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Are some evergreens insectivorous by proxy?

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By Chelsie Vandaveer

December 26, 2001

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Evergreens of the temperate and boreal areas have mutualistic associations with fungi generally called ectomycorrhizae [ek' toe my' co rye' zay]. Ectomycorrhizal fungi form a mesh of hyphae called a Hartig net around the roots of the tree. This net serves as an interface between the root and the fungus.

Trees benefit from the fungal relationship. In attempts to reforest grasslands, trees that have been inoculated with fungi grow better than trees not inoculated. It has always been believed that fungi absorb nutrients from decaying matter and pass the nutrients to the trees. But that is not the whole story with the fungus, Laccaria bicolor [lak car' ee a bi' col or].

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In Saint-Brisson, France, a study of Douglas fir (Pseudotsuga menziesii) discovered that trees inoculated with Laccaria bicolor grew 60 percent more wood in eight years than trees that had to establish a fungal relationship on their own. The inoculated plots of trees also established a very diverse symbiotic microflora in the soil.

In experiments using Norway spruce (Picea abies), seedlings were grown in a laboratory with and without Laccaria bicolor. For the first 15 days after the treatment, the seedlings with L. bicolor slowed their root elongation by 26 percent, but then increased root growth by 136 percent. The treated seedlings grew 8 times larger and had twice the number of needles.

When de-rooted seedlings were treated with the fungus, most were able to re-grow a root system. These growth rates have been attributed to the fungi's ability to produce indole-3-acetic acid (IAA), a plant hormone.

But a study using white pine (Pinus strobus) at the University of Guelph in Ontario revealed that L. bicolor may have an even stranger relationship with evergreens. Springtails (Folsomia candida) were added to the pots of pines to see how the pine/fungus relationship would be affected. Surprisingly, the springtails did not eat the fungus.

Laccaria bicolor paralyzed the springtails, grew hyphae into the insects, and digested them. Less than 5 percent of the springtails were able to survive beyond 2 weeks in the presence of the fungus. Some of the nitrogen from the insects' bodies was passed through the fungus to the white pine. In return, it was discovered the fungus benefits by getting carbon compounds from the pine.

M. Snowarski's Fungi of Poland has posted a beautiful photograph of Laccaria bicolor taken by Bohdan Cienciata. To view the fruiting body of this fungus, click on the link:

http://www.grzyby.pl/gatunki/Laccaria_bicolor.htm

Photographs of springtails, Folsomia candida, have been posted by Dr. Steve Hopkin of the University of Reading in the United Kingdom. To view the photographs, click on the link:

http://www.stevehopkin.co.uk/collembolamaps/Entomobryomorpha/261FOcan/

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