Reply to Robson et al.

Robson et al.'s commentary1 on our article, 'Floral pigmentation has responded rapidly to global change in ozone and temperature'2, questions the study's conclusion that floral ultraviolet (UV) pigmentation has responded to global change, particularly to total column ozone (TCO). Robson et al.1 claim that our study spanned a time frame in which ozone was not declining and suggest no biological relationship between UV-B exposure and UV floral pigmentation. To support their claims, they selectively remove and reanalyze data. We respond with a critique of their interpretations of our results, and analyses of temporal patterns of TCO data from Koski et al.2. Despite Robson et al.'s concerns, our study continues to support a link between temporal changes in ozone and temperature, and temporal changes in UV floral pigmentation.

Floral Pigmentation Has Responded Rapidly to Global Change in Ozone and Temperature.

Across kingdoms, organisms ameliorate UV stress by increasing UV-absorbing pigmentation. Rapid ozone degradation during the 20th century resulted in elevated UV incidence, but pigmentation responses to this aspect of global change have yet to be demonstrated. In flowering plants, UV exposure favors larger areas of UV-absorbing pigmentation on petals, which protects pollen from UV-damage. Pigmentation also affects floral thermoregulation, suggesting climate warming may additionally impact pigmentation. We used 1,238 herbarium specimens collected from 1941 to 2017 to test whether change in UV floral pigmentation was associated with altered ozone and temperature in 42 species spanning three continents. We tested three predictions: first, UV-absorbing pigmentation will increase temporally and be correlated with reduced ozone (higher UV) when accounting for effects of temperature; second, taxa that experienced larger ozone declines will display larger increases in pigmentation; and third, taxa with anthers exposed to ambient UV will respond more strongly than those with anthers protected by petals. Globally, the extent of petal UV pigmentation increased significantly across taxa by ∼2% per year. However, temporal change was species specific-increasing in some taxa but declining in others. Species with exposed anthers experiencing larger declines in ozone displayed more dramatic pigmentation increases. For taxa with anthers enclosed within petals, pigmentation declined with increases in temperature, supporting a thermoregulatory role of UV pigmentation. Results document a rapid phenotypic response of floral pigmentation to anthropogenic climatic change, suggesting that global change may alter pollination through its impact on floral color, with repercussions for plant reproductive fitness.