The timing of biological events varies across individuals within a population. This variation is important because it can determine the ability of populations to respond to environmental change. Outside of a few model organisms, it is generally unclear how much of this phenotypic variation is accounted for by genetic variation within populations. We therefore used a common garden experiment to ask whether intraspecific variation in the timing and duration of events in the life cycle of coastal willow (Salix hookeriana) was explained by genotype. We measured phenological events in an S. hookeriana common garden at Humboldt Bay National Wildlife Refuge in northern California, USA. The common garden was planted in a randomized design with genotypes from a single population from Humboldt Bay in 2009. Phenological events were assessed on individuals of 10 different genotypes. There were seven individuals per genotype, for a total of 70 trees in our study. The following six phenological events were assessed every two weeks in 2014, following the monitoring scheme of the USA National Phenology Network: flowering, leaf budbreak (hereafter budbreak), fruiting, leaf expansion, leaf coloration, and leaf senescence.
Results/Conclusions
Salix hookeriana flowers in the spring, 10 days before budbreak (7 and 17 March, respectively, at our study site). One month later (17 April), fruits mature and leaves emerge. Leaves lose their green color and senesce in early November. Genotype explained significant variation in the timing of five of the six events in the life cycle of S. hookeriana, with some genotypes peaking early and others later (flowering, budbreak, fruiting, leaf coloration, and senescence, but not leaf expansion). Genotype also explained significant variation in the duration of each event, with some genotypes exhibiting shorter duration events and others longer. The study of the genetic basis of phenological events outside of model lab species is only beginning to unfold, and our study shows that individuals of the same genotype are much more likely to conduct phenological events at the same time and for a similar duration than individuals of different genotypes. Although phenological events are often considered to be heritable, the duration of life cycle events are typically considered to be strongly influenced by the environment. Our study does not rule out the role of the environment, but instead highlights that the duration of life cycle events can be genetically determined.