Larch (<em>Larix sibirica</em>) and Poplar (<em>Populus laurifolia</em>) in Refugia: Growth and Migration into the Mongolian Desert

In the ecotones, trees growth and population are the most sensitive to the changing hydrothermal regime. Here we analyzed Larix sibirica and Populus laurifolia response to the moisture changes in the unique refugia that bordered the Mongolian desert in the southern Siberia. The age of old-growth trees (A&gt;500 y) suggests that refugia have existed throughout the Holocene. We aimed to analyze (1) larch and poplar growth dependence on the climate variables, (2) treelines shift into the desert and (3) ground cover GPP (gross primary production) dynamics. We used on-ground survey, dendroecological analysis and remote sensing data. Beyond the refugia, trees were established after the warming onset in the study area (c. 1980). Since that, growth of both species has increased and has been controlled by atmospheric and soil droughts (measured by the SPEI and scPDSI indices, correspondingly). Summer winds impair trees’ growth via increased evapotranspiration. We found that both larch and poplar treelines shifted into southward sandy dunes. Although poplar is a less drought-resistant species, its treeline was shifting ahead of the larch one with a mean speed of 5.6 m/y vs 0.8 for larch. The mean and max treeline shifts were 260 and 450 m for poplar and 35 m and 70 m for larch. During the warming, the poplar population has dramatically increased (+300% vs + 46% for larch). P. laurifolia occupied climate-caused new niches ahead of drought-resistant L. sibirica due to its high anemophily and seed production. We found that increasing GPP trends in both refugia and in adjacent sandy dunes caused phenomenon of “desert greening”. The treelines migration into the desert contradict the predicted shrinkage of the tree range within its southern boundary. However, the projected increase of moisture deficit at the 2080–2100 may impair that phenomenon. Nevertheless, current changes in the hydrology regime are favorable for trees growth and expansion into the adjacent Mongolian desert.