Precipitation regulates the responses of xylem phenology of two dominant tree species to temperature in arid and semi-arid forest of the southern Altai Mountains

Arid and semi-arid forests are important carbon sinks, with implications for the global carbon balance. However, the impacts of climate warming on the growth of arid and semi-arid forest tree species and ecosystem carbon sink dynamics remain uncertain because the effects of the complex interactions between precipitation and temperature on xylem phenology are not clearly understood. Here, we monitored xylem formation over two years in two dominant tree species (Siberian larch, Larix sibirica Ledeb.; Siberian spruce, Picea obovata Ledeb.) along the arid and semi-arid southern Altai Mountains of Central Asia. We determined that temperature interaction with precipitation plays a key role in regulating xylem phenology of these two species, with differences between species. Under rising mean annual temperatures, the growth of L. sibirica advanced as the onset of xylem formation was not limited by early season water availability. However, the earlier cessation of cell enlargement, likely due to legacy effects, compensated for such advancement. In contrast, water stress constrained the advancement of xylem formation under rising temperatures in P. obovata. Nevertheless, water stress was seemingly relieved later in the growing season and consequently did not lead to the earlier cessation of xylem formation. Our results demonstrate that precipitation drives species-specific response to rising temperatures and thus is a key driver of growing season length and carbon sink dynamics in arid and semi-arid forests under climate warming. Integrating the effects of temperature and precipitation on xylem phenology in climate models may improve estimates of climate-carbon feedback in arid and semi-arid forests under future warming scenarios.