Testing a General Theory for Optimal Flowering Time in Deciduous Perennial Plants as a Function of Growing Season Length

Effects of climate change on phenological timing, like flowering onset, are crucial for population fitness and community dynamics. Recent research has focused on plastic responses to earlier springs, but the optimal phenological timing should depend also on the growing season duration, within which entire annual life cycles must unfold. Optimal energy allocation theory can address life-history scheduling when this critical time window expands. Extending Iwasa and Cohen's (1989) framework, we predict a nonlinear relationship between growing season length and optimal flowering time of deciduous perennial plants measured from spring onset. Common-garden experiments with purple loosestrife (Lythrum salicaria) and European goldenrod (Solidago virgaurea) along Swedish latitudinal gradients strongly supported this a priori prediction. As climate change alters both start and duration of growing seasons, our finding suggests that optimal flowering time expressed as calendar day could stall before accelerating its advancement in response to climate warming at current high-latitude range margins.