Biogeographic and glacial history of walnut ( Juglans regia L.)

The present study deals with the glacial history of walnut to address questions related to past distributions through genetic analysis and ecological modeling of the present and the last glacial maximum (LGM) periods. A maximum entropy method was used to project the current walnut distribution model on to the LGM (21-18 kyr BP) climatic conditions. The current distribution model predicted southern Caucasus, parts of western and central Asia extending into southern Asia encompassing northern Afghanistan, Pakistan, northwestern Himalayan region, and southwestern Tibet, as the favorable climatic niche matching the modern distribution of walnut. The hindcast of distributions suggested the occurrence of walnut during LGM was somewhat limited to southern latitudes from southern Caucasus, central and southern Asian regions extending into southwestern Tibet, northeastern India, Himalayan region of Sikkim and Bhutan, and southeastern China. Both the Community Climate System Model (CCSM) and the Model for Interdisciplinary Research on Climate (MIROC) projections overlapped, except that MIROC projected a significant presence of walnut in the Balkan Peninsula during the LGM. In contrast, genetic analysis of the current walnut distribution suggested a much narrower area in northern Pakistan and the surrounding areas of Afghanistan, northwestern India, and southern Tajikistan as a plausible hotspot of diversity where walnut may have survived glaciations. Overall, the findings suggest that walnut perhaps survived the last glaciations in several refugia across a wide geographic area between 30 degrees and 45 degrees North latitude. However, humans probably played a significant role in the recent history and modern distribution of walnut.