Trees a Challenge of Perspective
We now realize that there are a number of cooperative activities in arboreal life processes and that trees normally, not by exception, depend on multiple other organisms of their own and diverse species for sustained health.
Horticulturist and Plant Records Coordinator
It’s all about competition. Or is it? For most of the past two centuries, we have been given a picture of forests as realms of fierce competition, and have looked upon human interference as constructive refereeing. Partly as a result, landscaping practices have traditionally treated trees as individuals and plants in general in isolated terms. Every sort of attempt has been made to protect trees from shrubs, herbaceous plants, insects, everything. “There’s a fungus among us,” goes the dark refrain.
Ecological and biochemical research in tree biology and plant communities has yielded endless new understanding and insight. We now realize that there are a number of cooperative activities in arboreal life processes and that trees normally, not by exception, depend on multiple other organisms of their own and diverse species for sustained health. Where biologists tended to study one species, they now often study communities. There is much to learn, but the emerging picture of trees suggests that we could better view them as we do coral reefs than as stand-alone specimens in a lawn or clumps in a parking island.
While the heady parts of science may seem remote to daily landscape practices, there is much that can be put to use. Treating the health of trees as isolated individuals is a practice that misunderstands the life functions of trees and the plant community. The tree growing wild in disassociation with fungi, shrubs, herbs, lichens, mosses, insects and other herbivores, their predators, and other trees is rare indeed. Not viewing trees as part of an integral community has many hazards. Pesky fungi can simply be hit with some heavy metals. Spray larvae with toxic alkaloids. Slow growth can be pushed with nitrogen, slow blooms with phosphorus. All these things seem practicable, with discretion, because the tree is thought to be viable on its own. The track record of human intercession against pests is very disappointing, and often including devastation of non-target species while the pest continues to thrive.
There is little evidence from nature that trees remain healthy on their own. What appears in research is close interdependence between trees and most or all the life forms that inhabit their native communities. What is also becoming apparent is that the removal of an organism from a community creates extended chains of disruptions that can lead to the disappearance of many other organisms. The picture is one where symbiosis and mutualism – organisms mutually benefiting from their interdependence- becomes the dominant force. Collaboration occurs within the same species and between organisms of different species, genus, order, and kingdom.
Current events in the IPM life of a landscape professional provide plenty of case examples pointing toward the importance of understanding trees as community organisms, and the dangers of tampering. In the case of gypsy moth infestation of oaks, economic loss has driven research on many levels in this pest-host relationship. Oaks often fuse roots in an extended group, and this is accomplished with mycorrhizal fungi as an intermediary sharing chemical messengers, defense toxins, and nutrients this way. Through this means and through volatile terpenes, oaks are able to signal the onset of a devastating infestation. Unaffected oaks are able to withdraw nutrients into their roots and thus cheat, and reduce the breeding success of, the gypsy moths. On the other hand, human intercession with B. thuringensis, while a natural pathogen from the moth’s native range, has devastated our native silk moths, like Luna and Cercropia (along with pesticides and introduced fungi). Then there are unresolved questions such as how nitrogen fertilizer and resulting excess tender growth (with low toxin concentrations) affects browsing selection by deer and ovipositing insects.
Birches and other members of their family, maples, willows and poplars are all known to engage in root fusion, and some are even known to fuse roots with trees outside their own genus. We know that stands sumac, aspen and beech tend to be clonal, one organism really. Root fusion in forest trees, sharing nutrients and defenses, questions whether we should view trees as individuals. As we see borers and miners wreak havoc on white birches in landscape plantings, these are contrasted by healthy and numerous white birches lighting up the hillsides of the northern states. Root fusion, pest-parasitoid life cycles, and fungal symbioses may all have a role in the final explanation as to why we see this. Furious pursuit of Sudden Oak Death in California has produced the observation that impact is greatest in forest edges and in disrupted groves. The Phytophthora agent’s genus has a nefarious history of creating destruction where humans have recently changed the landscape. Fungal pathogens in general suggest that an empty niche is the devil’s playground.
Pines and their use of resins, the complex of terpenes some of which create that wonderful piney smell, provide an object lesson on the inferior nature of human insect control. Pines are very diverse in the mixture of terpenes their resins contain, even in the same species and the same stand. Under intense herbivory from beetles, the pines retain their chemical diversity. The result is that the beetles cannot successfully adapt to any one array of chemical defenses. In addition, monoterpenes are released heavily into the air immediately as pines endure damage to their needles, and this creates a localized cloud of ozone and organic nitrates toxic to beetles. It is produced ‘on demand.’ The same cannot be said for any human treatment product. The range of actions on beetles of pine sap is astounding. Sap resins are a mix of simple and complex terpenes that have very distinct effects on insects and animals that feed on them. It has been shown that one pinene repels beetles while another in the same mix attracts, other elements of the same mix interfere with beetle pheromones, and others are directly toxic. Pines alter the mixture in response to infestations that form complex cycles over the life of the pest. Similar interactions have been studied with Scolytid beetles, spruce budworms (Christoneura spp.), and sawflies of Rhyacionia and Neodiprion. Despite well-funded research driven by the value of commercial timber, the host-pest cycles remain incompletely explained. Interceding at any point of this game can prevent the pines from completing their management of the pest life cycle. In other words, spraying can, and likely often does backfire. Or caterpillars are sprayed only to produce an outbreak of mobile spider mites because their more local predatory mites are gone.
Trees draw on soil bacteria and fungi for food, as well as defense. Predatory fungi hunt nematodes and others have a limiting effect on potentially pathogenic organisms. Parasitoids that control potential arthropod pests have life cycles that require a few to many species other than the tree of interest. Parasitoids often have their own parasites. A fungus that controls one pathogen may itself become pathogenic at another point in its life cycle. In this sense, a diverse community of species where all are held in check and balance by their interactions with a host of other species deserves consideration as the model for landscapers and gardeners to pursue.
The applications of the tree-as-community model are difficult to exhaust. In the general sense, using as many members of a naturally-occurring community as is practicable is a plan for success. In the more particular sense, thorny and/or toxic small shrubs provide a good encouragement for deer to move on past your highly edible large shrub or tree. Another shrub provides nitrogen-fixing ability, root resins that are toxic to a pathogenic fungus, or late winter protection from sun scald. In the clear sense, some trees can share chemical defenses with other species through their roots, or confuse the reproduction cycle of pests with leaf resins. In the intuitive sense, plants that share a wild community share a long list of more subtle interactions which sustain that community.
Planting by community requires change in how we sell, design and plant landscapes. Predesign attention to each plant’s long-term ideal conditions, interactions with agents of control, and manual methods of infestation control (not all are laborious/disgusting) become paramount. The notion of single focal plants needs to go, or specimens need to be nearly foolproof. Initial, successional, and replacement plant material costs can be higher (I found little data for comparison). Place this possibility against the cost of fertilizer, pesticides, higher maintenance hours, still the cost of replacements, and impact on family/community health.
However you look at trees and their relationship to other plants, successful long-term health relies on understanding their connectedness. While we know important pieces of a pest-host-parasitoid cycles, we have yet to exhibit in practice that understanding. Putting the pieces of what we know together, it becomes less useful to view an organism as a pest, host, or parasite. If we allow new insights to inform landscape practices, we can move toward preserving and recreating self-sustaining landscapes. Plantings that present naturally-occurring communities as completely as possible may become the trend of long-term success.
Holdenreider, Pantasso, Weisberg and Lonsdale “Tree Diseases and Landscape Processes: the Challenge of Landscape Pathology,” Trends in Ecology and Evolution, vol.19 No. 8, Aug. 2004.
Langenheim, Jean H. Plant Resins, Chemistry Evolution, Ecology, Ethnobotany, Timber Press, 2003.
Litvak, M. E. et al., “Herbivore-Induced Monoterpene Emissions . . . ,” Ecological Applications, vol. 9, 1999.
Money, Nicholas P. Mr. Bloomfield’s Orchard, The Mysterious World of Mushrooms, Molds and Mycologists, Oxford Univ. Press, 2002.
Ricklefs, Robert Ecology, WH Freeman, 2001.