Suzanne Simard and Colleagues Knew That the Same Mycorrhizal Fungal

Suzanne Simard and colleagues knew that the same mycorrhizal fungal species could colonise multiple types of trees. They wondered if the same fungal individual would colonise different trees, forming an underground network that potentially could transport carbon and nutrients from one tree to another (S. Simard et al. 1997. Net transfer of carbon between mycorrhizal tree species in the field. Nature 388:579-82) .
Pots containing seedlings of three different tree species were set up and grown under natural conditions for three years (Fig.A) . Two of the three species (Douglas fir, birch) could form ectomycorrhizal connections with the same fungal species, but the third species (cedar) could not form an ectomycorrhizal connection with the fungal species. In some of the pots, the researchers placed airtight bags over the Douglas fir and birch seedlings and injected carbon dioxide made from carbon-13 into the bags with the Douglas fir and carbon dioxide made from carbon-14 into the bags with the birch. (13C and 14C are different isotopes of carbon that can be detected and measured by researchers.) As the seedlings photosynthesised, the carbon dioxide was converted into sugars that could be tracked and measured by the researchers. The researchers measured whether the sugars in each plant contained only the carbon isotope that was in the air of their plastic bag or also the carbon isotope from the air around the other plant.

-Simard et al. (1997) further hypothesised that if reciprocal transfer did occur, it would be a source-sink relationship driven by photosynthetic rates. That is, if one seedling is in full sun and the other in deep shade, there will be a net movement of carbon from the seedling in full sun to the one in deep shade. If a shade was placed over the birch seedlings and the cedar, and the Douglas fir was left in full sun, what result could Simard and colleagues expect?

A) More 13C would be found in the birch than 14C in the Douglas fir.
B) The amounts of 13C and 14C would be equal in the Douglas fir and in the birch.
C) The most 13C would be found in the cedar.
D) The most 14C would be found in the cedar.

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