Changes in agro-climatic indices related to temperature in Central Chile.

Climate change has profound environmental and socio-economic implications. To analyze climate change in relation to crops, a wide variety of agro-climatic indices has been proposed by the scientific community. In this study, changes in a set of 12 agro-climatic indices related to temperature were investigated in Central Chile over a 56-year period (1961-2016). The indices were computed based on data referring to daily maximum and minimum temperatures (TX and TN). They were organized in two categories: (1) cold- and (2) heat-related indices. Cold-related indices consisted of first frost day (FFD), last frost day (LFD), frost period (FP), number of frost days (FD), accumulated frost (AF), and number of days when TN is below - 2 °C (FD-2). Heat-related indices included the growing degree day (GDD) index, calculated based on four thresholds which measure the available heat resources for a wide variety of plants with different thermal requirements, and two heat stress indices which quantify the number of days with TX above 25 °C (plant heat stress (PHS)) and above 30 °C (plant high heat stress (PHHS)). Changes in agro-climatic indices were investigated using the Mann-Kendall test and the Sen's slope estimator. The main results revealed that the FFD occurred later, while LFD occurred earlier, thus determining a shortening of the FP in the northern half of the studied area. Trends in FD, AF, and FD-2 indices generally indicated warmer conditions in terms of TN during the cold period of the year. Agro-climatic indices related to heat showed important changes in Central Chile. Thus, statistically, the majority of trends become significant and indicated enhanced condition for crops in respect of GDD indices. However, from the heat stress perspective, the analyzed indices showed that conditions become worse in most of the studied locations for crops sensible to temperatures higher than 25 and 30 °C.