Response of tree-ring width to climate warming and selective logging in larch forests of the Mongolian Altai

Aims The Mongolian Altai is an old settlement area, which is populated by pastoral nomads since 2000-3000 years. Forests in this region (at ca. 2300 ma.s.l.) are highly fragmented and border on steppe and alpine grasslands, which are used for mobile livestock husbandry. The climate in Central Asia is warming to levels clearly above the global average, which affects the vegetation. Furthermore, the transition from planned to market economy and the decollectivization of livestock 20 years ago has strongly changed land use practices in Mongolia, especially resulting in an increase in recent logging activities. We were interested in the question how climate warming and selective logging influence the annual stem growth and the stand structure. Methods The impact of climate and land use by the pastoral nomads on the annual stem increment of more than 1800 trees of Siberian larch (Larix sibirica) was analyzed. Different groups of trees with divergent growth trends depending on the social position and stand history were identified by non-metric multidimensional scaling and analysis of similarities. Long-term trends in the annual stem increment were analyzed by establishing separate regional growth curves for trees of different age classes. Important Findings Instrumental climate data substantiate an increase of temperature by 2.1 degrees C since 1940 at constant precipitation. Trees benefit from the increased temperatures. Climate-response analysis revealed that radial stem increment was promoted by the temperature in early summer, but also high precipitation in spring and in the year before tree-ring formation. Forest dynamics is also strongly influenced by anthropogenic activities. In addition to the natural forest dynamics, logging resulted in divergent growth trends within given age classes and habitats (forest interior and forest edge); overall, 22 groups of trees with different characteristics in the annual radial stem increment were identified. A tree-ring series-based reconstruction of logging intensity since 1935 suggests that moderate selective logging occurred throughout the study period. However, selective logging was strongly intensified after 1990 as the result of the breakdown of the Communist regime in Mongolia and the transition from centrally planned to market economy. Because tree stump densities showed that the ratio of felled to live trees was 2:1 in the interior or even 0.9:1 at the edges of the forests and most logging occurred during the past 20 years, it must be concluded that the forests of the Mongolian Altai are presently exploited far beyond the level of sustainability. A close correlation of the ratio of felled to live trees with the density of summer camps of pastoral nomads in the vicinity suggests that trees are primarily felled by the local population.