In this real-life model of forest resilience and regeneration, Professor Suzanne Simard shows that all trees in a forest ecosystem are interconnected, with the largest, oldest, “mother trees” serving as hubs. The underground exchange of nutrients increases the survival of younger trees linked into the network of old trees. Amazingly, we find that in a forest, 1+1 equals more than 2.
Dr. Suzanne Simard is a professor with the UBC Faculty of Forestry, where she lectures on and researches the role of mycorrhizae and mycorrhizal networks in tree species migrations with climate change disturbance. Networks of mycorrhizal fungal mycelium have recently been discovered by Professor Suzanne Simard and her graduate students to connect the roots of trees and facilitate the sharing of resources in Douglas-fir forests of interior British Columbia, thereby bolstering their resilience against disturbance or stress and facilitating the establishment of new regeneration.
Dr. Simard writes:
Mycorrhizal fungi form obligate symbioses with trees, where the tree supplies the fungus with carbohydrate energy in return for water and nutrients the fungal mycelia gather from the soil; mycorrhizal networks form when mycelia connect the roots of two or more plants of the same or different species. Graduate student Kevin Beiler has uncovered the extent and architecture of this network through the use of new molecular tools that can distinguish the DNA of one fungal individual from another, or of one tree’s roots from another. He has found that all trees in dry interior Douglas-fir (Pseudotsuga menziesii var. glauca) forests are interconnected, with the largest, oldest trees serving as hubs, much like the hub of a spoked wheel, where younger trees establish within the mycorrhizal network of the old trees. Through careful experimentation, recent graduate Francois Teste determined that survival of these establishing trees was greatly enhanced when they were linked into the network of the old trees.Through the use of stable isotope tracers, he and Amanda Schoonmaker, a recent undergraduate student in Forestry, found that increased survival was associated with belowground transfer of carbon, nitrogen and water from the old trees. This research provides strong evidence that maintaining forest resilience is dependent on conserving mycorrhizal links, and that removal of hub trees could unravel the network and compromise regenerative capacity of the forests.
In wetter, mixed-species interior Douglas-fir forests, graduate student Brendan Twieg also used molecular tools to discover that Douglas-fir and paper birch (Betula papyrifera) trees can be linked together by species-rich mycorrhizal networks. We found that the mycorrhizal network serves as a belowground pathway for transfer of carbon from the nutrient-rich deciduous trees to nearby regenerating Douglas-fir seedlings. Moreover, we found that carbon transfer was enhanced when Douglas-fir seedlings were shaded in mid-summer, providing a subsidy that may be important in Douglas-fir survival and growth, thus helping maintain a mixed forest community during early succession. This is not a one-way subsidy, however; graduate Leanne Philip discovered that Douglas-fir supported their birch neighbours in the spring and fall by sending back some of this carbon when the birch was leafless. This back-and-forth flux of resources according to need may be one process that maintains forest diversity and stability.
Mycorrhizal networks may be critical in helping forest ecosystems deal with climate change. Maintaining the biological webs that stabilize forests may help conserve genetic resources for future tree migrations, ensure that forest carbon stocks remain intact on the landscape, and conserve species diversity. UBC graduate student Marcus Bingham is finding that maintaining mycorrhizal webs may be more important for the regeneration and stability of the dry than wet interior Douglas-fir forests, where resources are more limited and climate change is expected to have greater impacts. Helping the landscape adapt to climate change will require more than keeping existing forests intact, however. Many scientists are concerned that species will need to migrate at a profoundly more rapid rate than they have in the past, and that humans can facilitate this migration by planting tree species adapted to warm climates in new areas. UBC graduate student Brendan Twieg is starting new research to help us understand whether the presence of appropriate mycorrhizal symbionts in foreign soils may limit the success of tree migrations, and if so, to help us design practices that increase our success at facilitating changes in these forests.