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ABSTRACT:Changes in several temperature-based agroclimatic indices in the central-eastern of Argentina, most of them located within the Pampas region were analysed for 39 meteorological stations. Trends of first (FFD), last (LFD) and number (NFD) of frost days, frost period (FP), start (SGS), end (EGS) and length (LGS) of the growing season, growing degree days (GDD), diurnal temperature range (DTR), chilling hours and lowest annual minimum temperature were computed for two periods, 1940-2007 and 1975-2007. The largest changes were observed for the whole period 1940-2007 and were mostly indicative of a long-term minimum temperature warming throughout the region. During this period, generalized decreases in the NFD and in the FP (i.e., a delayed FFD and an earlier LFD) were found. Although the trends in the growing season indices were not as large as in the frost indices, they were consistent with the overall warming: an earlier SGS and a delayed in the EGS. The trends of the GDD showed a large variability between months with a generalized increased throughout the year. The DTR showed the largest number of stations with statistically significant negative trends from austral late spring (November) to austral early fall (April). For the period 1975-2007, the behaviour changes in all analysed indices: the short-term trends weakened and in some cases reversed sign. The LFD tended to occurred later in the year, particularly for the southern Pampas. The EGS shifted from mostly positive to negative trends, resulting in a shorter LGS. These trend changes were not spatially homogeneous. Although those short-term trends were predominantly non-statistically significant, they could potential affect management decisions and crop yields. In particular, frost is still an important hazard in agricultural activities and within the context of our results short-and long-term characterization of frost risk need to be considered at local and sub-regional scales.

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Episodes of heat stress constrain crop production and will be aggravated in the near future according to short and medium-term climate scenarios. Global increase in cloudiness has also been observed, decreasing the incident solar radiation. This work was aimed to quantify the probability of occurrence of heat stress and cloudiness, alone or combined, during the typical post-owering period of wheat and canola in the Southern Cone of South America. Extended climate series (last 3-5 decades with daily register) of 33 conventional weather stations from Argentina, Brazil, Chile and Uruguay (23ºS to 40ºS) were analysed considering the period from September to December. Two different daily events of heat stress were determined: i) maximum daily temperature above 30ºC (T>30ºC), and ii) 5ºC above the historical average maximum temperature of that day (T+5ºC). A cloudiness event was de ned in our work as incident solar radiation 50% lower than the historical average radiation of that day (R50%). The T>30ºC event increased its probability of occurrence throughout the post-owering phase, from September to December. By contrast, the risk of T+5ºC event decreased slightly, just like for R50%, and the higher the latitude, the lower the probability of R50%. The T>30ºC plus R50% combined stresses reached greater cumulated probabilities during post-owering, compared to T+5ºC plus R50%, being 42% vs. 15% in northernmost locations, 26% vs. 19% in central (between 31ºS to 35ºS), and 28% vs. 1% in southernmost locations, respectively. A curvilinear relationship emerged between the monthly probability of combined stresses and the number of days with stress per month. In summary, T>30ºC was the most frequent thermal stress during post-owering in wheat and canola. Both combined stresses had a noticeable risk of occurrence, but T>30ºC plus R50% was the highest. Evidence of the recent past and current occurrence of heat stress individually, and its combination with cloudiness events during post-owering of temperate crops, serves as a baseline for future climate scenarios in main cropped areas in the Southern Cone of South America.

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