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Isolated patchy forests in the arid timberline of Inner Asia are facing increasing risk of forest mortality with increasing drought frequency. The allocation of nonstructural carbohydrates (NSC) helps forests adapt to increasing drought. Previous studies that focused on site-level observations of NSC responses to changes in seasonal rainfall suggested that NSC in branches and stems showed no difference under different levels of drought stress. We conducted large-scale sampling of Larix sibirica-dominated forest patches of different sizes at 7 sites with different mean annual precipitation (MAP) in northern Mongolia in August 2017. Results showed that MAP determines the leaf and stem soluble sugar (SS), leaf and branch starch concentrations of Larix sibirica. The branch stores more starch at lower MAP. A comparison of seasonal NSC dynamics between wetter and dryer sites in the arid timberline of northern China shows that starch in stems is significantly higher for both Pinus sylvestris var. mongolica and Betula platyphylla at drier sites in the dry year 2017 than the normal year 2016. Our two lines of evidence suggest that the enhanced shoot starch allocation is an immediate response of trees to drought, rather than an adaptation strategy to regional climate dryness, although a spatial pattern may exist.