Forecasting Responses of a Northern Peatland Carbon Cycle to Elevated CO2
 and a Gradient of Experimental Warming

Abstract. Competition is a major driver of carbon allocation to different plant tissues (e.g., wood, leaves, fine roots), and allocation, in turn, shapes vegetation structure. To improve their modeling of the terrestrial carbon cycle, many Earth system models now incorporate vegetation demographic models (VDMs) that explicitly simulate the processes of individual-based competition for light and soil resources. Here, in order to understand how these competition processes affect predictions of the terrestrial carbon cycle, we simulate forest responses to elevated atmospheric CO2 concentration [CO2] along a nitrogen availability gradient, using a VDM that allows us to compare fixed allocation strategies vs. competitively optimal allocation strategies. Our results show that competitive and fixed strategies predict opposite fractional allocation to fine roots and wood, though they predict similar changes in total net primary production (NPP) along the nitrogen gradient. The competitively optimal allocation strategy predicts decreasing fine root and increasing wood allocation with increasing nitrogen, whereas the fixed strategy predicts the opposite. Although simulated plant biomass at equilibrium increases with nitrogen due to increases in photosynthesis for both allocation strategies, the increase in biomass with nitrogen is much steeper for competitively optimal allocation due to its increased allocation to wood. The qualitatively opposite fractional allocation to fine roots and wood of the two strategies also impacts the effects of elevated [CO2] on plant biomass. Whereas the fixed allocation strategy predicts an increase in plant biomass under elevated [CO2] that is approximately independent of nitrogen availability, competition leads to higher plant biomass response to elevated [CO2] with increasing nitrogen availability. Our results indicate that the VDMs that explicitly include the effects of competition for light and soil resources on allocation may generate significantly different ecosystem-level predictions of carbon storage than those that use fixed strategies.

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Competition alters predicted forest carbon cycle responses to nitrogen availability and elevated CO<sub>2</sub>: simulations using an explicitly competitive, game-theoretic vegetation demographic model

10.5194/bg-2019-55 ◽

2019 ◽

Author(s):

Ensheng Weng ◽

Ray Dybzinski ◽

Caroline E. Farrior ◽

Stephen W. Pacala

Keyword(s):

Carbon Cycle ◽

Elevated Co2 ◽

Fine Roots ◽

Nitrogen Availability ◽

Optimal Allocation ◽

Plant Biomass ◽

Allocation Strategy ◽

Terrestrial Carbon Cycle ◽

Competition For Light ◽

Allocation Strategies

Abstract. Competition is a major driver of carbon allocation to different plant tissues (e.g. wood, leaves, fine roots), and allocation, in turn, shapes vegetation structure. To improve their modeling of the terrestrial carbon cycle, many Earth system models now incorporate vegetation demographic models (VDMs) that explicitly simulate the processes of individual-based competition for light and soil resources. Here, in order to understand how these competition processes affect predictions of the terrestrial carbon cycle, we simulate forest responses to elevated CO2 along a nitrogen availability gradient using a VDM that allows us to compare fixed allocation strategies versus competitively-optimal allocation strategies. Our results show that competitive- and fixed-allocation strategies predict opposite fractional allocation to fine roots and wood, though they predict similar changes in total NPP along the nitrogen gradient. The competitively-optimal allocation strategy predicts decreasing fine root and increasing wood allocation with increasing nitrogen, whereas the fixed allocation strategy predicts the opposite. Although simulated plant biomass at equilibrium increases with nitrogen due to increases in photosynthesis for both allocation strategies, the increase in biomass with nitrogen is much steeper for competitively-optimal allocation due to its increased allocation to wood. The qualitatively opposite fractional allocation to fine roots and wood of the two strategies also impacts the effects of elevated [CO2] on plant biomass. Whereas the fixed allocation strategy predicts an increase in plant biomass under elevated [CO2] that is approximately independent of nitrogen availability, competition’s effect on wood allocation amplifies plant biomass under elevated [CO2] with increasing nitrogen availability. Our results indicate that the VDMs that explicitly include the effects of competition for light and soil resources on plant strategies may generate significantly different ecosystem-level predictions than those that use fixed allocation strategies.

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Review of Weng et al. "Competition alters predicted forest carbon cycle responses to nitrogen availability and elevated CO2: ..."

10.5194/bg-2019-55-rc3 ◽

2019 ◽

Author(s):

Benjamin Stocker

Keyword(s):

Carbon Cycle ◽

Elevated Co2 ◽

Nitrogen Availability ◽

Forest Carbon

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Forecasting Responses of a Northern Peatland Carbon Cycle to Elevated CO2 and a Gradient of Experimental Warming

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Competition alters predicted forest carbon cycle responses to nitrogen availability and elevated CO<sub>2</sub>: simulations using an explicitly competitive, game-theoretic vegetation demographic model

Competition alters predicted forest carbon cycle responses to nitrogen availability and elevated CO<sub>2</sub>: simulations using an explicitly competitive, game-theoretic vegetation demographic model

Review of Weng et al. "Competition alters predicted forest carbon cycle responses to nitrogen availability and elevated CO2: ..."