and H is total tree height.

Spurr (1952 :141 ) noted extreme variations among reported volume tables and suggested the reason was not in the trees but in the construction of the tables. Tables are a "curve-fitting" response to needs. It utilizes easily-measured aspects of the forest. It seems likely that a theoretical approach may win out, one that develops a theory and then measures the required parameters to gain estimates of the volume of useful phytomass and its dynamics. Spurr (1952 :141) said that "the locality, type of growth, and site where a tree grows apparently do not affect total cubic-foot volume sufficiently to justify the development of more than one volume table for a given species." He added that one volume table (presumably based on theoretical grounds) may be all that is necessary, one adjusted by a percentage that differs by species.

Volume per acre is highly related to basal area and stand height (Spurr 1952:352). From combined plots Spurr (1952:187) presented an equation for loblolly pines (pure, even-aged, and normal stocking)as:

V = 342 + 0.385 B H

basal area and height, with a standard error of the estimate of 302 cubic feet. When mean values of B (134) and H (79) are used, the estimate of V is 4457 cubic feet per acre (reported) which may be compared to the computed value of 4418. Rounding and other errors abound. Spurr (1952:210) commented that the same factors that affect growth per tree are not necessarily the same factors that affect growth per acre, the latter being a function of site quality. The same growth can be distributed among many combinations of small and large trees.

Occupied Volumes

Soil Volume

These trees occupy area but also soil volume. We attempt to estimate the likely soil volume within the root zone. The zone is the product of the occupied area and a probable soil depth. Table 4. Where rooting depth is 2.5 inches, the volume of soil potentially occupied from surface to that depth is 9,075 cubic feet. For comparative purposes, note that for loblolly pine, yield of peeled, 2 inch diameter and larger green wood, is about 6,700 cubic feet at age 60.

The soil depth is influenced by many factors from depth to bed rock, depth to hard-pan, and depth to toxic substances.

The effective soil volume is that immediately around the roots. All is interlocked, however, by gravitational water and capillary action. Depth for any stand of trees must be estimated; a precise value with low variance is impossible to obtain for the relevant root-volume.

Soil Water Volume

Rainfall deficiencies in July and August of 1 inch contribute to outbreaks. Craighead (1925) said all outbreaks follow periods of rainfall deficiencies. Excessive shortages or excesses are "stress" and readily understood as related to tree dying and death and thus beetle presence.

The soil volume (the one-acre-area x estimated relevant soil rooting depth expressed in cubic feet) holds and carries the water used by the roots. Bulk density expresses the pore space as does water holding capacity. The proportion of the soil volume occupied by water needs to be estimated. Things that influence the water content of trees is their age, spacing, ground or understory vegetation, and rooting depth (influenced by hardpan, gravel and textural bands, and mottling). These and related soil conditions are described as series (texture and structure or widths of the layers), slope, and erosion. Not only root density but root system architecture may influence the pattern and extent of water uptake from the soil (Schulze et al. 1987). Schulze et al. (1987) observed about natural field conditions :"..more complex models are required to account for such phenomena as the intricate three-dimensional distribution of competing roots in the soil, the dynamic aspects of root growth and senescence, the non-uniform uptake of water per unit length of active root, and the shifting nature of soil water resources resulting from new wetting fronts within the soil profile." (Also see Cauldwell and Richards 1986.)

The rate of water loss by the canopy may be closely coupled with the moisture environment of the roots (Schulze et al. 1987:33). Loblolly pines transpire at about 25% of maximum rate when the soils are freezing. While pines transpire at 40% of maximum.

Pines have deeper roots than hardwoods, thus access to more stored water. Deep crowns and high stem densities reduce winds and light in dense stands, thus reduce water loss from the stand.

Stransky and Hall (1964) observed that the spring-flush height growth of 9-month old seedlings of loblolly was inhibited by matric potentials of -2 bars and stopped completely at -3.5 bars. The young flush began to wilt near -5 bars.

Site Index

Clutter and Lenhart(1968) observed that site index was the common way to express the productive capacity of the land being managed. It is the average height of the dominant and co-dominant tree at some age (25 selected by them). The equation (on file) shows site index, 40 to 80, as related to tree age and height.

Temperatures of the Tree Bole

Stomatal closure is a mechanism (along with leaf form etc.) for maintaining high tissue water contents during drought periods, though it has costs to the plant. Such closure under high irradiance may lead to high leaf temperatures and significant metabolic damage (Berry and Bjorkman 1980). These damages, we suggest are related to the evolving death of the pine tree and the emission of pheromone like substances of interest to pine beetles. Water flow in the tree is profound and governed by hydrostatic pressure gradients, not total water potential gradients (largely constant at the cellular level) (Sinclair and Ludlow 1985). Leaves of trees seem to die at a species-specific relative water content (rather than leaf water potential) suggesting water volume changes in tissues as the path of study rather than a thermodynamic model. They discuss stages of plant water balance and stage three " begins as the soil dries further, the root water uptake declines, and stomates lose the ability of keeping the transpiration rate approximately equal to the uptake rate. In this stage, the stomata are closed and the transpiration rate is dependent on the vapour conductance of the epidermis. When the root uptake rate becomes substantially less than the epidermal transpiration rate, the RWC [relative water content] falls and the leaf dies when the critical RWC is reached. Sinclair and Ludlow (1985) argue that water loss rate is not regulated by plants over a wide range of available soil water. They are a function of the available water.

Root Fungi and Tree Death

Root fungi grow on the dead roots of pines. Trees occurring too densely often have insufficient water for all trees. Trees having been planted on improper soils often have insufficient soil moisture. Large trees in competition with neighboring trees often have insufficient water. Trees over-heat without sufficient water and their leaves abscise, photosynthesis is halted, and the tree dies. That pines are "shade intolerant" is widely known. Perhaps not so well known is that mature pines are "moisture-stress intolerant " (Tolley and Strain 1985). Although Dixon and Osgood (1961) said that the beetle is " one of the most destructive insects of pine in the South and has been recognized as such since 1899", Heikkenen suggest, a century later, that it is not destructive at all. It is a scavenger of trees already dead.

Tree Mortality

"Similarly, young vigorous stands are generally less susceptible to catastrophic bark beetle epidemics, which typically originate in over-mature trees or those stressed by drought and other agents (although some species of root rots and bark-feeding insects readily damage young trees as well as old)."(Natl. Acad Sci. 1975)

Craighead and St. George (1940) thought beetles were inoculated sufficiently in fall to "cause the death of trees the following spring." That observation is consistent with the premise that beetles are seen when the trees in a group have laready died and sufficient time has passed for them to turn yellow. Doggett (1971) reported a delay between beetle attack and onset of tree fading of three to six weeks, depending on the season, for loblolly pines in Wayne County, NC.

Dixon and Osgood (1961:14) used the phrase that trees over 6-inch dbh were "attacked and killed." They said that pines attacked (p.16) were "in poor condition." Dixon and Osgood (1961) said that Hopkins (1899) has said that beetles do not attack the healthy, living trees but excavate their brood galleries in the living bark of trees..." He had added " ... but not killed by the attack of late broods the previous fall." that threw foresters and students of the beetle off the track. They do not attack living trees!

Specifying when a tree is dead is as difficult as stating the time of death of a human. Twigs, branches, and leaves die or fall. No new sprouts occur; there is evidence but the final criteria are missing. In pines the difficulty is great, and death is an essential observation. Failure to make the right observation may suggest improper causes of death. The Christmas tree owner knows that their base-severed tree is dead. It remains green for a time, the longer the better. The longer green, the more satisfactory the tree for the holiday. Similarly, a conifer in the wild may be dead, but remain green, death undetected. If death occurs at some heating threshold, and if the leaf abscission layer forms then, and alpha-pinines (from the fermenting fluids within the pines) are dispersed from their openings, then the mechanism for bark beetle attraction is present. It is now known that the sex pheromones of the bark beetle are similar to these emitted substances.

Heikkenen ( 1957:57) studied balsam fir and observed that spruce budworm outbreaks were correlated with extensive areas of mature trees (and presumably those dying or beginning the death sequence). In these early studies Heikkenen observed that the hemlock looper was reported to attack only after a decline in tree growth of several years (Carroll 1956). Susceptibility of stands to black-headed budworm increases with age and the proportion of balsam fir in the stands. The evidence for major insect outbreaks was building … that insect attacks are of stands that have reached their biological potentials and were no longer able to grow and replace rooting and moisture-collecting tissue fast enough to supply the growing volume of the tree mass, or unable to collect sufficient energy to grow roots or plant parts, or parts replacements in the now-large tree.

Heikkenen (1957 : 53) observed that balsam fir trees that die in the spring or summer appear to deteriorate faster than those that die in the winter (lower moisture, lower temperatures, reduced fungus spores, and reduced fungus and insect activity). This suggests that the timing of the stress that produces the tree or stand death can be very influential in when the trees are noted to be "dead." In brief, when the stand deteriorates rapidly, the beetles attack. This is counterintuitive to "beetles kill stands of trees."

Outbreaks are said to occur, but that may be simply the evidence of tree suffering stress-induced death. Blue-stain is said to be introduced by beetles -- perhaps the real financial loss to a stand other that the value lost by not harvesting at the proper time (before a probable stress event of the mature stand or foregoing returns from alternative investments.)

It seems reasonable that the same deterioration in branches and leaves occurs in roots. Excessive moisture or excessive dryness may cause death. The dead root part is subject to fungal attack. Wilted root-parts may not be able to re-grow and overcome the omnipresent or animal-transported fungi (by insects, isopods, earthworms, shrews, and moles). The wood losses to the fungi are typically clear in the so-called "over-mature" stands. Heikkenen (1957:42) observed that the importance of the wood-destroying fungi would diminish as the tendency to use smaller woods for pulp and shorter harvest rotations. His observations on tree and insect relations are reported here to suggest the evolution of the concept being described.

Dixon and Hodges (1961) said that initial beetle attacks on healthy trees sometimes cause a profuse flow of sap that may " pitch-out" the beetles. If sap flow is only moderate, the beetles move directly through the outer bark to the wood and construct galleries in the inner layers. Barras and Hodges (1974) said that parent males expend energy during mating and clearing the galleries of fras, but they seem to feed little.

Berryman and Pienaar(1973:456) observed that the beetle larvae feed for much of their lives in the outer bark. Emerging densities are 80 - 1,000 insects per square foot.

The multiple generations and other properties give a so-called tremendous potential for population increase and the capacity for increase "...explains why the outbreaks appear so suddenly when environmental conditions are favorable for the beetles " (Natl. Acad Sci. 1975:121). Perhaps there are other explanations. Dixon and Osgood (1961) observed " outbreaks frequently occur without apparent warning, yet populations may disappear as quickly as they appear." Barras and Hodges(1974) said that females normally find suitable host trees and initiate attack, after which their pheromone attracts male beetles, perhaps on a shorter flight line than theirs. The females thus use more lipids, substance which serve as an energy source during emergence. In sum, lipid content of pine bark is low. In early spring, lipid content of loblolly inner bark is low, thus beetles get little lipid energy directly from bark tissue. Attacking beetles nay use the normally high carbohydrate content of inner bark for that purpose. They said " After attack...carbohydrates decrease rapidly."New microorganism populations (e.g., mycangial fungi populations in phloem) are ingested by beetle larvae (said to be a symbiotic relationship.)

Where are the beetles? The population, wherever it exists, is undergoing limitations from intra-specific competition, said Berryman and Pienaar (1973) The beetles are crowded, having responded to population aggregating pheromones when a susceptible host is located (see multiple citations in Berryman and Pienaar 1973).

"The precise causes of the rapid changes in population size are not thoroughly understood" (Natl. Acad. Sci 1975:121). Perhaps they are aggregations of populations from a wide area attracted by the pheromone-like-substance-emitting stressed tree. The sequence of events may be misunderstood. The National Academy(1975:121) said that "These insects characteristically attack trees in massive numbers, so that an infested tree almost always dies. Mortality occurs rapidly, because of the associated penetration of blue-stain, Ceratocystis minor. The tree dries quickly, phloem is destroyed, and the foliage begins to turn yellow in about 3 weeks." (Natl. Acad. Sci 1975:121). The erroneous observation can be easily understood in light of the above concepts of the sequence of stress, death, and presence of abundant beetles.

Sampling using multifunnel traps with frontalin and turpentine have been used to sample beetles but Turchin and Odendaal (1996) found that the effective sampling area for a trap was less that 0.1 hectare and declines with the density of host trees. Earlier that found the dispersal distance was approximately 0.45 km.

So What: The Management Implications

The southern pine beetle is said to be a destructive pest of pine in the South. Research has been almost continuous since 1899 (Hopkins 1899). Dixon and Osgood (1961) reviewed publications to that date. A $20 million project with increases has been underway since the early 1970's. After much study, uncertainty remains, perhaps because an erroneous model for the insect has been followed.

In our view, the model used by Gumpertz et al. (2000) was erroneous. They predicted outbreaks within counties as if beetle attacks were unrelated to stand age, site conditions, and moisture relations. The probability of an attack within a county is related to the presence of stressed, mature trees in each county. Logistic regression or other techniques will not illuminate that model. Presence of susceptible trees on poor sites may be a function of economic conditions, foresters' recommendations, and proximity of the county to a point of sale. Treating attacks as if they are grossly probabilistic seems erroneous and misleading. It appears that " ...beetles attack in a complex, overlapping pattern, and may attack almost continuously during favorable weather (Natl. Acad.. Science, 1975:121). Maybe they do not attack at all but fly in droves to clumps of dead or severely stressed trees? Once dead, what of the trees? Beetle "killed" lodgepole pine in eastern oregon was removed and studied by Ince et al. 1984.They found gross energy balance that energy required by harvesting was about 3.4 percent of the gross energy content of the delivered products.

Notes to move........Several bird species, including one endangered species, forage on under-bark insects.

References

Barras, S.J. and J.D. Hodges. 1974. Weight, moisture, and lipid changes during life cycle of the southern pine beetle., USDA Forest Serv. Souther For. Expt. Sta, SO-178, New Orleans, La 5pp.

Berry, J. and O. Bjorkland. 1980. Photosynthetic response and adaptation to temperature in higher plants. Annu. Rev. Plant Physiol. 31: 491-543

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Notes

Ralston, C.W. and R.C. Chapman 1970-71 School of Forestry, Duke, studied primary productivity of loblolly.

Higginbotham, K. 1971. Duke, Botany Dept., studied growth dynamics of the pine canopy. Upper third has 3 leaf flushes, mid 2 flushes, and lower third has one leaf flush indicating varying rates of productivity within the crown.

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