ARCUS 13th Annual Meeting and Arctic Forum 2001

Cycles in the Forest: Mammals, Mycophagy, and Mycorrhizae.

Gary A. Laursen¹, Rodney D. Seppelt², Maggie Hallam^3
1Institute of Arctic Biology, University of Alaska Fairbanks, Department of Biology and Wildlife, 305A Bunnell, Fairbanks, AK, 99775-6100, USA, Phone 907/474 6295, Fax 907/474 6185, ffgal@mail.uaf.edu
²Biology, Australian Antarctic Division, Channel Highway, Kingston, Tasmania, 7050, Australia, Phone 011 61 (03) 62 , Fax 011 61 (03) 62 , rod.seppelt@antdiv.gov.au
³Biology and Wildlife, University of Alaska Fairbanks, 316 Bunnell, Fairbanks, AK, 99775-6100, USA, Phone 907/474 6295 , Fax 907/474 6185, mhallam@alaska.net

Depiction of the forest cycle involves symbiotic white spruce, Picea glauca (Moench) Voss var. albertiana (S. Brown) Sarg., as a host to numerous epigeous mycorrhizal fungi and 12 hypogeous ectomycorrhizal ascomycetes (4 sp.) and basidiomycetes (8 sp.), parasitic spruce broom rust fungi, Chrysomyxa arcotostaphyli Diet., and small mycophagous mammals, principally three species: Northern Flying (Glaucomys sabrinus) and Red (Tamiasciurus hudsonicus) squirrels, and the Tundra Redback vole (Clethrionomys rutilus).

Healthy white spruce live in mutualistic symbiosis with mycorrhizal fungi. Fungal mycelia engulf spruce root tips. Mycelia are much finer than either roots or root hairs and the spruce benefit by this increased surface area for the absorption of labile nutrients and water from otherwise nutrient poor soils. Spruce also gain physical protection for its root tips engulfed by the mycelium. This "gloved casing" provides a barrier from other microorganisms seeking to invade roots. Mycorrhizal fungi also produce antimicrobial compounds that deter competition from other fungi. In turn, the fruitbodies of mycorrhizal fungi benefit from a supply of sugars and amino acids from its host roots. Spruce may even be growing in more northern boreal forest locations where they would otherwise not persist without the advantages of the mycorrhizal relationship. Concomitantly, the mycorrhizal fungi would not be present without the spruce.

Parasitic fungi, and specifically Spruce Broom Rust (Chrysomyxa arcotostaphyli), occur abundantly in the boreal forests of interior and southeast Alaska. It is here that the range of spruce and kinnikinnick or mealberry (Arctostaphylos uva-ursi (L.) Spreng. var. uva-ursi ) coincide. Germinating rust spores on the spruce result is a perennial systemic infection. Fungus produced auxins cause prolific branching of the spruce and the limb mass is called a Witches Broom. Regions of the spruce other than the broom continue to grow normally. Fruiting of the rust fungus occurs on the broom's needles causing an orange coloration. In the fall, needles are shed and the broom appears as a mass of dead twigs. Northern Flying and Red squirrels take advantage of these dense branch clumps. Squirrels "hollow out" brooms, raise their young in these hollows, and then cache limb-dried epigeous and hypogeous mycorrhizal fungi for their winter food supply.

Trees ultimately die from repeated attack by parasitic rust fungi, insects, and mechanical damage. A host of decomposer heart and root rot fungi begin the process toward eventual felling of the dead trees. On the forest floor, the fallen spruce continues to play a critical role in mammal mycophagy and the mycorrhizal cycle by providing convenient raised walkways as "highways for travel" over the forest floor for the animals which in turn leave spore-rich feces as they go. Some spores even require this "right of passage" through a rodent's gut as a necessary precursor to germination, which completes this forest cycle.

This complex biological system is dynamically balanced through the physical environment where any changes will be reflected in the biology. Hypothesised increases in microbial activity can only exacerbate and increase concerns for altering the Arctic Carbon sink to further release unknown quantities of greenhouse gases. Therefore, there is need for more integrated research to fully understand and appreciate these high latitude ecosystems.

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