Early season temperature controls cambial activity and total tree ring width at the alpine treeline

Background: Temperature directly affects xylogenesis at high-elevation treelines. The low-temperature limitation of meristematic processes is thus key to understand treeline formation. Aims: We aimed to experimentally test in situ the direct low-temperature effect on wood tissue formation at the alpine treeline. Methods: We applied controlled Peltier-mediated cooling and warming (3 K) to branch segments in Pinus uncinata at the treeline in the Swiss Alps. In addition, we studied xylogenesis in untreated trees during the growing season by sequential micro-coring. Results: Micro-cores indicated that the cambial zone was fully developed by the time the cooling and warming treatment started, shortly after snowmelt. Presumably, because of this, experimental cooling of branches did not significantly reduce the number of cells produced per season. Warming extended the formation of early wood into the late season, and thus reduced the fraction of late wood. Conclusions: We conclude that temperatures very early in the season determine the width of the cambial zone which, in turn, strongly controls the number of tracheids produced during the remaining growing season. Temperatures later in the season mainly determine the early wood to late wood ratio. These data provide an empirical basis for the mechanistic understanding of tree growth at the treeline in response to temperature.