The effects of metabolic and functional traits on bud opening: Comparing warming and defoliation in conifers.

Developing buds are crucial carbon sinks that require nonstructural carbohydrates (NSCs) for growth. However, the trade-off between carbon production in the older internodes and the demand in the growing internodes for bud opening remains unknown. Here, we determined how NSCs and functional traits influence bud phenology in the saplings of 2 conifer species. To manipulate both source and sink, saplings of balsam fir (Abies balsamea, L. Mill) and black spruce (Picea mariana B.S.P. (Mill.) were exposed to 2 simultaneous treatments: warming (+2 °C) and defoliation. Balsam fir, the species with earlier phenology, exhibited greater shoot volume and specific leaf area, promoting water and carbon acquisition for primary growth. Heating led to an earlier phenology but did not affect the leaf traits for both species. Defoliation also led to an earlier phenology, mostly because of the decreased growing sink, with fewer needles and smaller specific leaf area needed for growth. Starch and sucrose levels in older needles and growing buds decreased under defoliation, but the sugar alcohol D-pinitol remained unchanged. Heating increased the D-pinitol concentration in the growing buds (+17%) compared to ambient conditions. Under warming, a high D-pinitol concentration in buds can act as a carbon sink in the vacuole, maintaining or increasing water absorption, and thus, resulting in faster needle expansion and bud opening. These data demonstrate that different physiological mechanisms explain earlier bud opening under defoliation and warming. Additional studies are needed to disentangle the roles of leaf traits and carbon allocation in regulating phenology.