Warming induced changes in wood matter accumulation in tracheid walls of spruce

The warming-driven increase of the vegetation season length impacts both net productivity and phenology of plants, changing an annual carbon cycle of terrestrial ecosystems. To evaluate this influence, tree growth along the temperature gradients can be investigated on various organization levels, beginning from detailed climatic records in xylem cells’ number and morphometric parameters. In this study, the Borus Ridge of the Western Sayan Mountains (South Siberia) was considered as a forest area under rapid climate change caused by massive Sayano-Shushenskoe reservoir. Several parameters of the xylem anatomical structure in Siberian spruce (Picea obovata Ledeb.) were derived from normalized tracheidograms of cell radial diameter and cell wall thickness and analyzed during 50 years across elevational gradient (at 520, 960, and 1320 m a.s.l.). On the regional scale, the main warming by 0.42°C per decade occurs during cold period (November–March). Construction of the reservoir accelerated local warming substantially since 1980, when abrupt shift of the cold season temperature by 2.6°C occurred. It led to the vegetation season beginning 3–6 days earlier and ending 4–10 day later with more stable summer heat supply. Two spatial patterns were found in climatic response of maximal cell wall thickness: (1) temperature has maximal impact during 21-day period, and its seasonality shifts with elevation in tune with temperature gradient; (2) response to the date of temperature passing +9.5°C threshold is observed at two higher sites. Climate change yielded significantly bigger early wood spruce tracheids at all sites, but its impact on cell wall deposition process had elevational gradient: maximal wall thickness increased by 7.9% at the treeline, by 18.2% mid-range, and decreased by 4.9% at the lower boundary of spruce growth; normalized total cell wall area increased by 6.2%–6.8% at two higher sites but remained stable at the lowest one. We believe that these patterns are caused by two mechanisms of spruce secondary growth cessation: “emergency” induced by temperature drop versus “regular” one in warmer conditions. Therefore, autumn lengthening of growth season stimulated wood matter accumulation in tracheid walls mainly in cold environment, increasing role of boreal and mountain forests in carbon cycle