Xylem anatomical responses to climate variability in Himalayan birch trees at one of the world's highest forest limit

The Himalayas is one of the most ecologically sensitive and fragile areas in the world. The climate of the region is dominated by the monsoon seasonality, with typical dry winters and abundant summer precipitations. Here, forest vegetation spreads up to the world’s highest elevations, where cold temperatures and early spring droughts represent the main limiting factors for growth. In this study, we applied a dendro-anatomical approach to assess xylem trait chronologies and their association to local climate variability in the diffused broadleaved Betula utilis D Don close to the treeline (above 3900 m a.s.l.). We measured tree-ring width on increment cores from 73 trees. On seven cores, we prepared 12 μm thick sections, which were analysed with ROXAS for the assessment of ring-based xylem parameters. We then built up the corresponding trait chronologies and analyzed them against the time series of monthly temperatures and precipitations. Mean ring width (MRW), mean vessel area (MCA_V) and ring-specific hydraulic conductivity (Kr) positively correlated with summer temperatures in the previous and current year. In addition, MCA_V was significantly associated with March precipitations. Instead, fibres’ area showed a widely negative correlation only with temperatures during the previous and current season suggesting that fibres get narrower when vessels are wider. These results revealed that Himalayan birch is well adapted to the monsoon seasonality and is responding positively to atmospheric warming, thus suggesting the potential for this species to further expand in altitude under the forecasted climate change scenarios.