In the highlands of Madagascar, upland rice growing has developed in recent years thanks to the availability of varieties suited to the prevailing low temperatures in this mountainous region. However, what repercussions is climate change likely to have on this crop, on which a large proportion of the island’s inhabitants depend? By simulating rice production over a century, depending on the extent of climate change and the cropping practices adopted, a team from CIRAD and FOFIFA came up with a surprising result: it was the most pessimistic climate scenario that enabled the best yields.
Global warming could have serious consequences for rice production, and as a result for food security. Precise data on the effects of global warming are few and far between, and primarily concern irrigated rice. Upland rice, on the other hand, has never been studied before.
A team of researchers from CIRAD and the Malagasy National Institute of Agricultural Research (FOFIFA) looked into the impact of global warming on upland rice productivity in the highlands of Madagascar, where the crop has developed recently. Their study covered a ninety-year period, from 2010 to 2099, depending on the cropping system adopted.
Two climate change scenarios
Rice yields were simulated using the CERES-Rice model, which was calibrated and then validated using the FOFIFA 161 rice cultivar, for which a set of experimental data was compiled over a six-year period. The cropping systems comprised two soil tillage systems – hand ploughing and no-tillage – and two nitrogen fertilizer rates – high and low.
In relation to the control, without climate change, two scenarios were tested. In the first, carbon dioxide emissions increased gradually up to 750 ppm and the temperature rose by 0.15 °C per decade. This was the optimistic scenario, in which the increase in carbon dioxide levels and the relatively moderate increase in temperature were supposed to foster rice growth.
In the second scenario, carbon dioxide emissions also rose gradually, but the temperature rose by 0.5 °C per decade and rainfall fell by 0.2 mm a day between December and February. This was the pessimistic scenario, in which the combination of a marked rise in temperature and a reduction in rainfall could have led to severe water stress in rice.
The analysis did not reveal any differences in yields between the soil tillage systems, irrespective of the degree of climate change and fertilizer rate. No-tillage did not improve yields compared to tillage, or the efficacy of water use or nitrogen uptake by the plant. It is likely that in order to significantly improve soil properties, no-tillage requires substantial dry matter production, which is impossible to achieve at the prevailing low temperatures in the region.
However, fertilization did have a significant effect on yields, with a gain of 1500 kg/ha of grain for nitrogen applications of 45 kg/ha. Nitrogen is a major constraint in this type of soil, in which its availability is reduced due to the soil’s poor anion exchange capacity and to leaching.
Rice yields, which were 5478 kg/ha on average, were markedly higher in the pessimistic scenario, with a gain of 576 kg/ha compared to the control. In that scenario, the increase in temperature speeded up flowering and grain maturity, in such a way that the demand for water and nutrients from the plant tallied better with their availability in the soil. Yield variability was lower, and the gap between this scenario and the others continued to grow over the years.
A positive effect on rice productivity
Although the initial hypotheses – crops without biotic constraints or marked weather events – limit the import of the results, global warming could have a positive effect on rice productivity in this cold region, where rice is grown at the lower limit of its temperature tolerance.
Unlike what it likely to happen in southern Asia, where rice is grown at the upper limit of its temperature tolerance and yields are likely to fall overall, the most “pessimistic” forecasts in terms of temperature could lead to a marked increase in yields in the highlands of Madagascar.
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