An Important Ecological Phenomenon
As ectomycorrhizal hyphae extend out from one plant's roots they often encounter the root systems of different plants. If those plants also associate with ectomycorrhizal fungi, the hyphae will often grow around the root and create a new mycorrhizal structure. In this way, two plants can be linked into what is called a "common mycorrhizal network". The networks can link individuals of the same species as well as create networks between individuals of different species. These networks were first observed in laboratory studies that used glass boxes, which allowed researchers to visually follow hyphae from one plant to another. The presence of networks in the field was assumed to occur, but until recently there was little specific evidence. The main reason for this is that hyphae are both small and fragile and any disturbance of the soil often breaks hyphal connections. So researchers were unable to dig into the soil to see the networks without destroying them in the process.
With the advent of molecular techniques, studying mycorrhizal networks became much more tractable. By taking small soil cores that contained roots of different species, scientists could extract fungal DNA from the roots and determine if they were the same species. If the same fungal species was present on the roots of different tree species, it provided strong evidence that the plant species were connected by a common mycorrhizal network. Although this method did not conclusively demonstrate a direct hyphal linkage between the plants, most fungal individuals extend over areas much larger than the diameter of a small soil core, so it was most likely that plant species were connected by hyphae of the same fungal individual. Many studies have used this method and almost all of them have found that common mycorrhizal networks are quite widespread. For example, a study at Pt. Reyes showed that Bishop Pine (Pinus muricata) and Douglas fir (Pseudotsuga menziesii) were connected by mycorrhizal networks and another on Mt. Tamalpis showed that Douglas fir and tanoak (Lithocarpus densiflora) were connected belowground. In the latter study, the researchers found that although common mycorrhizal networks were common, the fungal species connecting the two trees was different at different locations in the forest.
While demonstrating that common mycorrhizal networks were widespread was an important first step, big questions still remained about the ecological function of these networks. There has been lots of speculation in the scientific literature, particularly because many laboratory studies had shown that both carbon from the tree and nutrients taken from the soil by fungi could pass between plant individuals linked by a common mycorrhizal network. This research suggested that plants could potentially facilitate the growth of other plants (both of the same and different species), which could play a very important role in seedling establishment, forest succession, and other plant-plant interactions. Demonstrating this in the field though was a challenge, since one would need to set up an experiment where plants were grown with and without access to common mycorrhizal networks. One way to do this would be to dig trenches around some plants (which severs any hyphal connections) and leave other plants untrenched. The problem with this method, however, is that in addition to severing the common mycorrhizal network in trenched plots, the roots of other plants are also eliminated. So changes in growth of plants in those plots could be due to the absence of the common mycorrhizal network or lack of root competition from other plants. Because of this difficulty, understanding the ecological significance of these networks in the field remained relatively unresolved until very recently.
A field study by Kazuhide Nara has shed important new light on this subject. He used a novel approach of first establishing common mycorrhizal networks in glass boxes in the laboratory and then transplanting the boxes into the field. He also took advantage of working in a study area where there was no native mycorrhizal inoculum. He worked in a volcanic desert on Mt. Fuji, where there has been very little plant recolonization following a 1707 eruption. By having no inoculum, the networks he established in the lab were able to remain the only connections between seedlings in the field. He connected very young and older willow (Salix reinii) seedlings with eleven different ectomycorrhizal species independently. This allowed him to examine whether common mycorrhizal networks formed by different fungal species had different effects on the plants. He also had non-mycorrhizal control boxes, which contained the same seedlings but no common mycorrhizal networks.
To analyze the importance of the networks for seedling establishment, Nara focused primarily on the growth and nutrient status of the younger seedlings. In the boxes where there was no common mycorrhizal network, the young seedlings showed very poor growth and this appeared to be driven by competition with the older willow seedlings in the same box. In comparison, the growth and nutrient acquisition of the youngest seedlings connected to older seedlings by common mycorrhizal networks was significantly greater. So it appeared that competition was reduced by the mycorrhizal network. Interestingly though, growth of connected seedlings varied considerably depending on which fungus was used to create the common mycorrhizal network. Seedlings connected by Hebeloma leucosarx and Russula sororia did the best, while seedlings connected by Laccaria amethystina did the worst. In fact, seedlings connected by Laccaria amethystina did not grow any better than the seedlings that had no mycorrhizal fungi. This variation is very important ecologically because we know that the distribution of most fungi is quite patchy. Depending on where a seed lands, the mycorrhizal network that is formed may consist of different species, which may or may not provide benefits to the seedlings during their establishment.
Nara's work represents a major breakthrough is our understanding of how common mycorrhizal networks affect seedling establishment in field conditions. While the variation in the effects of different fungi was quite interesting, we know from many studies that plants are often colonized by multiple ectomycorrhizal species at the same time. Future studies that look at the effects of multiple networks on seedling establishment will help deepen our understanding of this phenomenon. However, due to the inherent difficulties in studying mycorrhizal networks, Nara's work gives us an excellent place from which to start.
- Kennedy, P.G., Izzo, A.I., and T.D. Bruns. 2003. There is high potential for the formation of common mycorrhizal networks between understory and canopy trees in a mixed evergreen forest. Journal of Ecology 91: 1071-1080.
- Horton, T.R. and T.D. Bruns. 1998. Multiple-host fungi are the most frequent and abundant ectomycorrhizal types in a mixed stand of Douglas fir (Pseudotsuga menziesii) and Bishop Pine (Pinus muricata). New Phytologist 139: 331-339.
- Nara, K. 2005. Ectomycorrhizal networks and seedling establishment during early primary succession. New Phytologist. In press.