Spread of walnut (Juglans regia L.) in an Alpine valley is correlated with climate warming

Since the Roman time, the walnut tree (Juglans regia L.) has been cultivated in gardens and orchards in large parts of Europe, but only since Karl The Great (742–814) the walnut tree has been introduced to Tyrol (Austria) and other Alpine regions in Central Europe. As late as the beginning of the 20th century no wild populations but only cultivated trees are reported for inner Alpine valleys of Tyrol. Recently, however a striking increase of growing sites and individuals of J. regia has been detected within the Inn valley and other inner Alpine valleys of the Eastern Alps. In addition, climate warming, specifically milder winters since 1970 have been recorded within the study area (Innsbruck, Tyrol, Austria). Therefore, a dendroecological study at four study sites in surroundings of Innsbruck dominated by Scots pine (Pinus sylvestris L.) and/or Norway spruce (Picea abies (L.) Karst.) forests was initiated to examine more closely, if influence of climate warming favoured spread of this deciduous tree.Together, the four study sites encompassed 30 ha, where in total 651 J. regia trees were analysed. Mapping of trees indicated that >90% grew on south and south-west-facing slopes. Evaluation of the age structure revealed that >48% of the analysed J. regia trees were younger than 10 years, >72% younger than 20 years and >92% younger than 30 years.Analysis of climate conditions in years of germination revealed a striking correspondence between the number of germinated trees and winter (December–February) minimum temperatures (r=0.43r=0.43; p<0.01p<0.01) and the number of frost days (December–February) <−5 °C (r=0.40r=0.40; p<0.01p<0.01), which can be related to frost sensitivity during seedling stage. In summary, results of this study provide evidence that climate warming after 1970 favoured establishment of J. regia within this inner Alpine valley.