A potato model intercomparison across varying climates and productivity levels.

A potato crop multimodel assessment was conducted to quantify variation among models and evaluate responses to climate change. Nine modeling groups simulated agronomic and climatic responses at low-input (Chinoli, Bolivia and Gisozi, Burundi)- and high-input (Jyndevad, Denmark and Washington, United States) management sites. Two calibration stages were explored, partial (P1), where experimental dry matter data were not provided, and full (P2). The median model ensemble response outperformed any single model in terms of replicating observed yield across all locations. Uncertainty in simulated yield decreased from 38% to 20% between P1 and P2. Model uncertainty increased with interannual variability, and predictions for all agronomic variables were significantly different from one model to another (P < 0.001). Uncertainty averaged 15% higher for low- vs. high-input sites, with larger differences observed for evapotranspiration (ET), nitrogen uptake, and water use efficiency as compared to dry matter. A minimum of five partial, or three full, calibrated models was required for an ensemble approach to keep variability below that of common field variation. Model variation was not influenced by change in carbon dioxide (C), but increased as much as 41% and 23% for yield and ET, respectively, as temperature (T) or rainfall (W) moved away from historical levels. Increases in T accounted for the highest amount of uncertainty, suggesting that methods and parameters for T sensitivity represent a considerable unknown among models. Using median model ensemble values, yield increased on average 6% per 100-ppm C, declined 4.6% per °C, and declined 2% for every 10% decrease in rainfall (for nonirrigated sites). Differences in predictions due to model representation of light utilization were significant (P < 0.01). These are the first reported results quantifying uncertainty for tuber/root crops and suggest modeling assessments of climate change impact on potato may be improved using an ensemble approach.

Can the agricultural AquaCrop model simulate water use and yield of a poplar short-rotation coppice?

We calibrated and evaluated the agricultural model AquaCrop for the simulation of water use and yield of a short-rotation coppice (SRC) plantation with poplar (Populus) in East Flanders (Belgium) during the second and the third rotation (first 2 years only). Differences in crop development and growth during the course of the rotations were taken into account during the model calibration. Overall, the AquaCrop model showed good performance for the daily simulation of soil water content (R2 of 0.57-0.85), of green canopy cover (R2 > 0.87), of evapotranspiration (ET; R2 > 0.76), and of potential yield. The simulated, total yearly water use of the SRC ranged between 55% and 85% of the water use of a reference grass ecosystem calculated under the same environmental conditions. Crop transpiration was between 67% and 93% of total ET, with lower percentages in the first than in the second year of each rotation. The observed (dry mass) yield ranged from 6.61 to 14.76 Mg ha-1 yr-1. A yield gap of around 30% was observed between the second and the third rotation, as well as between simulated and observed yield during the third rotation. This could possibly be explained by the expansion of the understory (weed) layer; the relative cover of understory weeds was 22% in the third year of the third rotation. The agricultural AquaCrop model simulated total water use and potential yield of the operational SRC in a reliable way. As the plantation was extensively managed, potential effects of irrigation and/or fertilization on ET and on yield were not considered in this study.

Can complementarity in water use help to explain diversity-productivity relationships in experimental grassland plots?

Positive diversity-productivity relationships have repeatedly been found in experimental grassland plots, but mechanistic explanations are still under debate. We tested whether complementarity for the exploitation of the soil water resource helps to explain these relationships. In the dry summer of 2003, evapotranspiration (ET) was assessed at the Swedish BIODEPTH site using two different approaches: snapshot measurements of canopy surface temperature and simulation of time-accumulated ET by means of a soil water balance model. More diverse plots were characterized by lower surface temperatures and higher accumulated ET. Transgressive overyielding tests revealed that ET in polycultures was higher than in the best-performing monocultures, but this pattern was reversed at high degrees of water stress. Our results indicate that a more complete exploitation of soil water by more diverse grassland systems is on the one hand likely to be a driver for their increased biomass production, but on the other hand causes the more diverse communities to be affected earlier by drought. Nevertheless, the results also suggest that productivity may (at least partially) be maintained under dry conditions due to the higher likelihood of including drought-tolerant species in the more diverse communities.