Influence of air–soil temperature on leaf expansion and LAI of Carpinus betulus trees in a temperate urban forest patch

•Maximum leaf area and leaf area index were achieved in early and late May, respectively.•Soil temperature had a significant influence on LA and LAI.•Influence of current year's air temperature was significant only for LAI.•Preceding year's mean air temperature was also effective on LA.•There was a significant correlation between the preceding year's sunlight duration and LA. Air temperature is effective meteorological parameter in overall leaf phenology of deciduous temperate forests. Not only the air temperature but also the soil temperature influences seasonal leaf development of deciduous trees. In order to determine the extent of air and soil temperature impact on the Carpinus betulus leaves, we analyzed two significant parameters; leaf area (LA) and leaf area index (LAI) as an indicator of leaf development. We monitored LA expansion through leaf harvesting with about weekly intervals from early April to late May, and LAI through hemispherical photographing from mid-March and mid-June. After the budburst and leaf unfolding between late March and early April, mean LAI and LA consecutively exhibited increasing patterns until their maximums (2.70 and 28.60cm2) in late May and early May, respectively. Both the LA and LAI trends were in accordance particularly with the soil temperatures of all layers. Namely, the coefficients (r) were at least 0.88 indicating high significance (P