Underground Connection: Fungi and Pines in Peril
By Dr. Cathy L. Cripps
In his book Diversity of Life, E.O. Wilson states that “Most life on land depends ultimately on one relationship: the mycorrhiza, the intimate and mutually dependent coexistence of fungi and the roots systems of plants.” His point is that the importance of these beneficial fungi should not be underestimated. This is especially true for forest ecosystems, and those of whitebark pine are no exception.
WHITEBARK PINE FORESTS IN PERIL
Whitebark pine (Pinus albicaulis) is North America’s only stone pine — pines with seeds that are not dispersed by wind — and it is primarily limited to the Pacific Northwest and the north-central Rocky Mountains. This five-needle pine is considered a keystone species at treeline elevations where other conifers have difficulty surviving. At these high elevations, it forms magnificent forests of gnarled old growth trees or low-tangled krummholz forms. However, whitebark pine populations are diminishing at an alarming rate because of record infestations of mountain pine beetles coupled with the devastating effects of the invasive white pine blister rust. The results are ghost forests of whitebark pine skeletons. The species has been granted endangered status in Canada and awaits this designation in the United States. Enormous efforts are underway to restore lost forests with large plantings of disease-resistant nursery-grown whitebark pine seedlings. But, can we also retrieve all the complex parts of this unique ecosystem? And what about the mycorrhizal fungi — how crucial are they to the survival of these forests?
A COMPLEX ECOLOGY
Above ground, the ecology of whitebark pine forests is a fascinating story of the interaction of trees, birds, squirrels and bears. In fall, when the round purple cones mature, the forests erupt in a riot of sound and a flurry of activity. Clark’s nutcrackers, large gray corvid birds with long bills, attract each other to the ripening food source with their raucous calls. Landing precariously in tree tops, they incise seeds out of cones like surgeons and gulp them into their crops. They fly off to bury their treasure as a food stash. Meanwhile, red squirrels chatter high above trying to beat the birds to the cones by clipping and dropping whole branches — an efficient way to gather a winter’s cone supply. Later, as winter approaches, grizzly and black bears raid the squirrel stashes for the fatty seeds. In spring, seedlings germinate in clusters from the un-retrieved seeds and, remarkably, this is the main dispersal mechanism for whitebark pine as a species; it depends on the forgetfulness of the birds.
What goes on below ground is more of a mystery. Here, a major portion of the trees exist as extensive and massive roots systems that push through hard soil in harsh high-elevation habitats just below treeline. This is the interface between the living and non-living world. It is a dark place where pale, living roots come into contact with minerals in the soil — or do they?
In a twist of nature, certain mycorrhizal fungi insert themselves between tree roots and soil to form a protective barrier around each root tip. In addition, the fungi boost the uptake of nitrogen into roots by extending their long thin bodies into the soil where they forage for scarce nutrients. The microscopic threads that comprise their bodies are called hyphae — or, when in mass, mycelium — and they act as conduits, moving nutrients and water from soil into roots. These fungi can also protect tree roots from heavy metals, tiny invertebrate grazers, and root pathogens hiding in the soil. The fungi themselves are able to eke out a living on photosynthetic scraps, sugars that leak from fine roots.
‘Mycorr-’ means fungus and ‘-rhizal’ means root and the unions of these two entities, called mycorrhizae, are found on the roots of more than 80 percent of the plant species on Earth. This mutualism is not the exception, but the rule in nature, and all forests — except perhaps those of mangroves — depend on mycorrhizal fungi. This has been true for thousands of years.
The group of fungi that attach themselves to woody plants, mostly trees, wrap themselves around the outside of roots to form what are called ectomycorrhizae. These tiny sock-like structures are found on each of the thousands of root tips throughout a forest. Although miniscule, ectomycorrhizae have a huge impact on the survival of trees, many of which could not maintain themselves in nature, especially in harsh climates, without these fungi. They are only visible when they reproduce and their fruiting bodies (i.e. mushrooms) push up though the soil to produce the spores that fly off in the wind to land, mate and continue the species’ life cycle.
NO TWO FUNGI ARE ALIKE
Just as each tree species is not interchangeable with another, so too one fungal species cannot be substituted for another without altering the biology of an ecosystem. There are thousands of species of ectomycorrhizal fungi in the world, each with its own unique ecology, physiology, and preference for certain tree hosts.
As an example, Cenococcum is a tough hardy fungus with dense black mycelium that is found everywhere, often at low levels just below the radar. But it comes into its own during periods of drought, when it thrives while other fungi die, all the while passing the benefits of drought tolerance on to its tree partner. Another example is Piloderma, a promiscuous fungus that attaches itself to many different tree species; it not only gains sugars from tree roots but also has the ability to decompose dead organic material and pass the nutrients along to living trees. Its bright yellow mycelium can be observed in rotting wood or organically rich soils as well as on roots. Douglas-fir alone associates with more than 2,000 different species of ectomycorrhizal fungi, and some are only found with this species. A single tree in a forest can host any number of ectomycorrhizal fungi simultaneously, with each providing its own unique set of benefits. Often, there is a succession of fungi found on a tree over its lifetime. Certain ectomycorrhizal fungi are important in a tree’s establishment as a seedling, and other fungi come in later to provide benefits in mature forest situations.
FUNGAL ALLIES OF WHITEBARK PINE
Until recently, we knew very little about ectomycorrhizal fungi in whitebark pine forests. Our mycology lab at Montana State University (MSU) has discovered that from the Greater Yellowstone Area (GYA) north into Alberta, these forests appear to host only a limited number of ectomycorrhizal species in great contrast to Douglas-fir forests. Some of these fungi are specialists and are only found with five-needle pines; some only occur with stone pines. There are several species of stone pines scattered throughout Europe and Asia, in addition to the whitebark pines of North America.
A well-known fungal inhabitant of stone pine forests in Europe (P. cembra forests) and Asia (P. pumila forests) is the so-called Siberian slippery jack (Suillus sibiricus). Remarkably, this fungus also occurs with five-needle pines in western North America, and we now report it from most of the whitebark pine forests studied in the GYA. These records show that the Siberian Suillus has a long evolutionary history with stone pines in the northern hemisphere that may be an indication of its importance to these pines; it does not occur with other conifers or on two/three-needle pines. Other examples of specialists with whitebark pine include S. tomentosus variety discolor and S. subalpinus; the latter is only known from the Yellowstone region. A variety of mammals, large and small, devour the mushrooms produced by these fungi as a natural part of the forest food chain.
Other special ectomycorrhizal fungi found in whitebark pine forests include Rhizopogon, commonly called “pogies.” These interesting fungi spend their whole life underground. Instead of mushrooms, they produce subterranean tuber-like fruiting bodies often called false truffles. These marble- to golf-ball-sized tubers have unique odors that attract squirrels, voles, deer and bears — including grizzlies — which sniff them out and dig them up as a food source. When ripe, these pogies can smell intriguingly musky, fermented fruity or sour — even like blue cheese. The spores located inside the pogies are capable of traveling through the digestive tract of a mammal unharmed and are eventually deposited in pellets and piles. In a sense, these fungi have co-opted mammals as their spore dispersal mechanism just as whitebark pines have co-opted birds to disperse their seeds. Ultimately, the spores germinate and reunite with a root to form an ectomycorrhizal bond after passing through the gut of a mammal. When greasy scat samples of grizzly bears were brought into the MSU lab, examination with a microscope revealed a mass of one kind of spore. The bears had feasted exclusively on one particular kind of pogie, no doubt with a very attractive odor!
Coming full circle, while we see whitebark pines providing habitat for the animals that eat or spread its seeds and rest or nest in boughs and shade for the understory plants that proliferate at their base, there are even more intricate ecological relationships among these organisms, including the ectomycorrhizal fungi that help maintain the forests.
A MISSING PIECE IN RESTORATION
Are these fungi a missing piece of the puzzle in whitebark pine restoration? Can they be used to jumpstart the whitebark pine seedlings being planted in such huge numbers to help a greater number survive?
In Austria, forests on steep slopes were clear-cut to extend pastureland many years ago. Subsequent efforts were made to restore these forests which included the planting of European stone pine seedlings, a practice that continues today. Fifty years ago, a wise mycologist named Meinhard Moser discovered that adding the spores or mycelium of Suillus sibiricus (and related species) to nursery seedlings could significantly boost the survival of these nursery seedlings when they were out-planted. Today, mature forests are flourishing on the alpine slopes of Austria and the Siberian Suillus is still used on the stone pine seedlings in the Austrian Federal Nursery. Inoculation of nursery trees with ectomycorrhizal fungi is now status quo in Europe.
At MSU, graduate student Erin Lonergan and postdoc Dr. Eva Grimme examined how this practice might be applied to nurseries in the western United States. From their research, we now know that S. sibiricus can form ectomycorrhizae efficiently on whitebark pine seedlings in the greenhouse and that colonization will occur if given enough time (a few months) and if a low nitrogen fertilizer is used. High nitrogen fertilizers and fungicides prevent mycorrhizal colonization. The age of the whitebark pine seedlings at inoculation is also important since prolific side roots need to develop before colonization can take place. When successful, hundreds of tiny, white sock-like ectomycorrhizae can be observed on the roots of containerized seedlings.
A large field study conducted in the International Peace Park, which includes Glacier and Waterton Lakes national parks, tested how the application of S. sibiricus spore slurries to whitebark pine seedlings in the greenhouse affected their survival in the field. This project was led primarily by conservation biologist Cyndi Smith from Waterton Lakes National Park in collaboration with Lonergan. Numerous park volunteers and the Glacier Park revegetation crew helped plant the 1,000 seedlings originally grown at the U.S. Forest Nursery in Coeur D’Alene, Idaho. The study also examined the use of shelter objects (such as stumps and logs) and burning (torching in this case) on whitebark pine seedling survival. Early results show an increasing trend over time for improved survival of the seedlings inoculated with S. sibiricus and planted on burns. Planting in microsites and on burns also improved seedling survival. Early results look promising and large-scale application for inoculating seedlings of whitebarks and limber pines is slated for nurseries in Canada in the near future. Effects appear to be site specific and inoculation with ectomycorrhizal fungi is most likely to make a positive difference in soils devoid of whitebark pine’s native mycorrhizal fungi; this would include severe burns, areas not previously populated by whitebark pine and possibly ghost forests.
As we work to save the vital whitebark pine from disappearing from the landscape, it is essential to use all available tools. Ectomycorrhizal fungi are an integral part of forest integrity, ecology and health; showing respect for these mighty microbes might just mean the difference between the restoration and death of a forest.
Dr. Cathy L. Cripps is an associate professor at Montana State University. Her website is: http://plantsciences.montana.edu/facultyorstaff/faculty/cripps/cripps.html
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