DP Biology (first assessment 2025)

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Question 23M.2.SL.TZ1.e

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Date May 2023 Marks available [Maximum mark: 1] Reference code 23M.2.SL.TZ1.e
Level SL Paper 2 Time zone TZ1
Command term State Question number e Adapted from N/A
e.
[Maximum mark: 1]
23M.2.SL.TZ1.e

The 2015 Paris Agreement sets out an international framework for avoiding dangerous climate change. A key aspect is conserving and enhancing sinks of greenhouse gases, including forests.

Free air carbon dioxide enrichment (FACE) experiments are being used to investigate whether increases in atmospheric CO2 concentration will cause biomass increases in existing forests. Three FACE experiments have been running for at least ten years in young, developing forests. Photosynthesis rates are measured in 25 to 30 m diameter plots. In control plots, carbon dioxide concentrations remain at current atmospheric levels (ambient CO2). In treatment plots, the CO2 concentration is raised by 50 % (elevated CO2).

The table gives some details of these experiments and the highest annual net primary production recorded during the period of the experiment. Net primary production is the mass of carbon absorbed and fixed by photosynthesis in plants that is not released due to plant respiration.

 

    Maximum annual
net primary production
/ kg C m–2 y–1
Site of experiment Dominant tree species Ambient CO2 Elevated CO2
Rhinelander, 
Wisconsin		 Populus tremuloides 
(deciduous angiosperm)		 0.81 0.98
Oak Ridge, 
Tennessee		 Liquidambar styraciflua 
(deciduous angiosperm)		 1.00 1.26
Duke, 
North Carolina		 Pinus taeda 
(evergreen conifer)		 1.21 1.55
 

 [Source: Walker, A.P., De Kauwe, M.G., Medlyn, B.E., et al., 2019. Decadal biomass
increment in early secondary succession woody ecosystems is increased by CO2 enrichment.
Nature Communications, [e-journal] 10, 454. https://doi.org/10.1038/s41467-019-08348-1.
Open access.]

State the effect of elevated CO2 on net primary production in these young, developing forests.

[1]

Markscheme

increases it/higher (maximum annual net primary production);

The answer must be referring implicitly or explicitly to NPP. 

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