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

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 28.60 cm2) 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 < 0.01) for all correlations. The correlations between LAI and air temperatures were also significant (P < 0.01) i.e. r = 0.76, 0.82 and 0.88 for maximum, mean and minimum air temperatures, respectively. However, the correlations between LA and air temperatures were insignificant. Unlike the current year, there were significant correlations of the LA with both the preceding year's mean air temperature (P = 0.05) and sunlight duration (P = 0.02). Consequently, the results revealed the importance of relatively less referred parameter; soil temperature, primarily for the urban forest trees which are susceptible to anthropogenic climate warming.