Climate response of tree-ring width in Larix sibirica growing in the drought-stressed forest-steppe ecotone of northern Mongolia

Siberian larch (Larix sibirica) has its southern distribution limit in Mongolia in an area of rapidly rising temperatures. Direct effects of climate on tree-ring formation due to drought stress or indirect effects via the control of insect herbivore populations are little studied. The hypotheses were tested that stem increment of Siberian larch is reduced by (1) drought during the growing season and (2) high snow cover, as the latter is thought to protect hibernating herbivores, including gypsy moth (Lymantria dispar). Tree-ring width increases with decreasing summer temperature, increasing precipitation during the growing season and decreasing winter precipitation. The susceptibility of stem wood formation to drought during the growing season suggests that a future climate warming will decrease productivity of Siberian larch, thus affecting its existence within the forest-steppe ecotone of Mongolia. Narrow tree rings in years following winters with low snowfall support the hypothesis that winter precipitation exerts an indirect effect on the growth of Siberian larch by controlling the survival rates of gypsy moth eggs.