The Editor’s Choice paper for Issue 105.3 is titled; A global test for phylogenetic signal in shifts in flowering time under climate change. Authors Nicole Rafferty and Paul Nabity have written a blog post about their paper…

Biological signals of climate change abound. One of the most conspicuous is changes in the timing of life history events, such as migration, reproduction, and senescence. These shifts in phenology are all around us, and many readers have probably observed plants in their own gardens flowering earlier with warmer springs or at unusual, novel times.

What is the evolutionary context for these phenological shifts? We know that Earth’s rapidly changing climate is challenging many organisms. Evidence suggests the pace of change is far more rapid than previous warming events in the past 66 million years (Zeebe et al., 2016). Although the last great warming period, the Paleocene-Eocene Thermal Maximum, saw an increase in temperatures greater than what we have experienced since the Industrial Revolution, far fewer extinctions have been attributed to past warming. Instead, some evidence indicates an increase in plant diversity during this ancient warming period (Jaramillo et al., 2010).

Wildflower community near the Rocky Mountain Biological Lab in Colorado, USA, where many species have shifted their flowering times.

Understanding how organisms adapt, rapidly or otherwise, to these climatic changes is one of the fundamental goals of evolutionary ecology. Plants, in particular, play an important role in mitigating current climate change by buffering some greenhouse gas accumulation, and the earth’s climate history tells us they have adapted before.

A restored tallgrass prairie in the University of Wisconsin Arboretum in Madison, Wisconsin, USA. Many prairie plants are flowering earlier with climate change.

Using evolutionary trees, several authors have previously assessed how plant communities vary in their phenological responses to a rapidly changing climate. In particular, tests of phylogenetic signal provide insight into whether closely related species tend to exhibit similar traits, in this case similar shifts in flowering time. In general, plant phenological traits, such as date of leaf out and date of flowering, show phylogenetic signal (e.g., Davies et al., 2013); however, few studies have tested whether shifts in phenology show signal, and those that have are limited to 1-2 communities and show mixed results (e.g., Willis et al., 2008; Davis et al., 2010).

Desert plant communities, like this one in the Mojave Desert in California, USA, also exhibit shifts in flowering time, with some species delaying their flowering because of changes in precipitation patterns.

Compiling data sets on flowering phenology from around the Northern Hemisphere, our paper took a global approach to understand how plant evolutionary history influences flowering time responses to warming. We found that both the direction of shifts in flowering and the magnitude of the shifts show phylogenetic signal. This means that if one species of plant is flowering earlier, other closely-related species are also likely to be flowering earlier. We also found that these shifts are best explained using a trait evolution model that accounts for selection around an optimum trait value, in this case, an optimal phenological responsiveness. What remains untested is whether the rapid pace of change will cause plants to shift into new adaptive zones, how interactions with other community members shape shifts, and what traits or other environmental data explain taxonomic differences.

Our study highlights the importance of long-term data on phenology for understanding the effects of climate change and its implications for biodiversity. In this regard, citizen science monitoring efforts (e.g. the USA National Phenology Network), long-term data collection by dedicated individuals, and herbarium records provide invaluable insight.

Paul Nabity (Department of Botany and Plant Sciences) and Nicole Rafferty (Department of Biology), University of California, Riverside, USA.