Climate Change and Distribution of Date Palms Figure 6. The climate (EI) for P. dactylifera in current time and projected using CLIMEX under the CSIRO-Mk3.0 GCM running the SRES A2 scenario and for 2030, 2050, 2070 and 2100 for Australia, and southern Asia. doi:10.1371/journal.pone.0048021.g006 Dry Stress The term ‘drought’ refers to a period of time without significant rainfall [14]. Water stress occurs as a consequence of water loss through transpiration or evaporation during a period of time when there is a lack of available water in the soil [14]. Different degrees of water stress can be seen in a plant. When water loss is prolonged, a significant disruption in the metabolism of the plant occurs [14]. However, the date palm has developed a number of strategies to prevent dry stress. These include maintaining a high level of hydration, the ability to function while dehydrated, increasing the amount of water absorption (i.e., keeping a high level of osmotic pressure) by using abscisic acid, and by the development of an extensive root system [14]. Dry stress was not used in this study for the above reasons. Wet Stress August to October are the critical months when rain damage can inflict serious economic damage to the date crop [11]. A recent study observed that a total of 78.74 mm of rainfall during an 8-day period caused a greater than 50% loss in date palm yields while 86.36 mm of rainfall in 10 days led to 15% losses in date palm farms in some countries [11]. Date palms are known to suffer wet stress easily. The wet stress threshold (SMWS) was set to 0.9 and the accumulation rate (HWS) set at 0.022 week21 to allow the species to grow well in arid and semi-arid regions such as Algeria, Morocco, and southern Iran. Temperature Index Phoenix dactylifera has been cultivated in areas with a mean annual surface temperature greater than 16uC, such as southern Iran [8,28,29], south-eastern Iraq, eastern Pakistan [11,38], and northern and central Algeria [11,39]. Western Pakistan’s climatic parameters are comparable to other places suitable for date palm cultivation with the exception of its annual surface temperature, which is 13uC. Thus, the limiting low temperature (DV0) should be between 13uC and 16uC. Fourteen degrees Celsius was selected due to providing the best fit to the observed distribution of date palms in North Africa and Asia. Summer temperatures in locations which are highly climatically suitable for this species rarely exceed 46uC, thus the limiting high temperature DV3 was set at 46uC [13]. The lower (DV1) and upper (DV2) optimal temperatures were set at 20uC and 39uC, respectively, because temperatures between 20uC and 39uC are cited as favorable temperatures for date palm, depending on the varieties [14]. These numbers also provided the best fit to the observed distribution in South America, Asia, South Africa and Australia [11]. Moisture Index In terms of soil moisture, the lower moisture threshold (SM0) was set at 0.007, to represent the permanent wilting point [27]. Furthermore, this number provided a good fit to the observed distribution of date palms in South America, Asia and the Middle PLOS ONE | www.plosone.org 6 October 2012 | Volume 7 | Issue 10 | e48021