Recognizing the importance of Mother Trees and the vast below ground network connecting all trees has added to our understanding of how forests work.
These connections have been known for a long time by Aboriginal peoples and the research of Dr. Simard and others has demonstrated scientifically that forests are deeply connected and collaborative places.
In mapping the fungal network, our research has shown that the biggest and oldest trees are the most connected nodes in the forest. These highly-connected hub trees, also known as Mother Trees, share their excess carbon and nitrogen through the mycorrhizal network with the understory seedlings, which can increase seedling survival. These Mother Trees in this way act as central hubs, communicating with the young seedlings around them. In a single forest, a Mother Tree can be connected to hundreds of other trees.
Our research has shown that kin seedlings receive more carbon from Mother Trees than stranger seedlings do. The Mother Tree sends carbon to other seedlings in the neighbourhood as well.
Additional research has shown that large hub trees (that we call Mother Trees) transmit carbon through the mycorrhizal network to young seedlings in need. The carbon moves along a source-sink gradient, where the larger tree is the source of carbon and the seedling is the sink.
Network model showing the linkages between Douglas-fir trees through the mycorrhizal network. The arrow points to the most highly connected tree. Diagram from Beiler et al. 2010.
Trees can also form complex relationships with other species, similar to partnerships or friendships. Birch can compete with young neighbouring conifer trees for light and reduce their growth. However, in contrast to conventional thought, birch can also help Douglas-fir by sharing carbon through mycorrhizal networks. The more birch shades Douglas-fir, the more carbon it delivers through the mycorrhizal networks. As well, cutting out birch trees stimulates infection of neighboring Douglas-fir with root disease. The trees also trade resources seasonally – Douglas-firs shares excess sugars with the leafless birches in the spring and fall, and in return the birches provide the Douglas-firs with sugars in the summer.
Illustration from National Geographic magazine explaining how trees in a Douglas-fir forest use the mycorrhizal network to relay stress signals and share resources with one another.
View Graphic (PDF)As forests become stressed (e.g., under hotter and drier climates), seedlings are more dependent on mycorrhizal networks for establishment and survival. In arid ecosystems, Mother Trees provide germinants with essential carbon, water and nutrients through mycorrhizal networks. In less stressed ecosystems, these exchanges still occur but the extra help is less crucial to survival because resources are naturally more abundant.
Traditionally, forestry has treated trees as individuals and not as though they live in communities. However, the understanding that trees are linked below ground to their kin and extended community has the potential to influence the way we practice forestry.
The many hub trees and overlapping networks in a forest provides resilience – one or two hub trees can be removed without causing the network to collapse. The remaining hub trees within the network still allow for the flow of communication and the trading of resources. As a result, the death of one tree does not mean that the whole forest unravels, in the same way that a human community can sustain the loss of a community member without collapsing.
Our experiments have also shown that when forests are harvested, the retention of Mother Trees helps the forest regenerate. Seeds from the Mother Tree germinate nearby and quickly tap into the fungal web and receive resources that boost their chances of survival.
Other experiments have shown that connections between conifers and deciduous species like aspen and birch are important for mitigating disease and insect outbreaks. Protecting deciduous trees rather than removing them by cutting or spraying them with herbicides also makes forests more resistant to fire because the broadleaves hold more water, contain less resin, and are less flammable than conifers. Many other studies have shown that protecting old trees and a deciduous component helps maintain biodiversity of birds, animals, plants, fungi, and other organisms.
Additionally, experiments have shown that the retention of old trees and their extensive networks can reduce loss of carbon from the ecosystem—both above and below-ground. The Mother Tree Project is testing how different retention levels of Mother Trees could protect existing carbon pools.
The Mother Tree Project is investigating how the connections between Mother Trees, seedlings and other plants enhance resilience of the forest community. Keeping communities of trees intact by maintaining the network of connections should help them resist the stress of climate change and recover more rapidly when climate-related disturbances do occur. In other words, connected forests are better able to cope with climate change and be productive, healthy and diverse and around for many generations to come.
Video from National Geographic explaininghow trees in a Douglas-fir forest use the mycorrhizal network to relay stress signals and share resources with one another.
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