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.