Phenology of wood formation in larch (Larix decidua Mill.) trees growing along a 1000-m elevation gradient in the French Southern Alps

Abstract

Spring temperature increase is the main driver of larch tree wood formation onset along a 1000-m elevation gradient in the Southern Alps, while its cessation is more probably controlled by water stress at the lowest elevation and photoperiod at higher ones. The survival of perennial plants depends on their adaptation to changing environment and specially temperature, which in trees is notably implemented through wood formation process. Our main objective is to understand how the phenology of wood formation is related to environmental factors and to temperature in particular. We monitored the xylogenesis of 60 larch trees, distributed in four stands along an elevation gradient of 1000 m in the French Southern Alps. Cambial activity started around mid-May at the lowest site (1350 m) and around mid-June at the highest one (2300 m), showing a delay of 5.4 days per °C. The onset of wall-thickening and mature phenophases followed the same linear trend with a delay of 5.2 and 3 days per °C, respectively. Phenophase cessations followed a parabolic trend with trees from the lowest site finishing their growth the first, while those from 1700 m finished the last. Our results show that the onset of xylem formation is mainly driven by spring temperature increase, while its cessation is more related to photoperiod, with water shortage being able to hasten it. Future climatic changes will most probably increase growing season length (but not necessarily wood production) and shift upwards the optimal elevation for larch growth in the Southern Alps.