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...

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

No abstract

Norway spruce (Picea abies L.) is among the most sensitive coniferous species to ongoing climate change. However, previous studies on its growth response to increasing temperatures have yielded contrasting results (from stimulation to suppression), suggesting highly sitespecific responses. Here, we present the first study that applies two independent approaches, i.e. the non-linear, process-based Vaganov-Shashkin (VS) model and linear daily response functions. Data were collected at twelve sites in Slovenia differing in climate regimes and ranging elevation between 170 and 1300 m a.s.l. VS model results revealed that drier Norway spruce sites at lower elevations are mostly moisture limited, while moist high-elevation sites are generally more temperature limited. Daily response functions match well the pattern of growth limiting factors from the VS model and further explain the effect of climate on radial growth: prevailing growth limiting factors correspond to the climate variable with higher correlations. Radial growth correlates negatively with rising summer temperature and positively with higher spring precipitation. The opposite response was observed for the wettest site at the highest elevation, which positively reacts to increased summer temperature and will most likely benefit from a warming climate. For all other sites, the future radial growth of Norway spruce largely depends on the balance between spring precipitation and summer temperature.

Significant alterations of cambial activity might be expected due to climate warming, leading to growing season extension and higher growth rates especially in cold-limited forests. However, assessment of climate-change-driven trends in intra-annual wood formation suffers from the lack of direct observations with a timespan exceeding a few years. We used the Vaganov-Shashkin process-based model to: (i) simulate daily resolved numbers of cambial and differentiating cells; and (ii) develop chronologies of the onset and termination of specific phases of cambial phenology during 1961–2017. We also determined the dominant climatic factor limiting cambial activity for each day. To asses intra-annual model validity, we used 8 years of direct xylogenesis monitoring from the treeline region of the Krkonoše Mts. (Czechia). The model exhibits high validity in case of spring phenological phases and a seasonal dynamics of tracheid production, but its precision declines for estimates of autumn phenological phases and growing season duration. The simulations reveal an increasing trend in the number of tracheids produced by cambium each year by 0.42 cells/year. Spring phenological phases (onset of cambial cell growth and tracheid enlargement) show significant shifts toward earlier occurrence in the year (for 0.28–0.34 days/year). In addition, there is a significant increase in simulated growth rates during entire growing season associated with the intra-annual redistribution of the dominant climatic controls over cambial activity. Results suggest that higher growth rates at treeline are driven by (i) temperature-stimulated intensification of spring cambial kinetics, and (ii) decoupling of summer growth rates from the limiting effect of low summer temperature due to higher frequency of climatically optimal days. Our results highlight that the cambial kinetics stimulation by increasing spring and summer temperatures and shifting spring phenology determine the recent growth trends of treeline ecosystems. Redistribution of individual climatic factors controlling cambial activity during the growing season questions the temporal stability of climatic signal of cold forest chronologies under ongoing climate change.