Insects and diseases only recently have been recognized as contributors to forest health and productivity. Epidemics of these organisms are triggered by factors that increase host susceptibility and availability and could be used as indicators of declining forest health and diversity. Increased productivity often follows pruning, thinning and increased soil fertility caused by insects and pathogens, compensating over time for the more conspicuous tree growth losses and mortality. Increased diversity results from modification of environmental conditions or habitat and food resources. Increased diversity inhibits continued insect and disease epidemics, increases nutrient acquisition and cycling, and stabilizes forest species interactions. Forest health and productivity may be enhanced for decades after epidemics.
~ T. D. Schowalter: Roles of insects and diseases in sustaining forests*
Non-linear relationships between spruce budworm and fir trees in the Blue Mountains of northeast Oregon and southeast Washington
This article presents a discussion of the state of the forests in the Blue Mountains of northeast Oregon and southeast Washington. Stretching from central Oregon eastward to the Hells Canyon country, and intruding northeastward into the rich Palouse soils of eastern Washington, these mountains are both geologically and floristically diverse. On the west they grade into the largest juniper forest on the planet, while to the east, high-elevation grasslands edge the deep canyons which finger down to the Snake River.
Elevations range from 3500′ to almost 10,000′, with extensive montane forests covering the slopes in between when the aspect and soils allow. They provide abundant habitat for species as diverse as bull trout, peregrine falcon, Rocky mountain elk, and mountain quail. In the last few decades after having weathered a series of insect outbreaks they became the focus of a debate over forest health.
I decided to write down my ideas because of all the conjecture and debate revolving around possible management alternatives. There is a need to state plainly some of what we know and don’t know, and to place it in the context of the ongoing debate on the possible role of non linear1 relationships in ecological theory. The changes we have wrought in the last one hundred years are making themselves felt throughout our western ecosystems. Grazing, fire-suppression, commercial timber harvesting, and even seemingly benign activities such as firewood cutting and mushroom picking have had and will have a continuing impact on our forests. Understanding the combined effects of our actions can be complicated at best (Belsky and Blumenthal 1997) and even counter-intuitive as you will see as my story unfolds.
In 1993, the outbreak of western spruce budworm Choristoneura occidentalis, a major defoliator of Douglas fir and true fir (a hybrid of grand fir and white fir in the mountains of the interior Pacific Northwest), collapsed even more quickly than it had started seven or eight years before. Here “collapse” is used in the quantitative sense: the number of budworm per square meter of foliage over time has dropped to a background level, which is below population levels recorded when the outbreak started. In some places the number of insects has been reduced by two orders of magnitude in a few years.
Entomologists have now had their first chance to watch an entire outbreak cycle in this part of the western United States, from beginning to end. The pattern and length of the irruption bear a close resemblance to those of the closely related eastern species of budworm Choristoneura fumiferana (Royama 1985). Western scientists, as their eastern counterparts before them (Tothill 1922), are searching for answers to the dramatic collapse at the end of the outbreak, looking for clues as to why the population irrupted initially, how it was maintained, and why it collapsed.
There is a “mathematical taxonomy” to the budworm’s behavior: populations fluctuate in an irregular fashion yet they grow and decline in a few years or less at the beginning and end of the outbreak cycle. The background and outbreak levels are quite distinct and robust. To say it another way: the population exists in one of two mutually distinct states, and is relatively constant in either one. In both states, the population oscillates somewhat randomly, but in a bounded way. At background levels, the population tends to stay at background; at outbreak levels, it tends to stay at outbreak. The transition between these states may be triggered by a non linear relationship between the budworm and some factor or set of factors in its environment.
The system may be exhibiting what is called chaotic or near chaotic behavior. The word chaos is used to describe systems that are not absolutely predictable in their behavior, and that are capable of changing their course in this abrupt fashion. The relationships that tie the parts of these systems together can be simple yet still result in sudden shifts in behavior. This behavior will also be very sensitive to changes in the starting point of the system. A slight difference in these initial conditions might lead to a very large difference in the state of the system over time. (Crutchfield; Farmer; Packard, and Shaw 1986).
In the case of the budworm, the background and outbreak population levels might be the signatures of these different movements. To verify whether or not this is the case, the recommended procedure is to watch the system for a few hundred cycles and then to create a diagram mapping out these movements in time. (Kot and Schaffer 1984).
The problem is that it may be 30 40 years or so between outbreak cycles. This rounds out to 3,000 to 8,000 years of sampling! Proof will be hard to come by, and may only be arrived at indirectly, through tree coring, pollen records, and other paleobotanical evidence.
Along this line, Boyd Wickman, now retired from the Forestry and Range Sciences Lab in La Grande, Oregon, recently showed the results of years of tree ring work (Wickman; Mason, and Swetnam 1994). Employing the sophisticated software developed at the University of Arizona’s tree ring laboratory and with old growth Ponderosa Pine to factor out the effects of climate2, he has identified what may be a regular cycle of western spruce budworm outbreaks every 40 years or so, in the Wenaha-Tucannon wilderness of the northern Blue Mountains. There are also clear signs of tussock moth outbreaks, the two distinguished by their distinct signatures. These signatures are available thanks to careful monitoring of the recent outbreak, and to 24 years of tussock moth population data gathered by Dr. Richard Mason, also at the La Grande lab.
Another scientist at the lab, Dr. Catherine Parks, has done work just as fascinating. In the plantings she did as part of her doctoral research (Parks 1993), she found that dry periods enhanced budworm defoliation. The water-stressed trees that were most susceptible to defoliation were also those that most readily redirected their metabolism into enhanced root storage and development. She also found indications that these young defoliated trees might be the ones best adapted to survive the outbreak, since they offered less of their future metabolic gains for the budworm to take. Those trees that continued to put out green needles, with less of their development going into their root systems, eventually showed the most stress and mortality.
Such evidence suggests that we may be looking at a tightly co evolved system, with the budworm performing a crucial role by applying evolutionary selection pressure on several major tree species. In turn, this herbivory could be part of a feedback loop which then controls the budworm population.
The idea that simple non linear relationships could lead to such interesting behavior has stimulated scientists to explore the implications using mathematical models. They’ve found that model populations with even simple predator prey relationships can develop complex spatial patterns ranging from regular to chaotic (Hassell; Comins, and May 1991) like what we observe in budworm populations and the populations of other western defoliators as well. Needless to say, some of these ideas are quite controversial and they have sparked a healthy debate about the nature and meaning of fluctuations in insect populations (Logan and Hain 1991).
Why would such tight feedback loops evolve? Climate change could certainly be one reason. Recent corings of the Greenland ice sheet reveal a remarkable record of variation in the temperature over the last 100,000 years, with temperature shifts sometimes very rapid (Taylor; Lamorey; Doyle; Alley; Grootes; Mayewski; White, and Barlow 1993;Alley; Meese; Shuman; Gow; Taylor; Grootes; White. J. W. C.; Ram; Waddington; Mayewski, and Zielinski 1993). These climatic changes would have to be reflected in plant communities as well.
Our western conifers have been around for a long time. We have to assume that they have the necessary genetic diversity to be able to respond very rapidly to such modifications in climate, moving into sites that have changed to fit their needs. There are arguments about how fast trees can migrate into “open” environments (see for example (Gear and Huntley 1991)); but there’s no doubt that plant communities reorganize themselves in ways that we find difficult to imagine, given what we currently see around us (Spaulding 1984).
What about the trees occupying a site to which they were now less adapted? Fire and insect outbreaks could provide a means by which otherwise long-lived trees of a given species would be removed from an area that had become drier due to climate change, opening the way for better adapted individuals or members of another species.
It is my hypothesis that by removing old growth Ponderosa pine and attempting to suppress all fires, we may have sent a false climate signal. In the absence of competition and fires, the Douglas fir and true firs have occupied the landscape at a density more characteristic of wetter sites or times. The pumping action of thousands of trees per hectare, many now reaching optimum growing age, has induced droughtiness in the soil and a nutrient shortfall. Enter the budworm to return the ecosystem to balance. Such ideas about the role of insects in forest ecology are now gaining currency (Schowalter 1992).
All of this was amply brought home during a visit to a friend’s land right as the outbreak ended. He told me of his amazement at the disappearance of the budworm and at how trees that appeared to be dead because of their total defoliation were growing new crops of needles. We walked around the immediate vicinity of his cabin and noticed quite a few 10′ – 20′ trees in the understory which were showing no signs of life. But almost all of the 30′ 40′ trees had new crops of needles from 2/3 to 3/4 of the way up. A natural thinning appeared to be taking place, and while other insects may eventually cause the death of some of the “survivors”, clearly the outbreak was nowhere near as catastrophic as originally conjectured.
Implications of this relationship are that, first and foremost, the budworm is an integral part of the ecosystem and has been evolving along with its tree hosts for a long time. Second, the forests are probably not going to disappear, since they are adapted to respond to insect outbreaks. Finally, without fire, forest insect populations become the primary control on the tree density in fir-dominated stands of the Blue Mountains.
The first and last of these can be linked via a mathematical argument: Before the arrival of white settlers with their emphasis on fire suppression, the fire return frequency in the Blue Mountains was about 10 20 years. Of course this is just the average in a statistical distribution. Some areas would have burned more often, and some less often. It seems natural to assume that generalized organisms, such as the spruce budworm or tussock-moth populations, would step forward to recycle the nutrients bound up in heavily stocked stands.
This argument makes it clear that economic imperatives have to be tempered by ecological and evolutionary reality. The tenets of industrial forestry are simple. The emphasis is on heavy machinery, uniform harvesting techniques, large expenditures of capital and maximizing profit through greater efficiencies3 of scale. Maximum volume increment, a corollary to maximum profit, is seen in all cases and everywhere as a good thing. This logic would lead foresters to select for trees with the ability to continue producing green needles, a sign of growth, during budworm outbreaks.
There is only one immutable law in biology however, the law of survival. The only quantity that is maximized is the probability that an individual will live on to produce more of its kind. This is what makes the argument above quite plausible: the idea that evolution would have directed Douglas fir and true fir into hedging their ecosystem bets. The husbanding of valuable metabolic resources in below ground biomass, rather than above ground tree growth, may signify the genetic diversity necessary to deal with rapidly changing environments. If this is the case, then forest economics should be yoked to these ecosystem realities.
In no way do I intend to minimize the current problems facing forest managers. There are pockets with quite a few dead trees. In such areas, the potential for stand replacement fires does exist. The point is that care should be taken with wide scale thinning or harvesting regimes. In the immediate aftermath of an outbreak, it can be difficult to tell which trees will make it and which won’t, and it’s the survivors that provide grist for the evolutionary mill. Given the past history of rapid climate change, we must preserve as much of the naturally occurring genetic diversity as possible in the trees of these forests.
1Non linear means that the quantity we’re trying to predict, such as an insect population, is affected by variables that can “feedback” to the quantity so as to amplify or damp the change in population level.
2Ponderosa pine is not affected by insect pests of Douglas fir and true fir. It does respond to changes in climate just as those species would. This allows scientists to distinguish growth suppression caused by climate shifts from that brought about by insect outbreaks.
Alley, R. B.; Meese, D. A.; Shuman, C. A.; Gow, A. J.; Taylor, Kendrick C.; Grootes, P. M.; White. J. W. C.; Ram, M.; Waddington, E. D.; Mayewski, P. A., and Zielinski, G. A. Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event. Nature. 1993 Apr 8; 362:527-529.
Parks, Catherine Gray. The influence of induced host moisture stress on the growth and development of western spruce budworm and armillaria ostovae on grand fir seedlings. Corvallis, Oregon: Oregon State University. p. Ph.D.; 1993.
Schowalter, T. D. Roles of insects and diseases in sustaining forests. Proceedings of the Society of American Foresters National Convention, Bethseda, Maryland, USA: Society of American Foresters; 1992: 262-267.
Spaulding, W. Geoffrey. The last glacial interglacial climatic cycle: its effects on woodlands and forests in the American west, Utah State University. Eight North American Forest Biology Workshop; Utah State University. 1984.
Taylor, Kendrick C.; Lamorey, G. W.; Doyle, G. A.; Alley, R. B.; Grootes, P. M.; Mayewski, P. A.; White, J. W. C., and Barlow, L. K. The `flickering switch’ of late Pleistocene climate change. Nature. 1993; 361:432-436.
Wickman, Boyd E.; Mason, Richard R., and Swetnam, T. W. Searching for long-term patterns of forest insect outbreaks. Article: S. R. Leather; K. F. A. Walters; N. J. Mills, and A. D. Watt, Editor. Individuals, Populations and Patterns in Ecology. Andover, United Kingdom: Intercept Press; 1994; pp. 251-261.