Sunshine as culprit: It induces early spring physiological drought in dark coniferous (Pinus sibirica and Abies sibirica) alpine forest

Abstract

Trees comprising high mountain forests have different requirements for site conditions (such as the water supply), thus current climate warming leads to varying reactions of upper forest boundaries depending on the site conditions and ecophysiological features of species. Positive reactions to an increasing heat supply during vegetative season may be hindered for drought-sensitive species by a water deficit in a cold environment, particularly during late winter or early spring. We investigated the radial growth of dark coniferous forest species Siberian stone pine (Pinus sibirica Du Tour) and Siberian fir (Abies sibirica Ledeb.) growing on slopes of different orientation (south-west, east, and north) near the upper forest boundary in an area undergoing fast climate warming: the Western Sayan Mountains (South Siberia, Russia), near a massive water reservoir. Correlations of tree-ring width chronologies with moving 21-day temperature series were used to more precisely determine the timing of temperature influence; an analysis of extreme and optimal years and multifactor regression modeling were applied to assess the most favorable / unfavorable thermal conditions in the study area and to estimate the tree growth reaction to the current climatic trends, respectively. Despite relatively low variation in growth (standard deviation less than 0.2), a significant common climatic signal in tree-ring width was found at all sampling sites. On the shaded northern slopes, P. sibirica is only mildly limited by summer temperatures and has a low similarity (correlations and synchronicity of extreme / optimal years) with other sites. Conversely, the growth of A. sibirica and P. sibirica on two sunlit slopes is similar (r = 0.44-0.81) and has a common pattern of regulation by temperatures before (r =-0.17…-0.40; April 3-May 4 on average) and more significantly during vegetative season (r = 0.31…0.44; May 17-August 27 on average). We hypothesize that both species, being drought-sensitive, undergo water stress in the early spring, and exposition of previous years' needles to severe temperature variation may lead to partial defoliation and xylem embolism, thus suppressing growth. The patterns of climatic response and species distribution on slopes indicate that A. sibirica is more sensitive to this physiological drought and needs gentler slopes than P. sibirica. Temperature increase in the study area during the last decades have occured about five times faster in the early spring (being enhanced by the reservoir) than in summer. This combination of spring and summer warming leads to an increase in P. sibirica radial growth on the northern slope and the stable growth of both species on sunlit slopes, i.e. providing a tentatively optimistic assessment of the dark coniferous forests' near future in the region.