Spatio-temporal variation in phenological response of citrus to climate change in Iran: 1960–2010

Abstract

Recent studies investigating floral and faunal phenological responses to climate change have highlighted the extent to which these relationships are species and location specific. This study investigates temporal responses of citrus peak flowering to climate change in the cities of Kerman, Shiraz and Gorgan, Iran. Phenological data comprise peak flowering dates of five citrus types: orange, tangerine, sweet lemon, sour lemon and sour orange, collected daily from government heritage gardens located within each of the cities over the period 1960–2010. For the same period, daily Tmax, Tmin and rainfall data were acquired from the Iranian Meteorological Organization. Time trend analyses were undertaken for both the phenological and meteorological data, followed by linear regression to determine the nature and extent of relationships between these variables. We find that the mean peak flowering dates, and their long-term trends over the 51-year period, are similar across the five citrus types within each city, but demonstrate significant differences between cities. Flowering date advances of 0.12–0.17d/yr are recorded for Kerman, and more rapid advances of 0.56–0.65d/yr for Shiraz. Notably, progressive delays in flowering dates occur in Gorgan (0.05–0.1d/yr). The peak flowering dates in the former two cities demonstrate strong relationships with mean annual Tmin, ranging from r=0.47–0.61 (p=0.0045; p<0.0001) for Kerman to r=0.53–0.67 (p=0.0386; p<0.0001) for Shiraz, and equate to peak flowering advances of 3.15–3.39d/°C and 4.34–5.47d/°C, respectively. By contrast, the strongest relationships between peak flowering dates and annual climate in Gorgan are with rainfall (r=0.02–0.3, p=0.8874; p=0.0528), indicating a weak phenophase response of 0.01d/mm. For Gorgan, the strongest relationships (r=0.43–075, p=0.0002 to p<0.0001) are between peak flowering date and mean Tmax for May, the month during which peak flowering occurs, with a delay in flowering of 1.26–1.86d/°C cooling. This suggests a relatively more influential climatic role directly preceding peak flowering, which may be associated with anomalous cooling in May. However, Kerman and Shiraz demonstrate more consistent strength in correlation between peak flowering and climate variables across the months of the year, with only slight peaks for the months flanking peak flowering. Our study highlights the importance of considering location-specific phenophase shifts within given regions, as dissimilar trends may occur within a country; this has important implications for future agricultural planning and fruit crop supply to local and international markets.