Detecting flowering time response to climate change in Denmark

Richard B. Primack

 

“Let us spend one day as deliberately as Nature, and not be thrown off the tracks by every nutshell and mosquito’s wing that falls on the rails.” Henry David Thoreau in Walden.

 

Lots of new phenology data is becoming available from digitized herbarium specimens, citizen science programs, and photographs uploaded to the iNaturalist app. But how do these data sets compare in terms of detecting the effects of temperature and climate change on flowering times?  

 

A new study compares three separate phenology datasets from Denmark: i) herbarium specimen data spanning 145 years, ii) one year of flowering data collected from a citizen science program, and, iii) data derived from a year of photographs uploaded to iNaturalist. Each dataset includes flowering dates of three common plant species: Allium ursinum (ramsons), Aesculus hippocastanum (horse chestnut), and Sambucus nigra (black elderberry).

 

Photo:  Ransoms, horse chestnut, and black elderberry.

 

The iNaturalist dataset provided the most extensive geographic coverage across Denmark and the largest sample size, and recorded peak flowering in a way comparable to herbarium specimens. The directed citizen science dataset recorded much earlier flowering dates because the program objective was to report first flowering, and so was less comparable to the other two data sets. 

 

Figure 1: Distribution of data points across Denmark for: A) the herbarium dataset; B) citizen science dataset; and C) iNaturalist dataset. The herbarium data shows the widest range of temperatures. The iNaturalist data set has the most observations.

Herbarium data demonstrated the strongest effect of spring temperature on flowering in Denmark, possibly because it included year-to-year variation in temperature, while the other datasets only included one year of spatial variation in temperature across Denmark. Combining herbarium data with iNaturalist data provides an even more effective method for detecting climatic effects on phenology. Phenology observations from citizen science programs and iNaturalist will increase in value for climate change research once additional years are available.

 

This article is published as: 

Iwanycki Ahlstrand N, Primack RB, Tøttrup AP. 2022. A comparison of herbarium and citizen science phenology datasets for detecting response of flowering time to climate change in Denmark. International Journal of Biometeorology https://doi.org/10.1007/s00484-022-02238-w