The Science of Canada’s Symbol, the Beaver

2018BIGCOINSUB-7The beaver, Castor canadensis, is an official symbol of Canada, somehow representing our sovereignty. Each time we pick up a 5 cent-coin, the so-called nickel, which except for special collectors’ editions is about 95% steel and only 2% nickel, we see an illustration of a beaver. But how much do we know about the natural history and ecology of our icon?

Like many humans, beavers are monogamous and mate for life. They also impact both the physical landscape and biological diversity in their habitat.  Their exact impact varies from one site to another, depending on the location, relief and habitat type—again parallel to the non-uniform ecological footprint of our societies.

During dry periods, as much as 30% of  water in certain watersheds could be held in beaver ponds. This can decrease erosion when water flow increases to higher levels. If a beaver dam however collapses, the opposite effect can occur. Flooding was  caused by such an occurrence in Alberta the 1990s and in British Columbia in the summer of 2000.

MT386
A beaver chewing on a cottonwood. This will lead eventually to the tree’s production of shoots rich in protective compounds.

The presence of beavers is important for shaping the littoral communities in certain lakes of the Canadian Shield increasing the population of fish, crayfish, diving beetles, large bugs, tadpoles, newts and leeches. This happens not just from the changing water levels but because dams concentrate nutrients.

They are also engaged in a fascinating coevolutionary relationship with the type of trees they use to build dams. Regrowth of cottonwood trees felled by beavers results in the synthesis of much higher levels of phenolic glycosides. These plant compounds then serve as a defence against other mammalian herbivores and beaver themselves, ensuring the long term survival of the cottonwoods. Another beaver-target, the quaking aspen, also uses a chemical defence against beavers. Younger trees, although easier to take down, are avoided by beavers because juvenile suckers contain higher concentrations of salicin, salicortin, tremulacin, and tremuloidin. Juvenile suckers are asexual shoots produced by trees that have been cut down but which still have living roots.

glycosides
Notice that each of the above compounds consists a simple sugar linked to a phenolic compound by replacement of a hydroxyl group in the sugar molecule—hence their name: phenolic glycosides, which protect trees against herbivores. The compounds’  concentrations was measured by HPLC-analysis after methanol extraction. (Structures from chemspider.com)

In the ecological web of mammals, it’s not surprising to see beavers play a more direct role than the consequences of their influence on plant biochemistry.  The world’s second largest rodent is an important food source for wolves and black bears. Abandoned beaver lodges can provide breeding shelters for bobcats and winter shelters for badges and red foxes.

Sources

Ecological impact of beavers Castor fiber and Castor canadensis
and their ability to modify ecosystems Mammal Rev. 2005, Volume 35, No. 3&4, 248–276

Optimal central-place foraging by beavers:
Tree-size selection in relation to defensive chemicals
of quaking aspen https://www.researchgate.net/publication/30848534_Optimal_central-place_foraging_by_beavers_Tree-size_selection_in_relation_to_defensive_chemicals_of_quaking_aspen

Beaver Behaviour and Biology
http://www.beaversolutions.com/beaver-facts-biology-behavior/

Catastrophic Failure of Beaver Dam At Chusnulida
The importance of beaver lodges in structuring
littoral communities in boreal headwater lakes http://www.nrcresearchpress.com/doi/pdf/10.1139/z97-121

Justice Laws Website http://lois-laws.justice.gc.ca/eng/acts/N-17/page-1.html

Enduring the 5-cent coin http://www.mint.ca

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The Science and Spirituality of Trees

P1090816At the risk of sounding like a nineteenth century Romantic painter or poet, I feel that trees echo a part of the human spirit that otherwise remains silent in our day to day urban lives. Although their leaves exist to provide more surface area than what green branches alone would provide, trees assume a wide variety of hues in variable sunlight. Their matrices of lignin and cellulose form towering trunks spreading an intricate canopy over our view of the sky. From dawn to dusk, or from spring to autumn, the smells they evoke are not constant. With different wind speeds, a spectrum of rustling and swaying sounds are created. The symphony of colors, fragrances and sound waves of trees along with their imposing strength can interact with our memories and inner feelings. Although they are not of the intensity brought on by another human, trees evoke notes within us that no other living thing can replicate.

Existential issues aside, from the point of view of smaller organisms, a tree is in a sense its own forest. Over the ridges of the bark of some species, there are lichen—  algae and fungus, mutually helping each other* survive and serving as an indicator of pollution in cities. All tree-species provide niches and shelter for insects, birds and small mammals. Most trees would not survive if their roots were without the company of fungi known as mychorrhizas that help them absorb nutrients in exchange for carbohydrates.

And how they produce carbohydrates is a scientific wonder. Photosynthesis occurs in chloroplasts, structures which on a microscopic and evolutionary level reveal another partnership. A billion years ago, endosymbiosis, a process by which large cell engulfed smaller ones without killing them, led to the formation of plasmids. These evolved into the modern set of membranes and genetic material serving as the sites of photosynthesis in all plants.

The overall reaction of photosynthesis mocks what is actually occurring in the cells of trees: 6 CO2 + 6 H2O –> 6 O2 + C6H12O6 . Mix the reactants in vitro, and you would get nothing but carbonic acid! How does a tree or any plant manage to come up with a gas that is more often than not at the opposite end of the reaction arrow? How does it generate something sweet that flows through veins know as phloem, veins that the tree itself constructed from the same building blocks that it made with sunlight, with mychorrihizas’s transferred ions and the two official reagents water and carbon dioxide? Essentially through a network of cooperating cycles, chloroplasts absorb light frequencies in order to eject electrons from chlorophylls. These electrons are returned after they are ultimately taken away from water and transferred to molecules that use them to bond carbon dioxide. But the energy of the sun is not only invested in an electricity-like movement but in creating a voltage by temporarily isolating the hydrogen ions that also result when electron-yielding water splits into oxygen.

In the tropics there are at least 40 000 tree species but possibly more than 53 000. Temperate Europe, in contrast,  has only 124. Although forest cover has improved in Europe since the Middle Ages and has continued to do so recently, elsewhere on the planet areas with the most biodiversity have experienced the most loss. Here is a map showing which areas have done well and which haven’t between 1999 and 2012.mapping_world_trees

Landsat 7 data from 1999 through 2012 were obtained from a freely available archive at the United States Geological Survey’s center for Earth Resources Observation and Science (EROS).  More than 650,000 Landsat images were processed to derive the final characterization of forest extent and change.

http://www.umdrightnow.umd.edu/news/umd-leads-1st-local-global-mapping-forest

From the vantage points of economics, carbon footprints and time management, it would make more sense for urban dwellers (now 54% of the planet) to access wooded areas as close to home as possible.

PittsburghtreeCover
from National Geographic

In the United States, Pittsburgh, formerly known as the Steel City, has steadily reforested its surrounding hillsides, which had been previously cleared for logging and mining. Currently, this new growth forest along with four large parks occupy 42% of the urban area in Pittsburgh. Not only do the trees help filter pollution, avoid soil runoff in the sloped areas, they provide citizens a chance to have their spirits uplifted in an alternative way.

  • Postscript. Many biologists argue that the relationship only helps the fungus and not the algae. And yet some lichens show a three-way symbiosis involving a yeast. See https://phys.org/news/2016-07-yeast-emerges-hidden-partner-lichen.html  The previous reference gives no credit to Goward, a naturalist who first realized that a 3rd partner had to be involved in order to explain a mystery:  Bryoria fremontii, is hairlike, often brown and eaten by Northwestern indigenous peoples, but the lichen, Bryoria tortuosa,  is often yellow- green and toxic, with high levels of vulpinic acid. Yet both species had the same alga and fungus. It turned out that the toxic species had a lot more yeast.

 

Symbiosis and Biodiversity’s Roles in Pine Beetle Epidemic

There have been suggestions that climate change has played a role in the epidemic outbreak of mountain pine beetle in Western Canada.  University of Alberta’s Justine Karst ‘s research on symbiotic fungi sheds light on another factor which makes lodgepole pines more vulnerable. It also serves as another reminder of the importance of biodiversity. What follows is a “transcript” from last week’s interview with Karst by host Bob Macdonald on Quirks and Quarks. After listening to the podcast, I edited the conversation, filling in some details and adding some references.

BOB:   Exactly how does the pine beetle actually kill a mature pine tree?

KARST: When reaching an epidemic population, the females attack pine trees and then send out pheromones into the air. These invite males. After mating eventually the females burrow into the bark and lay eggs, which hatch and feed off the sugar-containing tissues of the tree (phloem). The beetles also carry fungi that attack and plug the xylem (water-conducting tissues). The combination of these events is what kills the trees.

BOB:  So the beetles literally starve the tree by eating its energy source out of its veins?

KARST: Yes, and the fungi interfere with water flow, which aside from preventing photosynthesis also compromises its defense mechanism.

BOB:  And what impact has it had, so far, on the forest?

KARST: There has been widespread loss of pine in British Columbia and parts of Alberta where entire landscapes have lost pines.

beetlerange(Natural Resources Canada points out that since the current Mountain Pine Beetle outbreak started in British Columbia in the early 1990s,  50% of the total volume of commercial lodgepole pine in the province have been killed. A 2006 outbreak in BC fuelled a massive migration into Alberta. Worse, the beetle is now also reproducing in jack pine, the dominant pine species of the boreal forest.)

BOB:  What was your interest in studying the pine beetle?

KARST: I am not an entomologist but interested in mycorrhizal fungi, which colonize roots of most trees in the boreal forest, including pines. These organisms rely on a living tree host, which supplies them with sugars. When I arrived in Alberta and saw entire sections of the forest dying, I wondered what was going on with the associated fungi below the ground and also about the next generation of pines.

BOB: If the fungi is relying on the tree, what does the tree in return get from the fungi?

KARST: These fungi, having small, web-like tissue growing around the fine root tips, are able to access nutrients, which otherwise would be unavailable to the tree. They are also good at fetching water from pores of the soil–that would also not  be accessible to the roots. So it’s typically a mutualism: nutrients and water in exchange for sugars made through photosynthesis.

BOB: How did you go about  studying what was happening below the surface?

KARST: There were two components to this study: first we sowed pine seeds of lodgepole pine in disturbed pine stands and in areas not affected by the beetle. The fate of those seeds was tracked for two years. What we found is that seeds growing in the beetle-killed strands had a lower survival rate than those growing in an undisturbed forest. That was the first stage.

We also noticed that the mycorrhizal community changes after widespread loss of trees. Not only is there a loss in the number of fungi but there ‘s an impact on their biodiversity. So in the second stage of our research, we took samples of soil from both affected and healthy forests and brought them back into the greenhouse. We used this to inoculate seedling grown in pots. After letting them grow for 8 months, we measured monoterpene levels to see if the source of the soil and their associated fungi affected the seedling defense –response.

BOB:  What did you find?

KARST: The seedlings grown in soil from beetle-infested forests had much lower defenses (secreted less monoterpenes) than those from unaffected forests.

BOB:  So what unusual thing is happening in the soil of affected forests?

MS. KARST:  The community changes; species are lost. There is a  certain class of fungi — Leotiomycetes. — belonging to the Helotiales order that live on decaying matter. Their increasing numbers  are probably responsible for the lower defense mechanisms.

BOB:  Where did these fungi come from?

KARST: They were already present but were not as abundant as what you’d find in a forest with widespread mortality.

BOB:  What implications does that have for the regeneration of a forest?

KARST: It might imply that seedlings growing in such forests need some kind of intervention. By triage we might have to identify which strands of the forest have high concentrations of this particular fungus. They have to be inoculated with some other type of mycorrhizal fungi to help them through that window of time when they are  more vulnerable to attack.

BOB:  Without intervention, could other species of trees move in to take over where pine seedlings are struggling?

KARST: White spruce seem to be less sensitive to the species-composition of mycorrhizal fungi. So a shift in tree composition is possible in the future. In fact, that might be a good reason for planting a diverse forest. So in case of another insect outbreak, other tree species can carry the necessary mycorrhizal fungi through to prevent complications in subsequent seedling growth.

BOB:  Biodiversity. Strengthening biodiversity.

KARST: Yes, I think that’s a good message.

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