19 Dec 2022
 | 19 Dec 2022
Status: a revised version of this preprint is currently under review for the journal BG.

Tropical Dry Forest Response to Nutrient Fertilization: A Model Validation and Sensitivity Analysis

Shuyue Li, Bonnie G. Waring, Jennifer S. Powers, and David Medvigy

Abstract. Soil nutrients, especially nitrogen (N) and phosphorus (P), regulate plant growth and hence influence carbon fluxes between the land surface and atmosphere. However, how forests adjust biomass partitioning to leaves, wood, and fine roots in response to N and/or P fertilization remains puzzling. Recent work in tropical forests suggests that trees increase fine root production under P fertilization, but it is unclear whether mechanistic models can reproduce this dynamic. In order to better understand mechanisms governing nutrient effects on plant allocation and improve models, we used the nutrient enabled ED2 model to simulate a fertilization experiment being conducted in a secondary tropical dry forest in Costa Rica. We evaluated how different allocation parameterizations affected model performance. These parameterizations prescribed a linear relationship between relative allocation to fine roots and soil P concentrations. The slope of the linear relationship was allowed to be positive, negative, or zero. Some parameterizations realistically simulated leaf, wood and fine root production, and these parameterizations all assumed a positive relationship between relative allocation to fine roots and soil P concentration. On a thirty-year timescale, under unfertilized conditions, our model predicted the largest aboveground biomass (AGB) accumulation when relative allocation to fine roots was positively related to soil P concentration. However, this result was mostly driven by increased water use rather than decreased nutrient limitation. On a thirty-year timescale with P fertilization, the assumption of a positive correlation between relative allocation to fine roots and soil P concentration led to over-investment to fine roots and reductions in vegetation biomass. Our study demonstrates the need of simultaneous measurements of leaf, wood, and fine root production in nutrient fertilization experiments. Models that do not accurately represent allocation to fine roots may be highly biased in their simulations of AGB, especially when simulating a range of sites with significantly different soil P concentrations.

Shuyue Li et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-243', Anonymous Referee #1, 14 Jan 2023
    • AC1: 'Reply on RC1', David Medvigy, 20 Apr 2023
  • CC1: 'Comment on bg-2022-243', Katrin Fleischer, 03 Feb 2023
    • AC2: 'Reply on CC1', David Medvigy, 20 Apr 2023
  • RC2: 'Comment on bg-2022-243', Anonymous Referee #2, 23 Apr 2023

Shuyue Li et al.

Shuyue Li et al.


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Short summary
We challenged an ecosystem model to successfully simulate the carbon cycle of a tropical forest subject to nutrient fertilization. The model simulations only matched the observations when it prescribed increasing fine root production with increasing soil phosphorus. This result is consistent with and helps explain recent empirical studies, but differs from what might be expected from ecological theory and from the ways that fine root production is typically handled in ecosystem models.