Testing simulations of intra‐ and inter‐annual variation in the plant production response to elevated CO₂ against measurements from an 11‐year FACE experiment on grazed pasture

Abstract

Ecosystem models play a crucial role in understanding and evaluating the combined impacts of rising atmospheric CO2 concentration and changing climate on terrestrial ecosystems. However, we are not aware of any studies where the capacity of models to simulate intra- and inter-annual variation in responses to elevated CO2 has been tested against long-term experimental data. Here we tested how well the ecosystem model APSIM/AgPasture was able to simulate the results from a free air carbon dioxide enrichment (FACE) experiment on grazed pasture. At this FACE site, during 11 years of CO2 enrichment, a wide range in annual plant production response to CO2 (-6 to +28%) was observed. As well as running the full model, which includes three plant CO2 response functions (plant photosynthesis, nitrogen (N) demand and stomatal conductance), we also tested the influence of these three functions on model predictions. Model/data comparisons showed that: (i) overall the model over-predicted the mean annual plant production response to CO2 (18.5% cf 13.1%) largely because years with small or negative responses to CO2 were not well simulated; (ii) in general seasonal and inter-annual variation in plant production responses to elevated CO2 were well represented by the model; (iii) the observed CO2 enhancement in overall mean legume content was well simulated but year-to-year variation in legume content was poorly captured by the model; (iv) the best fit of the model to the data required all three CO2 response functions to be invoked; (v) using actual legume content and reduced N fixation rate under elevated CO2 in the model provided the best fit to the experimental data. We conclude that in temperate grasslands the N dynamics (particularly the legume content and N fixation activity) play a critical role in pasture production responses to elevated CO2 , and are processes for model improvement.

Constraints to nitrogen acquisition of terrestrial plants under elevated CO2

“…While increased N sequestration in long‐lived plant biomass and soil organic matter and slower decomposition of litter with lower N concentration may contribute to PNL in eCO 2 (Rastetter et al ., ; Luo et al ., ), there are thus also other processes by which eCO 2 may sustain increased plant N acquisition over long time periods (#P1‐3 in Table ). There are examples of long‐term free‐air CO 2 enrichment (FACE) experiments with declining (Norby et al ., ), approximately constant (Schneider et al ., ; McCarthy et al ., ; Reich & Hobbie, ; Li et al ., ; Talhelm et al ., ) or increasing (Schneider et al ., ; Reich & Hobbie, ) temporal response trends of eCO 2 ‐induced productivity enhancement. For forests, it has earlier been indicated that the initial productivity enhancement in eCO 2 declines rapidly during the first few years, but data available at that time only allowed for analysis of temporal response trends of up to five years (Körner, ).…”

Detection of historical changes in pasture growth and attribution to climate change

“…Decreases are less likely where N inputs are maintained either through fertiliser application or by biological nitrogen fixation (Hu et al 2006), as was the case at the Winchmore site. We have previously shown that simulations of grassland responses to CO 2 are very sensitive to the legume content (Li et al 2014), hence the decision to use the measured legume content in the modelling for this study. Further study of other sites varying in soil N availability would be necessary before any general conclusions can be drawn about historical CO 2 × soil interactions.…”

“…In particular, the inability of the model to capture the full range of interannual variability in NHA; while it does not concern us if we are simply looking at mean responses, this does become important if the object of the study is to examine changes in variability over time. Variability in NHA is a significant issue for farm management (Smit et al 1996, White et al 2010 and is expected to be a dominant influence under climate change (Li et al 2014). The second issue is the difficulty the model shows in simulating actual changes in legume content.…”

“…AgPasture provides a module that has been specifically designed to model temperate pastures with a mixture of plant species (Li et al 2011); it is based on the plant physiological principles outlined by Thornley & Johnson (2000), and implemented in the Ecomod model by Johnson et al (2003). The addition and validation of a CO 2 response function to AgPasture is described by Li et al (2014). Parameter settings and a test of model predictions against the Winchmore irrigation trial data may be found in Li et al (2011) for AgPasture and in White et al (2008) for EcoMod (which is relevant, because it shares many of the same functions as AgPasture).…”

Responses to atmospheric CO2 concentrations in crop simulation models: a review of current simple and semicomplex representations and options for model development

“…Also, Li et al . () suggested improving the representation of plant N demand responses (i.e. higher NUE), pasture legume content and possibly N fixation by legumes in AgPasture.…”