Moose browse is more complicated than it seems
It seems a little early to be worrying about winter browse for moose on the Kenai Peninsula, but recent sightings of bare willow and alder stands are raising the eye brows of biologists at the Kenai National Wildlife Refuge. Large areas of subalpine willow show up as brown patches from the air on the Tustumena Benchlands (see photo). Closer to home, thinleaf alder (Alnus tenuifolia) has been defoliated along the road system from Kasilof to Moose Pass.
Needless to say, for shrubs that store up food for the winter by photosynthesizing carbohydrates in their leaves, these observations don’t bode well for moose once the snow flies. Both willow and alder are browsed on by moose during the winter, and the various kinds of willow constitute the primary food of Alaskan moose in general.
The insects that graze on alder and willow leaves are very different in their life history. Willows are attacked by the caterpillars of several species of Geometrid moth including the autumnal moth (Epirrita autumnata) and Bruce spanworm (Operophtera bruceata). The former is an exotic from Europe but the latter is a moth species native to Alaska. We most likely have the spanworm here on the Kenai. Both species are well adapted to cold weather and have even been seen flying during snow storms!
Geometrid moths commonly appear and abruptly disappear mysteriously. Most outbreaks last less than three years and seldom cause permanent damage to the shrub host. However, you can imagine that a shrub that photosynthesizes poorly during the summer may not have a lot of nutritional value for moose later that winter.
Alders, in particular the thinleaf alder, are attacked by the caterpillars of the green alder sawfly (Monsoma pulveratum). This insect is an Old World species, first found in North America in the mid 1990s and first detected in Alaska in 2005. Unlike Geometrid moths, this introduced sawfly may ultimately kill its shrub host since it appears capable of consuming woody tissue in addition to leaves.
The potential to cause permanent damage is increased by the fact that two other sawfly species, the woolly and striped alder sawflies, may also attack the same alder simultaneously. Furthermore, infestation by the Alder canker, most often caused by the fungus Valsa melanodiscus, can also result in death of the twigs and sometimes the stem.
To make matters worse, two Phytophthora species may be associated with the canker in Alaska. Phytophthora, which in Latin means the “plant destroyer”, is a pathogen that causes root rot. A species of Phytophthora caused the infamous potato blight in Ireland in the mid 1800s. One of the two Alaskan species is unnamed and new to science, while the other (P. alni subsp. uniformis) is involved in alder disease in nearly a dozen European countries. The 2007 discovery of P. alni uniformis in Alaska was the first time that this pathogen had been confirmed in North America. While mortality has not yet been detected in Alaska, Dr. Jim Kruse, an entomologist with the U.S. Forest Service, suggests that the two species may have existed benignly beneath alder or been introduced from Europe, where Phytophthora species devastate alder.
Alder dieback along our streams could have a serious impact on our salmon populations. At a time when a warming climate is raising the temperatures of some streams on the Kenai above the spawning threshold for short periods of time, they need all the shade they can get. Alder is also a natural nitrogen fixer, and contributes directly to stream productivity.
Believe it or not, snow depth may also influence the nutritional quality of shrubs. The protein content of shrubs is increased by higher rates of nitrogen mineralization in the soil. Nitrogen mineralization is increased by higher soil temperatures during winter, and soil temperatures are directly impacted by the depth of snow. Snow acts as an insulating layer, so deep snow means more protein in winter browse for moose.
On the North Slope, research published in 2005 in the journal BioScience indicates that higher temperatures and more snow due to a warming climate have resulted in the spread of shrubs on the arctic tundra. Here on the Kenai, however, even as woody shrubs such as willow and dwarf birch invade 8,000-year-old peatlands due to a warming climate, it’s possible that their nitrogen content may be low if the average snow pack declines over time due to less snow, more rain, or winter temperatures that cause more snow to melt. That’s a little speculative at this point, but it’s the kind of ecological phenomenon that ensures job security for biologists.
So, later this winter, when the moose are browsing on your prized ornamental bush in the back yard and you’re cursing for not having taken the time to fence it in, be appreciative of the unusually tough times that our moose may be experiencing. If that doesn’t work for you, at least appreciate that the science behind this is more complicated than the first glance at a simple twig might suggest.
John Morton is the supervisory biologist at Kenai National Wildlife Refuge. You can find more information about the refuge at http://kenai.fws.gov or http://www.facebook.com/kenainationalwildliferefuge.