SummaryClimate change scenarios forecast increased aridity in large areas worldwide with potentially important effects on nutrient availability and plant growth. Our results suggest that negative effects on plant [N] and [P] are alleviated with extended duration of drought treatments and with drying-rewetting cycles. Availability of water, rather than of N and P, may be the main driver for reduced plant growth with increased long-term drought stress.

An annually resolved and absolutely dated ring-width chronology spanning 4,500 y has been constructed using subfossil, archaeological, and living-tree juniper samples from the northeastern Tibetan Plateau. The chronology represents changing mean annual precipitation and is most reliable after 1500 B.C. Reconstructed precipitation for this period displays a trend toward more moist conditions: the last 10-, 25-, and 50-y periods all appear to be the wettest in at least three and a half millennia. Notable historical dry periods occurred in the 4th century BCE and in the second half of the 15th century CE. The driest individual year reconstructed (since 1500 B.C.) is 1048 B.C., whereas the wettest is 2010. Precipitation variability in this region appears not to be associated with inferred changes in Asian monsoon intensity during recent millennia. The chronology displays a statistical association with the multidecadal and longer-term variability of reconstructed mean Northern Hemisphere temperatures over the last two millennia. This suggests that any further large-scale warming might be associated with even greater moisture supply in this region.

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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...