Tropical Dry Forest Response to Nutrient Fertilization: A Model Validation and Sensitivity Analysis
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: open (until 20 Apr 2023)
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Shuyue Li et al.
Shuyue Li et al.
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This study integrated a representation of phosphorus-dependent relative allocation to root tissues into the ED2 model. The model was simulated at a site in a tropical dry forest in Costa Rica for +N, +P, and +NP fertilization treatments. Modelled results were compared to empirical observations. The model was then simulated over 30 years to examine the influence of the new process representation over a longer time scale. The comparison between a model and empirical observations of experimental manipulations of nutrient input is very useful for model development. However, it was unclear whether the process that was represented in this study (increasing allocation to fine roots with increasing soil P) is prevalent in systems outside of this site, what its underlying mechanisms are, and how it relates to other central processes (such as the relationships between allocation to fine roots and soil nitrogen or water). Furthermore, the statistics used to establish the results were unclear.
Line 81 “While models have rarely be validated on these time scales” I would argue that models are often evaluated over the past several decades (1960s to present).
Production units should be kg m-2 yr-1.
Table 3 is challenging to interpret. Could this be transformed into a figure?
Include other parameterizations in Figure 5 (additional panels).
Include other treatments in Figure 8 (additional panels).
Figure 6: Clarify if this is averaged across treatments or if this is the control treatment only.