Little Eden Nature Reserve & Hostel, 605 Gardiner Rd., RR3 North Augusta, Ontario, Canada, K0G1R0

Introduction

Structurally the boreal, or northern, forest is composed mostly of conifers, which include all naked-seed trees or gymnosperms. This is an old group, older than many of the Jurassic dinosaurs. These trees grow an enormous amount of living and dead fuel, as well as tree-living lichen, and when ignited by lightening or human activities, the oxidation of built-up biomass results predominantly in the loss of:

1. needle-leaves and tree lichens
2. summer-dried dead twigs and branches
3. surface litter of the forest-floor organic layer, sometimes called "pine straw" and organic layer "duff"
4. dead and dry stems or trunks, standing or fallen.1

The Fine Fuel Moisture Code of Foresters:

Foresters have thought up several useful predictors of the likelihood of ignition and fierceness of wildfires. They have created one called the FFMC or Fine Fuels Moisture Code. Fine fuels like dead needle-leaves and dead twigs dry quickly in direct sun or when humidity is low. This is why we hang our laundry outdoors. It can dry quickly but it can also pick up moisture from the air if conditions turn humid, as at night. I see this physical process at home. My wood-burning stove is a little harder to light on wet mornings as the matches, paper, and wood kindling all become damp from absorbing atmospheric moisture. This observation explains why forest fires slow down at night when fuels are cooler, when they get no added radiation heat from the sun, and when higher humidity levels dampen fine fuels. This observation also explains how fires can roar through forests in spring sunshine even while the deeper litter-duff layer is still damp from spring snowmelt and rains. The fine surface fuels are dried in the spring sun, and this material plus the living tree needle-leaves are sufficient to promote fierce fire in gusty spring weather. The moisture condition of forests can be viewed nowadays through special satellite imaging. These great maps on computer screens help land managers.

So yes, the weather and the fine physical dimensions of fuels are important factors in predicting the outcome of fire ignition, spread, and energy release, but there is another contributing factor, a much deeper reason that takes us very far back in time giving an understanding of the pressures driving the evolution of trees.

Ancient Trees, Ancient Insects

The gymnosperms here in our boreal forest and many I have seen in tropical and dry lands are loaded with insecticides and most of these materials are very flammable indeed. A never-ending war goes on between arthropod bugs—mainly insects—and trees (which are not as passive as we may think). Need I say the words: defoliator, stem-borer, termite, or insects sealed in amber? Stand quietly in a summer forest and hear the frass or insect dung fall on the dry leaf litter. Trees are sedentary and cannot escape their annoyers/killers and so some like spruce, fir, larch, cedar, juniper, and pine produce high-energy wax, resines, and terpenes from which people make varnish, violin rosin, disinfectant, incenses, and turpentine. The forest air has the fresh scent of the lighter evaporating molecules. The meat of moose, hare, and spruce grouse has the gamey taste of these terpenes, incorporated in their diet of plant life. No doubt some of the dinosaur herbivores, like Brontosaurus, had the same gamey terpene taste, while other dinosaurs rejected the gymnosperm foods 2. It is as if our trees are loaded with diesel or gasoline; in fact, they contain biodiesel. In contrast maples, ash, birch, willow, and most of our other hardwood angiosperm flowering trees produce very little of these materials and the fire frequency and fierceness in hardwood forests is much lower. In hardwood forests spring sunshine dries the exposed leaf litter before new tree leaves have unfolded. Thus fire moves along slowly through the litter but rarely climbs the trunks. In fact, a fresh maple leaf cannot be ignited with a lit match. Try it yourself. Maple contains too much water and too little volatile chemical.

Tropical and Canadian Tree Defence

Relatives of our native Christmas trees are also loaded with insecticides. A good example, the Norfolk Island Pine, is found off the east coast of Australia, and is in fact, an Araucaria and not a pine. It is widely planted in the tropics and grown as an indoor plant in Canada. Another is the large and striking Monkey Puzzle Tree, Araucaria arucana of temperate Chile-Argentina, and planted further north in Andean Ecuador and even in mild England, Ireland, and coastal British Columbia. In the humid tropics, pressure from insects has been so great that some of the later-evolving flowering trees, angiosperms, have retained and evolved the same or similar physical-chemical strategies. For example, the fruit tree Almesca, Protium heptaphyllum, Burseraceae, common in tropical Brazil, produces copious amounts of sticky resin under the bark which glues and suffocates insect bodies or gums their mouth parts. These resins are also chemically toxic to many insects and some fungi and bacteria. Camphor, Cinnamomum camphora, and costly Aloeswood trees, Aquilaria agallocha, from east Asia have a similar strategy; Eucalyptus produce leaves with highly flammable oils, and the Palo Santo Tree, Bursera graveolens, found in dry lands from Argentina-Ecuador, is amazingly aromatic. Even the extinct tree lycopod Lepidodendron produced resin that has been transformed to Baltic amber. 3 Citrus trees and our two native Myrica bushes produce terpenes. If any citrus fruit peel is squeezed near a candle flame, the fine sprays of oil will flare. Try it and show this to children.4 The chemicals in conifers also repelled many, but not all, vertebrate herbivores throughout their evolutionary history. Cattle, llama, sheep, beaver, and even the green iguanas, one of the few remaining herbivorous reptiles, all reject conifers as food. I have tested this personally. Incidentally, beaver also reject our native Winter Holly, Ilex verticilata, as food, and so this uncommon bush persists in wetland, at the expense of the willow that the beaver favours.

Fire and Growth Drives Long-Term Forest Cycles or Pulses

Natural selection had no forethought in plant evolution and little in animals until we humans began manipulating wolf-dogs, edible plants, and our own offspring through mate selection.5 And so when ancient insects attacked or selected ancient gymnosperm populations the surviving adult trees contributed, without forethought or intent, to their own destruction by fire. 6 The forest process of year-by-year addition of fine woody biomass loaded with flammable chemicals nearly always leads to the death of the mother/father tree. Luckily, seeds of the offspring often survive in compact cones.7 So consider that some gymnosperms, in their struggle with insects, were set forth on a path of growth, burn, and seedling regeneration that we see today, many millions of years later. These trees have evolved a few anatomical changes like thick bark and compact cones to cope with expected fires. The argument I have presented here is the more complete and deeper prehistoric answer to the question of 'Why the Boreal Forest Burns.' This discussion points to the reality that forests must be studied in long cycles of 100 years or more, and not for the short periods of most university degree programs.

More Reading

Ideas like these concerning Boreal Forest process and evolution are presented in the discussion section of my short M.Sc. thesis (Day 1981). There are copies at the University of New Brunswick, Memorial University, the College of the Atlantic Corner Brook, and in the libraries of Newfoundland and Labrador's two large national parks—Terra Nova and Gros Morne. I wrote a similar short note in The Osprey a few years ago (Day 2005), and while researching that piece, I found through direct testing that the fine, crispy, whitish Reindeer Lichens in old barrens are the main biomass of flammable fuel. Readers may want to compare the different processes in barrens and conifer forest. In Lichen Woodlands, so common in Labrador and northern Quebec, both lichen and conifer biomass contribute to flammability and the process of fire cycles.

This note was written on the biologically diverse and archaeologically rich coast of Ecuador where I made interesting friends in Porto Lopez, Jama, Bahia, and Machala.

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1- The trunks and branches of living conifers are not consumed in flame. They are just too wet and a heat sink for the flame.
2-Brontosaur flesh tasting of terrenes had to be. This is not conjecture. The same goes for most of the large fish and meat-eating marine reptiles. Their flesh had to taste like seal, whale, and seabird meat both oily and fishy.

3-Amber is not a petrified fossil that has become stone. Amber is a collection of oil-soluble chemicals where the light molecules have been lost, leaving a hard nodule. Amber can be burned and can also be dissolved in a light oil or ether.

4- Forethought in selection is evident when we humans began manipulating wolf dogs, edible plants, and our own offspring through mate selection. Some molluscs, arthropods, fish, birds, reptiles, and other mamals also practice mate selection, and it is likely practiced by microbial life at a chemical or membrane level.

5- Because of the presence of these oils in citrus juices, these drinks should never be served in styrofoam cups as the oils dissolve the styrofoam structure. Demonstrate this to children by bending the peel and spraying a styrofoam cup surface with the citrus juice. In addition, conifer litter and sawdust should never be mixed with garden soils as the terpenes are antimicrobial.

6- Imagine if humans had evolved similar chemicals in our skin protecting us from fleas, lice, scabie mites, bedbugs, blackflies, deer flies and mosquitos. Imagine our evolutionary history without mosquito-born malaria, worms, dengue, and yellow fever. Then contemplate our cultural evolution with a flammable skin.

7-There are exceptions, for example, the very thick airy and insulsating bark protects old Red Pines from fire, and, remarkably, Pitch Pine, Pinus rigida, from the eastern US is able to resprout from buds beneath the bark. Balsam Fir is quite different: it is vulnerable to fire because of a thin unprotected bark, and the cones and seeds are often destroyed by fire. Some fir manages to survive forest fire in wetter pockets of boggy land, often bypassed by flame.

References:
Day R.T. (1981). The vegetation and organic layer in Kalmia Spruce postfire communities. MSc. thesis Memorial U. of Newfoundland.
Day R.T. (2005). Why do Kalmia Barrens burn? The Osprey 36(3): 64-65.

copyright Robin T. Day June 30 2008. Will appear in summer issue of The Osprey magazine, 2008, St. John's Newfoundland & Labrador, Canada.

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