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The Sound of Light in Trees: The Acoustic Ecology of Pinyon Pines

David Dunn's new CD is now out! Thanks to a grant that David has received to cover costs of publishing the disc, he is donating 100% of all revenues on sales to the Acoustic Ecology Institute. Thus your purchase of the disc will go directly to supporting all facets of our work.

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The Sound of Light in Trees

David Dunn

All of the sounds heard in this recording occur within the interior of one species of conifer tree, the common twoneedle pinion of the southwestern United States, Pinus edulis. What I mean by “the interior” are the layers of phloem and cambium between the outer bark and inner xylem of the tree. Within this narrow realm of cellulose, air, and fluid occurs an almost unknown acoustical world and an extraordinary array of living sound makers. While the majority of these sounds are made by one species of small insect (about the size of a grain of rice) known as the pinion Engraver Beetle, Ips confuses, there are possibly others such as bark beetles of the Dendroctonus genus, other species of the Ips genus, and the larvae of several species of miscellaneous invertebrates, most notably those of the many different species of Longhorn Beetles known collectively as Round Headed Wood Borers (Cerambycidae).

My intention in the composing of this collage was to convince the listener of the surprising complexity of sound occurring within one species of tree as emblematic of the interior sound worlds of trees in general. It is also intended to demonstrate the rich acoustical behavior of a single species of small insect and to suggest how sound is a much more important aspect of how it organizes its world, and interacts with its surrounding ecosystem, than previously suspected. The composition was organized around the idea that it would be possible to hear all of these sounds within one large tree if enough sensors could be simultaneously placed throughout its myriad branching structures.

I first began to focus my attention on these trees and their principal invaders (Ips beetles) as the demise of the pinions where I live in northern New Mexico became quite evident. In recent years there has been a major outbreak and it is estimated that a majority of these trees will die within another few years. Ips confusus are similar to the thousands of different known species of beetles that attack damaged or stressed trees throughout the world. Usually they maintain an equilibrium with the various kinds of conifer trees found in the western states but in recent years, whether due to local drought conditions, global climate change or other factors, bark beetles have outstripped the capacity of the trees to defend themselves and we are witnessing an extraordinary level of infestation. There have also been immense losses of many other species of conifers such as the ponderosa pine forests of Arizona and California and the spruce forests of Alaska and the Pacific Northwest.

If these infestations are a result of global temperature changes then the consequences are potentially dire. A further warming trend in the west may allow the beetles to invade the five-needle pines at higher elevations that have been largely immune to them. The result may be a spread of infestation to the huge unaffected population of jack pine in the northern states and Canada. Furthermore, a serious infestation of the high elevation conifers could substantially reduce the snow-fence effect (windrows of captured snow) that these trees provide in the conservation and distribution of water from the Rocky Mountains. Since the Rocky Mountains serve as headwaters for major river systems in North America, and the fact that most of their source water accumulates as winter snow, such tree loss may prove catastrophic.

Insects sense through what are generically called setae, hair-like attachments and protrusions. Even amongst the thousands of bark beetle species (Scolytidae) there is incredible variation. The consensus scientific view is that beetles use certain setae as mechanoreceptors that function through stretching, bending, and compression for sound perception. The simplest are true hairs often in the form of what are called trichoid sensilla (hairs that bend and rock in a socket). Another type is referred to as campaniform sensilla where the setae are replaced by a pit covered by a dome of cuticle. These are common at the base of the wings of some insects. Other insects (cicadas, dung beetles, grasshoppers) have tympanic organs attached to various parts of their anatomy.

All of these recordings were made with a custom-built vibration transducer inserted between the outer bark and the interior phloem where the bark beetles attack and colonize. Even though these sounds are extremely low in amplitude, they are transmitted as vibration across the interior surface of the bark structures for quite a distance. In most cases these sounds are not audible to the human ear or conventional air microphones. The sound producing mechanism in many bark beetles is most likely a “pars stridens” organ that functions as a friction based grating surface. In Ips beetles it is located on the back of the head and stroked by a “plectrum” on the under side of the dorsal anterior edge of the prothorax.

My testing in a variety of trees has verified that the acoustical signature of different locations in different trees is correlated to the density of infestation (i.e. size of population), the life cycle stage, and the condition of the phloem. Acoustical activity also occurs with varying intensity in trees at various stages of beetle attack, including some that have no apparent signs of infestation such as “pitch tubes” or needles changing color.

Studies of the larger Dendroctonus bark beetles (D. pseudotsugae, D. rufipennis, D. brevicomis, D. ponderosae) have revealed sonic stimulation of anti-aggregative pheromone release between genders and it has been established that in some species both possess a stridulatory organ. Mathematical analysis of bark beetle attacks on bark surfaces by Ips typographus has shown what may be a “minimum allowed distance” between attack points. It has been theorized that in some bark beetle species, stridulation may be a mechanism for controlling attack spacing through the induction of pheromone inhibitor release.

Out of the many hundreds of scientific papers published over the past three decades on bark beetles, the vast majority has concentrated upon their chemical ecology with the hope that something might lead to chemical control strategies. Only a handful of studies have been conducted on their bioacoustic behavior and almost all of these studies have been made in laboratory settings and not as field observations. One of my underlying intentions has been to create a true synthesis of art and science where my field studies of these insects and tree interiors through sound monitoring could not only yield fascinating sound art sources but some novel scientific insights along the way.

My working hypothesis for this project reads more like science fiction than science fact. I am theorizing that a much more complex micro-ecology exists between bark beetles, various fungi and their host trees. Sound probably has a much more profound role in regulating the dynamics of these relationships than previously suspected and I am trying to demonstrate this through a cross-disciplinary synthesis between the facts of what we already know combined with a few new experimental tests.

For instance, we do know that one of the most common techniques for evaluating drought stress in trees is to measure their cavitation events through ultrasound monitoring. As the tree’s vascular system becomes stressed from insufficient fluid transport, discontinuities in the integrity of its vascular conduits cause small partial vacuum bubbles to form. These can implode with such tremendous instantaneous force that, under laboratory conditions, they have been measured to produce temperatures up to 5,000 degrees centigrade. When these cavitation events occur, they release both light and ultrasound signals. Under extreme conditions, some trees produce these events as an almost continuous ultrasound signature.

I am not the first person to wonder if this phenomenon is “audible” to the beetles and can play a role in their large-scale infestations. So far no one has been able to prove the direct connection but we do know that, every year now, many more species of flying insects have been shown to possess ultrasound perception that probably evolved due to predation pressure from insectivorous bats. These insects possess a startle response of flight avoidance when hearing bat echolocation chirps. These echolocation chirps occur over a fairly wide ultrasonic range that probably overlaps with those of tree cavitation sounds, not to mention that bark beetles are a common food of bats throughout the world. Perhaps they have evolved a similar adaptation that serves the dual functions of signaling both bat presence and tree vulnerability (repelled by FM sweeps but attracted to steady-state signals)? One thing that is clear from the recordings of Ips confuses that I have made is that many of them have a very strong ultrasonic component. Communicative systems seldom evolve through investing substantial energy into a part of the frequency spectrum that an organism cannot both generate and perceive.

Many bark beetle species also carry various forms of “bluestain” fungi that more quickly kill the trees than the beetles alone. Some individual beetle species can sometimes carry as many as three or four different species of fungi in specially adapted transport cavities.  The fungi are essential in expediting the reproductive cycle of beetles by weakening and clogging the tree’s vascular system and, therefore, possibly increasing the incidence of cavitation. I have also wondered if the characteristic blue color of the fungi is a clue that they too might have a deeper relationship to these cavitation events. The blue is the part of the light spectrum reflected by the fungi and not used metabolically. At ambient temperatures it is known that cavitation ”sonoluminescence” releases light in a lower frequency spectrum than ultraviolet. Many species of fungi require different parts of the light spectrum (ultraviolet or infrared) to trigger their reproductive cycles. Perhaps there is a micro-ecological engine at work that, in conjunction with the well-established facts of chemical ecology, can explain why these tiny creatures have such complex sound behaviors.

Entomologist Philip S. Callahan has put forth the controversial idea that insects have a generalized high sensitivity to the infrared energy spectrum and that they are perfectly designed to optimize the transmission and detection of free-floating scent molecules by “pumping” them into a coherent non-linear infrared emission phase through the use of sound. He states, “as long as sound is studied in one corner of the lab and scent in another, the mechanisms of these sound-modulated scent molecules will not be understood…”

Less controversial is the role of the analogous stridulatory sounds of ants. Recent thinking postulates that ants employ nearfield (an acoustic transition zone surrounding a small source) airborne sound. The acoustic receptors are believed to be concentrations of trichoid sensilla on the two antennae. Such an adaptation would not only allow individual ants to hear each other but also determine both distance and direction to another source within the nearfield. This idea is also supported by the fact that inside of the tight spaces of a nest, such close-range communication might prove useful because chemical communication can be difficult to modulate at close range. The same would be true for bark beetles within a tree.

Around and around it would go. The sounds of drought stressed trees attracting beetles that invade with fungi requiring the light inside the tree to sustain reproduction. The subsequent demise of the tree would allow the beetles to complete their reproduction and then move on to other trees. All the while this is regulated by the exchange of complex chemical and acoustical signals between beetles, fungi and trees, transforming them into new forms of energy: nutrients, scent molecules, sound, and light.

Just as the beetles transform the trees that they consume into terpenoid molecules for their chemical communication system, when the sounds of their stridulations are slowed down (and temporally stretched) they become indistinguishable from the sounds made by the tree’s own movement of swaying branches and creaking limbs. It is as if the slow motion of the tree’s vibratory substrate is transformed into the manic metabolism of the beetles. Sound becomes a literal expression of physical scale. Life weaving mind from the computational fabric of space, time, and matter.

I readily admit just how fanciful my flights of hypothetical imagination might be, not to mention my lack of scientific credentials, but I also happen to think that this is one of the most important roles for artists in forging a new collaborative relationship with science: science fiction that might lead to science fact. Speculation and imaginative synthesis are the kind of thinking that artists, when they are thinking at all, do best. Unfortunately there is currently not much room for this kind of thinking in science because it is regarded as a threat to professional disciplinary rigor. At its worst, generalism is regarded as amateurish and flaky. It is, however, one of the few social benefits of being an artist that we can be tolerated, even encouraged, for our flights of imagination. I just happen to believe that it is more useful when those synthetic fantasies are grounded in science. It is, however, essential that one not be confused for the other. As a sound artist I can reveal new auditory phenomena, and propose new explanations for it, but ultimately what I propose must be handed over and evaluated by real science to be truly useful.

My argument for a fruitful exchange between art and science not only includes the traditional interpretive one as posited by E.O. Wilson’s concept of ”consilience” but includes a vision for how artists might influence the evolution of science by providing richer imaginative and metaphorical tools. Art can contribute to real world problem solving by enriching the communication of what science reveals through seeking the facts of nature. I believe this to be of historical necessity because of where we currently stand in relationship to an earth in crises. How can art participate in the discovery of solutions that can accelerate or extend those of science?

One of the delightful aspects of this kind of synthesis between art and science is that even if my speculations and research prove to be misguided, the sounds that I have recorded remain intrinsically interesting. You don’t need to know what all of these sonic machinations might mean to find them aesthetically engaging. Perhaps of equal importance to the pure revelation of this phenomenon is its generative working method and the possibility that the strategies employed by sound artists, for creating a compelling sonic experience out of the sounds of the natural world, might have a deeper application within science itself.

The way in which this collage was constructed allows for the temporal compression and juxtaposition of individually recorded sounds that, if otherwise presented in a conventional lexical manner, would only provide a shallow experience of the true nature of the sounds. When strung out in a linear fashion, two years of field recordings only allow the listener to focus on their immediate nature and not their interaction over time. By juxtaposing them, while respecting their local sonic integrity, we perceive the richness of how they reside within a more complex and resonant context. We become much more aware of their true interrelationship and diversity within their arboreal environment.

All of this was reinforced when a friend of mine told me about speaking to a Pueblo Indian elder about the bark beetle problem. The old man told him, “the beetles come when the trees begin to cry.”

I wish to thank James P. Crutchfield, Jim Cummings, Bob Harrill, and my wife, Naomi Milne-Dunn, for extensive discussions of these ideas and encouragement of my eccentric working methods. Thanks are also due to the Delle Foundation for support of the scientific research associated with this project.



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