What are the first signs of spring?
How would you define when spring has sprung? “Oak before the ash, we’re in for a summer splash. Ash before the oak we’re in for a soak”.
Meteorological definitions mark the 1st March as the first day of spring. The astronomical definition is based upon equinox and solstice. There is a third, lesser known marker of spring: phenology.
Phenology is the study of the timing of natural events from year to year, especially in relation to climate and weather.
Robert Marsham is regarded as the founder of phenology, keeping a record of his 27 ‘indicators of spring’ from 1736. The changing phenology of wildlife is one of the first observed responses to weather and climate change. Marsham’s records provide a baseline from which to track changes in nature. By comparing the historical records to data now collected by citizen scientists, we can now relate observed responses to weather and climate change.
Nature’s Calendar (in association with Woodland Trust and Centre for Ecology and Hydrology) is a citizen science scheme allowing members of the public to submit recordings of a wide range of species and events. These include; first flowers, migratory bird sightings, first bud burst on trees, first nest building and flowing grasses. The database currently holds over 2.9 million records, proving a valuable source of data for scientific researchers.
Last year’s report showed that nearly all spring events in 2019 were recorded earlier. This was as a result of monthly temperatures recorded in February, March and April being above their 30-year averages. All tree species monitored had earlier bud burst, followed by early first leaf fall, compared to the baseline year of 2001. Flowering plants were emerging earlier as they take advantage of milder winters, with snowdrops recorded in November and December 2018. In 2019 (relative to 2001) the average first recorded date for all insects was over 23 days early. This pattern continued across all indicators of spring. As global temperatures increase, will spring continue to arrive earlier?
UK Spring Index
The UK Spring Index, adopted by the government, looks at four biological events: first flowering of hawthorn, first flowering of horse chestnut, first recorded flight of an orange-tip butterfly and first sighting of a swallow. The index has revealed that since 1999, events have occurred around six days in advance of the average dates in the early 20th century. The advancement of phenology is strongly linked to March and April temperatures.
Collecting data on phenology allows us to monitor changes over time, but also the impacts it has on the wider natural world. For example, early flowering may result in a mismatch with dormouse phenology, impacting their ability to build reserves before winter dormancy. Furthermore, the early spawning of frogs has the potential to cause increased predation by newts. The milder, earlier springs have allowed birds to nest sooner which can result in a mismatch between their phenology and food availability. Different species respond at different rates to changes in our climate. With phenology advancing it is important to understand how nature is being impacted by climate change so that the future effects of increasing temperature trends can be predicted
There is a clear relationship between temperature and the spring emergence of plants, with plant species demonstrating an advancing phenology of five days with every one degree increase in spring temperatures. Another study concluded for 385 plant species the average first flowering date in 2000 had advanced by 4.5 days from records starting in 1954. Some scientists are suggesting that the season of spring is arriving up to eleven days earlier than in the 19th Century.
As for the rhyme mentioned at the start of this article, first leaf for oak is approximately 2 weeks earlier and ash a week earlier, than 30 years ago. Ash first leaf records have rarely been before oak in the last 50 years, a result of warmer springs.
Jess Snow, Ecologist