Climate effects on vegetation vitality at the treeline of boreal forests of Mongolia
Authors: Klinge Michael , Dulamsuren Choimaa , Erasmi Stefan , Karger Dirk Nikolaus , Hauck Markus 
Published: March 2018

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 sensitive to climate change and
human impact. Detailed investigations on the limiting parameters of forest
and steppe in different biomes provide necessary information for
paleoenvironmental reconstruction and prognosis of potential landscape
change. In this study, remote sensing data and gridded climate data were
analyzed in order to identify main distribution patterns of forest and steppe
in Mongolia and to detect environmental factors driving forest development.
Forest distribution and vegetation vitality derived from the normalized
differentiated vegetation index (NDVI) were investigated for the three types
of boreal forest present in Mongolia (taiga, subtaiga and forest–steppe),
which cover a total area of 73 818 km2. In addition to the
forest type areas, the analysis
focused on subunits of forest and nonforested areas at the upper and lower
treeline, which represent ecological borders between vegetation types.
Climate and NDVI data were analyzed for a reference period of 15 years from
1999 to 2013. The presented approach for treeline delineation by identifying representative
sites mostly bridges local forest disturbances like fire or tree cutting.
Moreover, this procedure provides a valuable tool to distinguish the
potential forested area. The upper treeline generally rises 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 rises southward to 2500 m a.s.l. The latitudinal gradient of both
treelines turns into a longitudinal one on the eastern flank of mountain
ranges due to higher aridity caused by rain-shadow effects. Less productive
trees in terms of NDVI were identified at both the upper and lower treeline
in relation to the respective total boreal forest type area. The mean growing
season temperature (MGST) of 7.9–8.9 ∘C and a minimum MGST of
6 ∘C are limiting parameters at the upper treeline but are
negligible for the lower treeline. The minimum of the mean annual
precipitation (MAP) of 230–290 mm yr−1 is a limiting parameter at the
lower treeline but also at the upper treeline in the forest–steppe ecotone.
In general, NDVI and MAP are lower in grassland, and MGST is higher compared
to the corresponding boreal forest. One exception occurs at the upper
treeline of the subtaiga and taiga, where the alpine vegetation consists of
mountain meadow mixed with shrubs. The relation between NDVI and climate data
corroborates that more precipitation and higher temperatures generally lead
to higher greenness in all ecological subunits. MGST is positively correlated
with MAP of the total area of forest–steppe, but this correlation turns
negative in the taiga. The limiting factor in the forest–steppe is the
relative humidity and in the taiga it is the snow cover distribution. The
subtaiga represents an ecological transition zone of approximately
300 mm yr−1 precipitation, which occurs independently from the MGST. Since the treelines are mainly determined by climatic parameters, the rapid
climate change in inner Asia will lead to a spatial relocation of tree communities, treelines and
boreal forest types. However, a direct deduction of future tree vitality,
forest composition and biomass trends from the recent relationships between
NDVI and climate parameters is challenging. Besides human impact, it must
consider bio- and geoecological issues like, for example, tree rejuvenation,
temporal lag of climate adaptation and disappearing permafrost.