Enhanced secondary growth induced by warmer temperatures in early summer advances autumn leaf senescence in temperate saplings

Phenological events in temperate plants are highly dependent on external factors such as temperature and photoperiod, but also on internal factors linked to the plant's development. Recent studies have demonstrated a strong correlation between the timing of autumn leaf senescence in temperate trees and temperature conditions before the summer solstice that alter the rate of plant development. However, experimental evidence to specifically test this hypothesis is lacking. Here, we examined how accelerated growth in early summer induced by warmer temperatures affects the timing of senescence in three common European trees: Carpinus betulus, Tilia cordata and Acer platanoides. Potted saplings were grown under "ambient" open top chambers (OTCs), i.e. with similar temperature as outside the OTC, and under actively warmed OTCs, i.e. +5°C above the ambient, from January until the summer solstice. At the summer solstice, all the potted trees were transferred to an unheated greenhouse, sharing the same climatic conditions until leaf fall in autumn. Plant diameter and height were measured before bud burst, at the summer solstice and after leaf senescence. In addition, gas exchange measurements were conducted before and after the summer solstice to assess leaf performance. Finally, leaf senescence was measured weekly, visually and using a chlorophyll meter, from August until leaf fall.

Leaf-out dates were strongly advanced in the warming treatment for all species (14–28 days). Primary growth was not affected by warming for any species whereas secondary growth was significantly enhanced in the warming treatments for C. betulus and T. cordata. Interestingly, the initiation of leaf senescence occured significantly earlier for the saplings that were subjected to warmer temperature in early summer for C. betulus and T. cordata but not for A. platanoides, suggesting an effect of secondary growth rate during early summer in regulating the start of leaf senescence. Chlorophyll content, photosynthesis and SLA also showed significant differences between the two temperature conditions applied in early summer for C. betulus and T. cordata, even when measured several months after their transfer to the same conditions. Specifically, at the end of summer, leaves that were exposed to the warming treatment in early summer were less efficient for carbon assimilation, had lower chlorophyll content, showed higher SLA (T. cordata only) and started coloration earlier than the ones kept under ambient conditions during early summer.

Our results suggest that there is a strong link between growth development in spring and the timing of leaf senescence onset in autumn as recently shown at large scale in the northern hemisphere. More studies focusing on molecular and/or on tree physiological mechanisms should be conducted to identify the underlying mechanisms responsible for the link between secondary growth rate in early summer and the onset of leaf senescence.