Phenological responses of vegetation to climate, in particular to the ongoing warming trend, have received much attention. However, divergent results from the analyses of remote sensing data have been obtained for the Tibetan Plateau (TP), the world's largest highelevation region. This study provides a perspective on vegetation phenology shifts during 1960-2014, gained using an innovative approach based on a well-validated, process-based, tree-ring growth model that is independent of temporal changes in technical properties and image quality of remote sensing products. Twenty composite site chronologies were analyzed, comprising about 3,000 trees from forested areas across the TP. We found that the start of the growing season (SOS) has advanced, on average, by 0. April-June and August-September minimum temperatures are the main climatic drivers for SOS and EOS, respectively. An increase of 1°C in April-June minimum temperature shifted the dates of xylem phenology by 6 to 7 d, lengthening the period of tree-ring formation. This study extends the chronology of TP phenology farther back in time and reconciles the disparate views on SOS derived from remote sensing data. Scaling up this analysis may improve understanding of climate change effects and related phenological and plant productivity on a global scale.tree rings | cambial activity | plant phenology | climate change | Tibetan Plateau P henology has a profound impact on vegetation growth (1), carbon balances of terrestrial ecosystems (2), and climate change feedback mechanisms (3). The importance of phenology has prompted many studies, mainly using ground-based observations (4-7), which provide useful phenological information at the species level. However, such studies are also quite time-intensive and typically focus on a few individuals in restricted geographic areas, which often limits their applicability to larger spatiotemporal scales. Changes in plant phenology can be detected on larger spatial scales through near-surface remote sensing, using digital repeat photography (8), but this approach remains limited to the stand level. Another commonly used approach is satellite remote sensing, which can cover large areas (9-11); however, this method has yielded inconsistent results on the Tibetan Plateau (TP) (9, 12, 13).The TP, with an average altitude of over 4,000 m above sea level (a.s.l.), covers more than 2 million square kilometers and is strongly affected by ongoing climate change. Due to its vast area, and its position in subtropical latitudes with high incoming solar radiation, changes in vegetation period duration may have major consequences for regional climate and for carbon sequestration in SignificanceInconsistent results regarding the rate of change in spring phenology and its relation to climatic drivers on the Tibetan Plateau have been obtained in the past. We introduce and describe here an innovative approach based on tree-ring data, which converts daily weather data into indices of the start (and end) of the growing season. This method pro...

This paper describes variability in trends of annual tree growth at several locations in the high latitudes of Eurasia, providing a wide regional comparison over a 2000-year period. The study focuses on the nature of local and widespread tree-growth responses to recent warming seen in instrumental observations, available in northern regions for periods ranging from decades to a century. Instrumental temperature data demonstrate differences in seasonal scale of Eurasian warming and the complexity and spatial diversity of tree-growing-season trends in recent decades. A set of long tree-ring chronologies provides empirical evidence of association between inter-annual tree growth and local, primarily summer, temperature variability at each location. These data show no evidence of a recent breakdown in this association as has been found at other high-latitude Northern Hemisphere locations. Using Kendall's concordance, we quantify the time-dependent relationship between growth trends of the long chronologies as a group. This provides strong evidence that the extent of recent widespread warming across northwest Eurasia, with respect to 100-to 200-year trends, is unprecedented in the last 2000 years. An equivalent analysis of simulated temperatures using the HadCM3 model fails to show a similar increase in concordance expected as a consequence of anthropogenic forcing.

13It is generally assumed in dendroecological studies that annual tree-ring growth is adequately 14 determined by a linear function of local or regional precipitation and temperature with a set of 15 coefficients that are temporally invariant. However, various researchers have maintained that 16 tree-ring records are the result of multivariate, often nonlinear biological and physical processes. 17To describe critical processes linking climate variables with tree-ring formation, the process-18 based tree-ring Vaganov-Shashkin model (VS-model) was successfully used. However, the VS-19 model is a complex tool requiring a considerable number of model parameters that should be re-20 estimated for each forest stand. Here we present a new visual approach of process-based tree-21 ring model parameterization (the so-called VS-oscilloscope) which allows the simulation of tree-22 ring growth and can be easily used by researchers and students. The VS-oscilloscope was tested 23 on tree-ring data for two species (Larix gmelinii and Picea obovata) growing in the permafrost 24 zone of Central Siberia. The parameterization of the VS-model provided highly significant 25 positive correlations (p<0.0001) between simulated growth curves and original tree-ring 26 2 chronologies for the period 1950-2009. The model outputs have shown differences in seasonal 27 tree-ring growth between species that were well supported by the field observations. To better 28 understand seasonal tree-ring growth and to verify the VS-model findings, a multi-year natural 29 field study is needed, including seasonal observation of the thermo-hydrological regime of the 30 soil, duration and rate of tracheid development, as well as measurements of their anatomical 31 features. 32 33