Tree-tapping by our Native Orchids
This article appeared in the publication Die Orchidee 58, 2007 - 2 Translated (somewhat freely) by I. Schmidt-Ostrander. Mr. Dietmund Klärner had earlier published an article about this in the Feuilleton of the Frankfurter Allgemeine Zeitung, #275, under “Nature and Science” 24.11.2004, p.34.
He calls to our attention the fact that aside from tropical epiphytic orchids, there exist not only semi-parasites like our European Mistletoe (Viscum album) but he includes terrestrial orchids from our temperate forests like Cephalanthera, Epipactis and Neottia for instance, which utilize food sources in the soil and what can be gained from the roots of tall trees and shrubs.
With over 20,000 species, the orchids represent the largest plant family. But even though they are so varied in their appearances, the continued existence of each one depends on fungi. In order to grow to maturity, each seedling must secure such a partner. From then on, the plant lives in close relationship with its fungi. Like many other plants, they form micorrhiza: a web of fungus-threads surrounds the root tips. (According to Strasburger 2002, the ectrotrophic micorrhiza of trees can also serve as symbionts for the endotrophic micorrhiza of orchids) and there can exist a metabolic exchange. Only a few orchids will need the carbohydrates supplied in this manner. After all, most of them have green leaves which, in sunlight, will photosynthesize to supply their own supply of carbohydrates.
Scientists at the University of Berkeley, California and at the University of Bayreuth, Bavaria, have found that there are other variations in orchid feeding. There are orchids that grow terrestrially in deep shade and do have green leaves. But instead of only depending on their own ability to photosynthesise, they accept a part of their sugars via their fungal partners, which are in contact with tree roots. Thus these orchids tap into the supply of tree food.
The researches collected for their examinations samples of leaves and roots from different orchid species in Bavaria, Germany. They identified the fungi on those roots through molecular-genetic testing. In wet meadows, the researchers found the regular orchid-fungi on Dactylorhiza and Epipactis. However, in nearby forests, they found on six out of seven orchid species those fungi which are regularly found on tree roots. For instance, Cephalanthera had contacts from Inocybe and Cortinarius (both terrestrial, micorrhizal fungi occurring world-wide); on Epipactis they found Inocybe and Truffle fungi.
The fact that all six of these orchid species receive a part of their carbohydrate supply via the fungi can be deducted from the presence of the carbon isotope 13. The plants’ leaves contain this isotope in somewhat higher concentration than neighbouring plants. The nitrogen isotope 15 is remarkably high in these orchids. Of course, the highest concentration of these two isotopes was found in Neottia nidus-avis, which is a plant without any chlorophyll and lives as constant parasite.
From the presence of the two isotopes, the researchers were able to estimate the contribution of micorrhizal fungi in other orchids. In those from the wet meadows it was found that the orchids received a good 25% of their nitrogen through the fungal threads. They were found to be autonomic in their production of carbohydrates. It is assumed that they even donate some products of their photosynthesis to their fungus partners. Forest dwelling orchids, living close by the trees, appear to be more dependent on their fungal partners for all their nourishment. They receive not only two-thirds of their nitrogen supply but also up to one third of their sugars from their partners. The boundaries between self-sufficient orchids and those that are completely dependent on fungi are therefore not solid. How much an individual plant manages to produce for itself is not only dependent on which species it is, but also on how much light is available to each plant.
It is not a peculiarity of the green-leaved orchids living in Bavarian forests to utilize their neighbouring trees’ fungal partners. The scientists found the same thing going on in Great Britain and in North America from Quebec to California. Those orchids which are partially nourished via tree micorrhiza can flourish even under a dense canopy of trees without much access to sunlight. This can of course also be a negative arrangement. When the forest and its fungi begin to change, the orchids will also have to suffer the changes. As a result, a number of those forest-growing terrestrial orchids are now on the endangered list.
Ingrid Schmidt-Ostrander - Canadian Orchid Congress