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

## TL;DR

This study shows that the best time for plants to flower depends not just on when spring starts but also on how long the growing season is, with implications for how plants respond to climate change.

## Contribution

The study introduces a nonlinear relationship between growing season length and optimal flowering time, extending energy allocation theory for perennial plants.

## Key findings

- Experiments with purple loosestrife and European goldenrod confirmed a nonlinear relationship between growing season length and optimal flowering time.
- Optimal flowering time may stall before advancing in response to climate warming at high-latitude range margins.
- The mathematical framework can be used to predict phenological limits for other species based on their life history.

## Abstract

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.

Climate change affects both the start and duration of growing seasons, creating complex effects on optimal flowering timing that go beyond simple responses to earlier springs. Using optimal energy allocation theory, we found a nonlinear relationship between growing season length and optimal flowering time which was supported by two experiments with perennial species. This nonlinearity reveals inherent limits to phenological advancement under climate warming, with our mathematical approach providing a flexible framework for finding these limits for other species based on their life history.