Springtime blooms seen earlier now than in the past

Taking something of a back-to-the-future approach, biologists from Boston University have looked into the past to find that flowering plants growing today blossom more than a week earlier than a century ago. Their findings, being presented at the Society for Conservation Biology’s annual meeting in New York City July 30 – August 2, show that among the plants studied in Boston’s Arnold Arboretum, flowering times have moved forward over the decades, with the plants flowering eight days earlier on average from 1980 to 2002 than they did from 1900 to 1920.

What has influenced this rush to flower? Primarily temperature, says Richard Primack, a BU biology professor and head of the research team. Since 1885, Boston’s mean annual temperature has increased 1.5 degrees Celsius or nearly 3 degrees Fahrenheit. According to Primack’s team, this increase in mean temperature, especially in the months February through May, has influenced the shift in flowering times.

In addition to its scientific insights, the study may provide a model for public participation in climate change research. The relatively low-tech, data-from-the-community protocol used by the team might open such studies to participation by botanical gardens, zoos, museums, or even individuals who, over the years, have carefully collected and tended records on how biological organisms respond to their environment.

“It’s an untapped resource that could have widespread applications,” says Abraham Miller-Rushing, a graduate student on the BU research team. “There’s always a pressure to find out what’s happened in the past so as to better understand what’s happening today. This is a new and different way to find out what’s happened.”

Assisted by BU undergraduates Carolyn Imbres and Daniel Primack and by Peter Del Tredici, a senior research scientist at the Arboretum, the researchers combed herbarium records dating back to 1885 to determine when plants had flowered in the past. They focused on records for 229 plants, all of which are still alive and blooming in the Arboretum.

By comparing current flowering dates of the woody plants selected with their past flowering dates recorded in the Arboretum’s herbarium, the researchers found a significant trend toward earlier flowering. On average, they found the plants flowered eight days earlier during the period 1980 to 2002 than they did 100 years ago.

The data also showed the plants’ flowering times were quite sensitive to relatively small shifts in temperature, advancing 3.9 days per 1 C of temperature change. The team’s analyses showed the plants were most responsive to temperature changes in the months before and during flowering — February through May. In general, throughout the past century, the warming temperatures were found to have caused the Arboretum’s plants to flower about five days earlier.

Changes to Boston’s temperature, however, did not explain completely the trend toward earlier flowering over time. According to the team, the results suggest that more localized effects, such as the size and age of the plants and conditions in and around the Arboretum, including warming caused by roads and buildings, probably contributed another three days to the observed earlier flowering times over the years.

The researchers conclude that it is viable to use existing collections from herbaria, museums, or other such institutions around the world to measure regional effects of climate change on phenological events. Analyses of such data could allow scientists to clarify the extent and character of the variation in natural responses local species have to climate change. Such analyses also could help improve predictions of the effects that future climate change might have on biological communities.