Timing and duration of European larch growing season along altitudinal gradients in the Swiss Alps

growing season was monitored along two elevational transects in the Lötschental valley in the Swiss Alps. Phenological ob-servations and weekly microcore sampling of 28 larch trees were conducted between April and October 2007 at seven study sites regularly spaced from 1350 to 2150m a.s.l. on northwest- and southeast-facing slopes. The developmental stages of nearly 75,000 individual cells assessed on 1200 thin sections were used to investigate the links between the trees’ thermal regimes and growth phases including the beginning and ending of cell enlargement, wall thickening and matura-tion of the stem wood. Needles appeared ~3–4 weeks earlier than stem growth. The duration of ring formation lasted from mid-May to the end of October, with the length of the growing season decreasing along elevation from 137 to 101days. The onset of the different growing seasons changed by 3–4 days per 100 m elevation; the ending of the growing season, how-ever, appeared minimally related to altitude. If associated with the monitored altitudinal lapse rate of −0.5 °C per 100 m, these results translate into a lengthening of the growing season by ~7 days per degree Celsius. This study provides new data on the timing and duration of basic growth processes and contri-butes to quantification of the impacts of global warming on tree growth and productivity