Climate effects on the vitality of boreal forests at the treeline in different ecozones of Mongolia

Abstract. In northern Mongolia, at the southern boundary of the Siberian boreal forest belt, the distribution of steppe and forest is generally linked to climate and topography, making this region highly sensible to climate change. Detailed investigations on the limiting parameters of forest and steppe occurrence in different ecozones provide necessary information for environmental modelling and scenarios of potential landscape change. In this study, remote sensing data and gridded climate data were analyzed in order to identify distribution patterns of forest and steppe in Mongolia and to detect driving ecological factors of forest occurrence and vulnerability against environmental change. With respect to anomalies in extreme years we integrated the climate and land cover data of a 15 year period from 1999–2013. Forest distribution and vegetation vitality derived from the normalized differentiated vegetation index (NDVI) were investigated for the three ecozones with boreal forest present in Mongolia (taiga, subtaiga, and forest-steppe). In addition to the entire ecozone areas, the analysis focused on different subunits of forest and non-forested areas at the upper and lower treeline, which represent ecological borderlines of site conditions. The total cover of boreal forest in Mongolia was estimated at 73 818 km2. The upper treeline generally increases from 1800 m above sea level (a.s.l.) in the Northeast to 2700 m a.s.l. in the South. The lower treeline locally emerges at 1000 m a.s.l. in the northern taiga and is rising southward to 2500 m a.s.l. The latitudinal trend of both treelines turns into a longitudinal trend in the east of the mountains ranges due to more aridity caused by rain-shadow effects. Less vital trees were identified by NDVI at both, the upper and lower treeline in relation to the respective ecozone. The mean growing season temperature (MGST) of 7.9–8.9 °C and a minimum of 6 °C was found to be a limiting parameter at the upper treeline but negligible for the lower treeline and the total ecozones. The minimum of the mean annual precipitation (MAP) of 230–290 mm yr−1 is an important limiting factor at the lower treeline but at the upper treeline in the forest-steppe ecotone, too. In general, NDVI and MAP are lower in grassland, and MGST is higher compared to the forests in the same ecozone. An exception occurs at the upper treeline of the subtaiga and taiga, where the alpine vegetation is represented by meadow mixed with shrubs. Comparing the NDVI with climate data shows that increasing precipitation and higher temperatures generally lead to higher greenness in all ecological subunits. While the MGST is positively correlated with the MAP of the total ecozones of the forest-steppe, this correlation turns negative in the taiga ecozone. The subtaiga represents an ecological transition zone of approximately 300 mm yr−1 precipitation, which occurs independently from the MGST. Nevertheless, higher temperatures lead to higher vegetation vitality in terms of NDVI values. Climate change leads to a spatial relocation of tree communities, treelines and ecozones, thus an interpretation of future tree vitality and biomass trends directly from the recent relationships between NDVI and climate parameters is difficult. While climate plays a major role for vegetation and treeline distribution in Mongolia, the disappearing permafrost needs to be accounted for as a limiting factor for tree growth when modeling future trends of climate warming and human forest disturbance.