As someone who has spent hours upon hours counting flowers at the Rocky Mountain Biological Laboratory (RMBL) to measure flowering phenology, I often wondered how the changes in flowering time that we were uncovering might affect the ability of these plant species to persist long-term (e.g., Iler et al. 2013).
The research of my co-author David Inouye shows that some plants experience severe frost damage to their flower buds when they flower too early in the spring (Inouye, 2008). Our new Journal of Ecology study shows that this frost damage is not as bad for populations as increases in mortality under climate change. But how did we figure this out?
To determine whether reproductive losses from earlier flowering could have negative effects on population viability, we combined long-term demographic data for Helianthella quinquenervis (aspen sunflower), a species that experiences frost damage when it flowers early, with long-term records of spring snowmelt dates, an important aspect of climate in the Western USA. Thanks to long-time RMBL resident billy barr, we know a great deal about how the snow conditions have changed at our study site.
To strengthen the conclusions we could draw from our study, I wanted to compare results from the observational data with those from an experimental manipulation. I decided to ski out to the RMBL even earlier than usual to conduct a snow removal experiment with co-author Paul CaraDonna. Imagine shoveling snow at 2900m above sea level, one day after skiing 5 km with supplies on your back, all before you are acclimated to the high elevation. We were sore for days.
Fortunately for us, the responses of the plants to experimental snow removal were consistent with the patterns in the long-term data: early snowmelt lead to increased frost damage and an increase in reproductive effort in the following year. Once we parameterized our population models with field data, we saw that Helianthella populations decline as snowmelt dates become earlier.
We also saw that early snowmelt had the biggest effect on population growth rates via increases in mortality, and much less so via frost damage. We think, and our data suggest, that Helianthella experiences water stress when snowmelt is early. Early snowmelt extends an early-season dry period, leading to increased water stress in plants (Sloat et al. 2015). Thus, in Helianthella, and probably in other long-lived perennials, earlier flowering under climate change is unlikely to pose a large threat to population persistence. Our study highlights the value of place-based research and long-term ecological data, something for which the RMBL is known. Thanks to a legacy of ecological research in the same location, we were able to examine for the first time the effects of climate change-induced earlier flowering on plant population growth rates.
Amy Iler, Chicago Botanic Garden and Northwestern University, USA