Global warming is causing UK plants to flower earlier

The UK's plants are at 'unprecedented risk' if the climate continues to change, researchers have warned. 

A comparison of the first flowering date of plants and the UK's average temperature from the 1750s until today has revealed that plants are flowering around 26 days earlier than they used to, as average maximum temperatures at the start of the year increase.  

Increasing urbanisation could also have an impact, with plants in towns and cities flowering on average five days earlier than their rural counterparts. This may be due to the 'urban heat island' effect, where aggregated human activity raises local temperatures. 

The scientists behind the study, writing in Proceedings of the Royal Society B, warn that this could have a range of effects, including impacts on farm productivity, food availability for wild animals and even a longer hayfever season.  

Lead author Prof Ulf Büntgen says, 'I was not expecting to see a difference this strong. The first flowering dates for plants take place over a large portion of the year, and while many happen in spring, some plants flower in January with others later in June and July. We saw that no matter when they flower, they are all doing so earlier than they used to. 

'While a degree of warming is abstract, and hard to comprehend, this impact on flowering is something everyone can understand. A month is a lot, and a dramatic change like this really cements the impact this warming is having.' 

Setting a standard

While many studies can take place over a relatively short period of time, others require much longer to gather the data they need. To establish long term environmental changes, such as those associated with global warming, information from hundreds of years is often required to establish a firm baseline from which changes can be assessed. 

The UK is well-suited to these kinds of studies due to a long history of individuals and groups recording their observations. One of the longest running set of observations began in 1736 when naturalist Robert Marsham began taking annual notes of the 'Indications of Spring' at his Norfolk home, such as the growth of leaves on trees and the arrival of birds. 

Subsequent generations of the family kept adding to these records, until they stopped in 1958. However, other individuals such as Jean Combes have kept their own records of these natural events, and their work is continued by groups such as The Woodland Trust with community science projects. 

These historical and modern observations have now been compared with the Central England Temperature series, which began recording temperatures in 1659. The researchers used these to assess how the first day of the year in which plants flowered has changed, and its relationship with temperature. 

'Such high sample sizes ensure that even if the data is quite noisy, the overall number of records cancel out the individual noise and emphasise the common trend,' Ulf explains. 

The data revealed that after centuries of stability, the date of flowering has been creeping back by around five days a decade for the past 70 years. 

Growing out of control

Using 1986 as a baseline, the overall change for the 406 species before and after this date was around 26 days. This was not consistent across all groups, however, with climbing species only changing their flowering by three days while trees flowered around two weeks earlier. 

The largest change came with small plants, which dominated the species included in the study, flowering almost 32 days before the baseline. 

Over the same period the average maximum temperature for January to April rose by 1.1⁰C, leading the researchers to suggest rising temperatures as the best explanation for this phenomenon on the strength of the correlation. 

In addition to the impact on plants themselves, earlier flowering could cause knock on effects for pollinators and other animals which rely on the timely production of nectar, fruit and seeds. 

'The biggest risk is that plants flowering earlier are more likely to be hit by a late frost,' says Ulf. 'Normally, the onset of flowering comes so late this risk is essentially avoided, but moving the start of spring into March puts it in a stage where it is more harmful, as the plant is taking up water and nutrients that could be affected by a freeze.  

'What we are interested in now is working with The Woodland Trust's 3.5 million observations on not just plants, but insects, bird migrations and other aspects of the British Isles' ecosystem.  

'We are doing a similar analysis to see how different components respond, and at what speed. If plants respond at a different rate to their pollinators, for instance, this could lead to an ecological mismatch which can cause a serious problem.' 

The study adds to a growing body of work investigating how changing seasons impact nature. This includes research such as the Museum's Orchid Observers project, which made use of community observations as well as information held in the collection. 

John Hunnex, who curates the Museum's British and Irish Herbarium and was not involved with this study, says, 'The research shows the value and potential of the Museum's British and Irish Flowering Plant Herbarium, which holds around 750,000 specimens dating back to the early 18^th century.  

'Information recorded with the specimens by naturalists such as Samuel Dale and Joseph Andrews can give good indications of flowering times. An analysis of the data may indicate if there have been changes over periods of time.  

'We are currently looking to get as many of our specimens databased, imaged and uploaded onto our Data Portal through the work of our Digital Collections Programme to assist researchers with projects like these.'