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Q28 - Xylem Embolism and Hydraulic Safety Margins

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Water Transport & Transpiration

“Embolism” in xylem vessels means the presence of air-filled tracheids and/or vessels. Embolisms can result in a substantial impairment of xylem transport and thus hydraulic failure.

Environmental causes of embolism include water stress and winter freezing; potential consequences include reduction of growth and dieback.

Choat et al. (2012) examined the vulnerability of forests to drought-induced hydraulic failure. They measured the minimum xylem pressure in plants under natural conditions (Psi_min), and the xylem pressure (P50) at which 50% loss of conductivity occurs. They included both angiosperms and gymnosperms in different climate conditions: mean annual precipitation (MAP) ranged from 300 to 4,500 mm, and mean annual temperature from -4°C to 27°C (Figure a).

The difference between Psi_min and PX (at which X% loss of conductivity occurs) represents a “safety margin” within which the plant operates in a given environment (Figure b).

Scatter plot labeled ‘a’. X-axis: P50 (MPa), Y-axis: Psi_min (MPa). Blue dots (M = Angiosperms) and red dots (N = Gymnosperms) are plotted along two regression lines. A dashed 1:1 line represents zero safety margin. Gymnosperm points tend to be farther from the 1:1 line than angiosperm points, indicating larger safety margins. Figure a. Minimum xylem pressure (Psi_min) plotted against P50 for angiosperm (M, blue) and gymnosperm (N, red) species. Dashed line = 1:1 (zero safety margin).

Box plot labeled ‘b’. Y-axis: Safety margin P88 (MPa). Four biome categories on x-axis: B = Mediterranean and Woodland, C = Temperate forest, D = Tropical seasonal forest, E = Tropical rain forest. Arrow F indicates increasing mean annual precipitation. Blue boxes = Angiosperms (M), red boxes = Gymnosperms (N). Safety margins are similar across biomes, with gymnosperms absent from tropical forests. Bars with the same letter are not significantly different. Figure b. Hydraulic safety margins across forest biomes. M = Angiosperms (blue), N = Gymnosperms (red). B = Mediterranean/Woodland, C = Temperate forest, D = Tropical seasonal forest, E = Tropical rain forest. F = increasing MAP.

Q28.1. After maturity, xylem structure can no longer acclimate to environmental stress to reduce risk of embolism.
Q28.2. Gymnosperms show greater hydraulic safety margins than angiosperms.
Q28.3. The safety margin quantifies the degree of conservatism in a plant's hydraulic strategy (plants with low safety margins experience higher risk of hydraulic failure).
Q28.4. There is global convergence in the vulnerability of forests to drought (all forest biomes are approximately equally vulnerable to hydraulic failure regardless of their current rainfall environment).
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Question reproduced from IBO 2023, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF