Cambial phenology and xylogenesis of Juniperus przewalskii over a climatic gradient is influenced by both temperature and drought

While temperature is known to be an important factor determining cambial phenology in cold and humid climates, how it interacts with other factors in determining the onset and end of xylogenesis is still not fully understood. Here, we analyzed phenological traits related to observed cambial phenology and xylogenesis over the course of two extreme hydroclimatic years across six sites that span the spatial distribution of Qilian juniper (Juniperus przewalskii Kom.) in the northeastern Tibetan Plateau. Cambial phenology and xylogenesis were assessed weekly or biweekly from the microcores of 30 trees. We found that the onset of xylogenesis significantly correlated with annual mean and minimum temperatures, regardless of the moisture conditions, resulting in an advance of the onset of xylogenesis of 10.1 days degrees C-1. In contrast, the end of xylogenesis was related to both maximum temperatures and drought conditions. The contrast in drought conditions over the two growing seasons made the importance of the water balance on the end of xylogenesis quite clear. During the extremely dry conditions of 2013, xylogenesis ended up to 30 days earlier than during 2012, a wetter and cooler year. Our results indicate that strong shifts in water availability during the growing season are crucial to the rate of wood production. Also, the significant correlation between the total number of xylem cells and both the duration and rate of wood production, highlights how changes in the timing of xylogenetic processes and the growth rate can have a strong influence on the future growth and performance of J. przewalskii. Our findings indicate that while warming promotes an early growing season and, potentially, a longer growing season, warming and decreased moisture availability has a strong influence on tree growth and productivity in cold and arid regions.