Graves WR, Gimondo A. Phenology of annual dormancy release and its association with fruit set of Dirca occidentalis (Thymelaeaceae). Madroño. 2021 Dec;68(4):416-24.
Weather and climate may influence the phenology of Dirca occidentalis A.Gray (Thymelaeaceae) in ways that impact reproductive success. Dirca occidentalis blooms during winter, when the likelihood of entomophily may be low. Based on preliminary observations that the timing of dormancy release and growth resumption varies over years and among shrubs within years, we quantified fruit set among flowers that formed at different times and examined whether annual variation in autumnal precipitation and temperature during autumn and winter are associated with phenology. Fruit set was determined during 2007–2008 through 2011–2012 by tracking 37,461 flowers near or at anthesis early, midway, and late within the blooming period of D. occidentalis. In addition, measures of phenology of 18 individual shrubs were made each December 29, January 26, and February 23 of the five blooming periods, and fruit set of these shrubs was determined. Fruit set was low (<5%) among flowers present early (December 26–January 2), but increased significantly in all blooming periods, to as high as nearly 30%, among flowers at anthesis later. Phenology ratings, and lengths of newly formed stems and leaves, on December 29 increased linearly as the amount of precipitation from October 1–December 15 of the same year increased. Phenology ratings on February 23 increased linearly with increasing air temperature from November 1–February 23. Rankings of phenology of the 18 shrubs were highly correlated over years, and fruit set of individual shrubs over years was 1% to 52% and increased linearly as growth resumption and flowering became later. Our results demonstrate that low autumnal precipitation is associated with delayed growth resumption and flowering, which corresponds with increased fruit set of this rare species.