Treeline ecotones at high latitudes and high elevations are generally temperature-limited environments. However, there is evidence that temperature control on tree growth has recently decreased at treelines due to climate change, while water availability is increasingly reported as a seasonally important growth-limiting factor. Analyzing climate-growth responses at intra-annual (wood cell) level can help unravel these complex patterns, but long time series of wood formation data are often unavailable.
We compiled a dataset of tree-ring width series and wood formation data for seven polar and alpine treeline sites across Eurasia and North America and including eight species of three conifer genera (Picea, Pinus, Larix). We calibrated process-based (Vaganov-Shashkin) models of tree growth and simulated temperature- and moisture-driven growth rates and wood formation dynamics for 1950-today at daily resolution.
Our models showed that temperature has remained the main growth-limiting factor at all sites, but the number of days with prevailing moisture-limited growth has increased at many sites over the past decades. Furthermore, preliminary results indicate that growth resumption in spring has shifted to earlier dates by several days and thus growing season length has increased at most sites. Model validation with observation data is ongoing and generally confirms model findings.
Our analysis explicitly demonstrates how multiple climatic factors, namely temperature and water availability, can simultaneously limit tree growth at treelines in varying proportions. Our findings thus contribute to a better understanding of non-linear climate-growth responses and climate change effects on tree growth in treeline forest ecosystems.