Use the following information to answer the question.
Suzanne Simard and colleagues knew that the same mycorrhizal fungal species could colonize multiple types of trees. They wondered if the same fungal individual would colonize 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 photosynthesized, 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 hypothesized 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 were 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.

Question

Use the following information to answer the question.
Suzanne Simard and colleagues knew that the same mycorrhizal fungal species could colonize multiple types of trees. They wondered if the same fungal individual would colonize 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 photosynthesized, 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 hypothesized 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 were 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.

Quizplus · Accepted Answer

If a shade is placed over the birch and cedar, and the Douglas fir is left in full sun, then the Douglas fir should receive more sunlight and have higher photosynthetic rates than the shaded birch and cedar. Therefore, there would be a net movement of carbon from the Douglas fir to the shaded birch and cedar. Since carbon-13 was injected into the bags with the Douglas fir and carbon-14 was injected into the bags with the birch, there will be more carbon-13 detected in the birch than carbon-14 detected in the Douglas fir. Thus, the researchers would expect to find more ¹³C in the birch than ¹⁴C in the Douglas fir.

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