By Tim Radford, Climate News Network
This piece first appeared at Climate News Network.
LONDON—Two thirds of alpine flowers have changed their pattern of bloom in response to climate change. Half of them have begun to bloom weeks earlier than normal, one third are reaching their peak bloom well ahead of the traditional almanac date, and others are producing their last blooms later in the year.
The season of flowers – that feast for bees and butterflies, and a signal for insectivorous birds to make the most of their moment in the sun – is a month longer than it was four decades ago.
This conclusion comes with two qualifications. The first is that it is limited to one meadow in one location in Colorado’s Rocky Mountains in the US. But the other is that it is the product of a meticulous, painstaking 39-year-long study by one researcher. So it follows that since there is not much room for mistake or argument about the pattern in one well-studied location, then a similar pattern probably does apply in many upland temperate zone sites.
When David Inouye of the University of Maryland began his research, he was a graduate student who just wanted to know what sources of nectar were available for hummingbirds and bumble bees. So he started counting flowers about 3,000 metres above sea level in Crested Butte, Colorado, at the Rocky Mountain Biological Laboratory. And he carried on.
He and colleagues report in the Proceedings of the National Academy of Sciences that they chose 60 common wildflower species – most of them perennial herbs – and they specifically excluded the rarer species because there was not enough data. So they made their judgement on the basis of two million flower counts, during the 39-year interval in which summer air temperatures increased by about 0.4°C per decade and in which the spring snow melt advanced by about 3.5 days per decade.
And they also specifically looked at the entire pattern of spring and summer bloom: the big picture of what biologists call phenology, the timing of biological events, in one place.
“Most studies rely on first dates like flowering or migration, because they use historical data sets that were not intended as scientific studies”, said Professor Inouye. “First flowering is easy to observe. You don’t have to take the time to count the flowers. So that’s often the only information available. It has taken a lot of effort to get the comprehensive insights needed for this analysis which helps us understand how ecological communities are going to change in the future.”
Biologists around the world have begun to use phenological shifts as indicators of climate, and as a basis for future conservation plans, and all of them have observed a pattern of change.
European researchers confirmed that plants were either moving to higher latitudes, or blooming earlier in response to global warming, and that birds, butterflies and blossoms were actually heading to higher altitudes. Some have used historic observations by one of America’s literary giants as the basis for their research into climate change, and others have looked at the consequences of changes in the plant timetable for the grazers and predators that depend on specific plant communities.
But Inouye and colleagues now think that much of the phenological evidence so far has underestimated the numbers of species that have altered their flowering times, and probably overestimated the magnitude of change: what matters in the field or the meadow is the sum of all the changes, and not just the first dates of flowering.
Inouye and students divided the meadow into 30 plots, and counted flowers every other day for 39 years, for five months every year. So because of the initial basis of the research, continued for so many years, the scientists had sure data on changes for individual species, including the first flowering, the peak flowering and the last blooms, along with a measure of changes in abundance.
The date of first flowering has advanced by six days per decade, the spring peak is on average five days earlier per decade, and the last flower of autumn has been three days later every decade.