Shining a Light on Patterns of Photoperiod Sensitivity in Germination and Flowering Across Latitudes, Ecosystems and Functional Traits.

In photoperiod sensitive plants, the timing of phenological events depends primarily on day length rather than temperature, precipitation or other environmental variables. This may make these photoperiod sensitive species less able to respond to climate change as their phenologies are more tightly controlled by day length conditions, which remain constant into the future, than by changing climatic conditions. We measured germination under three light treatments (short-day, long-day and equal light and dark) to quantify species' germination photoperiod sensitivity. We developed a novel metric that provides a continuous measure of germination photoperiod sensitivity for 67 plant species across a range of locations, habitats and growth forms. Of the 67 species, four species had significantly more seeds germinate in the long-day treatment, three species had significantly higher germination in the short-day treatment, and four species had significantly higher germination in the intermediate-day treatment. We then used this metric to quantify the relationships between germination photoperiod sensitivity (as quantified from the laboratory tests and calculated metric) and phylogeny, seed size, growth form, latitude, leaf area index (LAI) and ecosystem type. We also quantified the relationships between flowering photoperiod sensitivity (as quantified by a literature review) and the same ecological/environmental predictor variables as were tested against germination photoperiod sensitivity. There were no significant relationships between photoperiod sensitivity in germination or flowering and species' biogeography, phylogeny or other functional traits. Our findings suggest that photoperiod sensitivity is likely to be important in a range of different locations and in different types of species.