Seasonal climate variations drive decoupling between the duration and amount of xylem growth along a hydrothermal gradient in the southern Altai Mountains

Arid and semiarid forests are vulnerable to climate change. Whether rising
temperatures would increase wood production by extending the growing
season in these relatively dry forests remains unclear.  Here, we
document a decoupling of duration versus the amount of wood production.
Despite the prolonged duration of xylem cell production induced by
warming, we did not observe a consistent increase in the amount of xylem
growth, based on a two-year (2018-2019) monitoring of wood formation in
Siberian spruce (Picea obovata Ledeb.) along a natural hydrothermal
gradient in the southern Altai Mountains of central Asia.  The
duration of cell production ranged from 43 to 93 days, with the total
number of xylem cells between 37 and 185. Notably, the warmest Altai
low-altitude site (AL) had the longest duration (82.6 ± 13.3 days) but the
lowest number of xylem cells (63.88 ± 15.6 cells) in 2018, indicating a
decoupling between growing season length and xylem growth. However, this
decoupling weakened in 2019 when spring temperatures were cooler and
summer precipitation was higher than in 2018.  Warmer spring
temperatures can extend the duration of wood cell production by triggering
an earlier onset of cambium activity. Additionally, sufficient summer
precipitation provides the water necessary for turgor-driven cell division
and expansion, enhancing the cell production rate, which plays a dominant
role in determining wood growth in arid and semiarid forests. Thus, warmer
spring temperatures may amplify the observed decoupling, whereas
sufficient summer precipitation may narrow it.  Synthesis. This
decoupling highlights the critical role of seasonal climates in regulating
wood formation dynamics in arid and semiarid forests. With ongoing climate
change, longer and warmer growing seasons may amplify the importance of
water availability, further constraining forest growth and carbon
sequestration when growing-season water conditions are unfavourable.