Models of low temperature and high irradiance and their application to explaining the risk of seedling mortality

Low temperature during the growing season is known to be a leading cause of stress and damage to tree seedlings and interactive effects with high irra diance have been recognised as enhancing the damage. Spatial variation in l ow temperature and irradiance for mountainous terrain was, therefore, hypot hesised to give rise to spatially variable potential for regeneration of tr ee seedlings and expansion of forest. Combined exposure to low temperature and high irradiance appears most likely on clear mornings following frosty nights. Geographic information system-based models of the spatial variation in minimum air temperature over clear-felled terrain and potentially inter cepted direct radiation energy were constructed and were used together with measured values of minimum air temperature to explain observed spatial var iation in mortality rates for planted tree seedlings in central Sweden. A l ocal-scale multiple regression model explained 78% of the measured spatial variability in minimum air temperature over 625 km(2) and a corresponding, but univariate, microscale model explained 54% of the measured spatial vari ation in minimum air temperature over 4 ha. Multiple logistic regression mo dels for the mortality rates of seedlings showed there was no significant e ffect of potentially intercepted amounts of direct radiation energy during the morning for either scale. There was, however, a significant effect (p<0 .0001) of measured minimum air temperature on seedling mortality rates for the microscale study, but no corresponding significant effect for the local -scale study. This confirms that low temperature is an important determinan t for seedling mortality. It is likely that other factors than low temperat ure may be important. However, our results indicate that effects of local-s cale variability in minimum air temperature, represented by the minimum air temperature at standard height above the ground (1.8 m), may be over-shado wed by minimum air temperature variability at seedling height in complex te rrain. This has implications for the use of standard meteorological data fo r studies into the responses of plants to abiotic factors. (C) 2000 Elsevie r Science B.V. All rights reserved.