In cold climates, seasonal changes in trees (phenology) is primarily controlled by temperature. Experiments inducing artificial heating of the stem in evergreen conifers during the quiescent stage showed that it is possible to induce reactivation of cell division, demonstrating that temperature is a trigger for the formation of wood (xylogenesis). The higher temperatures predicted for the future are expected to advance xylem formation in spring, although photoperiodic constraints could restrict the responses of some boreal and temperate species to climate warming. Most studies in temperate and cold climates have been performed in ecosystems in which snowmelt provides abundant water especially at the beginning of the growing seasons, precipitation is frequent in spring and summer, and water availability is not a limiting factor for xylem formation. Little is known about the effect of precipitation on the onset of xylogenesis in cold drought-prone areas.
A recent paper in Annals of Botany monitors xylogenesis in Juniperus przewalskii under extreme dry conditions on the north-eastern Tibetan Plateau and finds that precipitation in the early growing season can be a critical trigger of xylogenesis when the thermal conditions are favourable. Xylem growth shows a positive and significant response to precipitation but not to temperature. The delay in the initiation of xylogenesis under extremely dry conditions seems to be a stress-avoidance strategy against hydraulic failure.
Rising temperatures since the late 1970s have increased aridity in many continental regions of the globe, exposing semi-arid forests in particular to additional stress. Such drought stress may postpone the initiation of xylogenesis, reduce cell production or contribute to radial growth decline. Long-term monitoring is necessary to detect potential thresholds in precipitation or soil moisture for the onset of xylogenesis.
Ren, P., Rossi, S., Gricar, J., Liang, E., & Cufar, K. (2015) Is precipitation a trigger for the onset of xylogenesis in Juniperus przewalskii on the north-eastern Tibetan Plateau? Annals of botany, 115(4): 629-639
A series of studies have shown that temperature triggers the onset of xylogenesis of trees after winter dormancy. However, little is known about whether and how moisture availability influences xylogenesis in spring in drought-prone areas.
Xylogenesis was monitored in five mature Qilian junipers (Juniperus przewalskii) by microcore sampling from 2009 to 2011 in a semi-arid area of the north-eastern Tibetan Plateau. A simple physical model of xylem cell production was developed and its sensitivity was analysed. The relationship between climate and growth was then evaluated, using weekly wood production data and climatic data from the study site.
Delayed onset of xylogenesis in 2010 corresponded to a negative standardized precipitation evapotranspiration index (SPEI) value and a continuous period without rainfall in early May. The main period of wood formation was in June and July, and drier conditions from May to July led to a smaller number of xylem cells. Dry conditions in July could cause early cessation of xylem differentiation. The final number of xylem cells was mainly determined by the average production rate rather than the duration of new cell production. Xylem growth showed a positive and significant response to precipitation, but not to temperature.
Precipitation in late spring and summer can play a critical role in the onset of xylogenesis and xylem cell production. The delay in the initiation of xylogenesis under extremely dry conditions seems to be a stress-avoidance strategy against hydraulic failure. These findings could thus demonstrate an evolutionary adaptation of Qilian juniper to the extremely dry conditions of the north-eastern Tibetan Plateau.