Spring Index

Figure 1: Index of the timing of biological spring events (number of days after 31 December) in the UK, 1891 to 1947, and 1998 to 2023

Figure 2: Relationship between Spring Index and Central England Temperature (CET), 1891 to 1947, and 1998 to 2023

Relevance

Phenology is the study of the timing of recurring natural events in relation to climate, and such observations provide year-on-year information on how nature is responding to a changing climate. This indicator illustrates the stress that one aspect of climate change (spring warming) can place on biological systems. Differential responses among species may cause problems for life cycles (e.g. pollinating insects emerging out of synchrony with flowers opening in spring), increasing vulnerability to extreme events such as late frosts, disruption of food webs, and changing the balance of competition between species.

International/domestic reporting

The UK Biodiversity Indicators have been reviewed in response to the latest goals and targets agreed under the Kunming-Montreal Global Biodiversity Framework (GBF) of the Convention on Biological Diversity (CBD). The indicators are being adapted, with some new indicators being developed, to better align them with the monitoring framework for the GBF. The suite of biodiversity indicators is therefore expected to change ready for forthcoming UK national reports to CBD in 2026 and 2029.

Web links for further information

• Royal Meteorological Society website on the State of the UK Climate 2021
• IPCC website on the IPCC Sixth Assessment Report
• NASA’s website on NASA Global Climate Change
• World Meteorological Organization website on WMO State of the Global Climate 2023
• Bulletin of the American Meteorological Society on BAMS State of the Climate In 2021
• The Woodland Trust website
• Nature’s Calendar
• The UK Innovation and Research Forum website on Research & Innovation for our Dynamic Environment (RIDE) forum
• Living with environmental change website on Biodiversity Climate Change Impacts Report Card 2015
• Marine Climate Change Impacts Partnership website on Marine Climate Change Impacts Report Card 2020
• Convention on Biological Diversity website on Aichi Biodiversity Targets
• Convention on Biological Diversity website on Kunming-Montreal Global Biodiversity Framework 2030 targets
• Defra’s website on Biodiversity 2020: A strategy for England’s wildlife and ecosystem services

Methodology

The UK Spring Index is calculated from the annual mean observation date of the following four biological events: first flowering of hawthorn (Crataegus monogyna), first flowering of horse chestnut (Aesculus hippocastanum), first appearance of orange tip butterfly (Anthocharis cardamines) and first sighting of a swallow (Hirundo rustica).

These four events were chosen for the following reasons:

• good UK coverage
• well recognised and easily recorded species
• good historical data
• both early and later spring events are included
• the observations cover plants, a vertebrate, and an invertebrate
• a strong phenological response is observed

For data collected up to and including 2018 the overall index was compiled by calculating an annual mean across all sites where all four biological events were recorded. As there was no missing data, the UK Spring Index could be calculated in two ways: the average of the four events or the average of the locations (the same value will be returned). After the UK Spring Index was calculated, the raw data was not used in the calculation again.

In order to include more data in the calculation the method was changed in 2019 and the overall index was compiled by calculating an annual mean for each species using all available data and averaging these four means. This new method of calculation (see below) has been applied to all previous years retrospectively and has been submitted for external review.

The number of locations will vary from year to year. The UK Spring Index varies year to year mostly because of differences in the temperature.

The average date of these events is now (1998 to 2023) about 8.6 days in advance of the average for the period 1891 to 1947. The UK Spring Index shows a very significant relationship with mean March-April Central England Temperature, which appears to be stronger when mean temperatures exceed 7⁰C. For the UK Spring Index, the relationship in years with mean March-April temperatures below 7⁰C was:

UK Spring Index = 149.53 - 3.57 x Temperature, R² = 38.83%, p<0.001

The equivalent for years when mean March to April temperatures equalled or exceeded 7⁰C was:

UK Spring Index = 175.40 - 7.37 x Temperature, R² = 80.88%, p<0.001

Data for the 1891 to 1947 period comes from the phenological network of the Royal Meteorological Society as published in the Quarterly Journal of the Royal Meteorological Society. This was a voluntary network of recorders that submitted their phenological observations to a central coordinator. All these data have now been digitised and loaded into the Nature’s Calendar database. The UK Phenology Network/Nature’s Calendar project runs along broadly similar lines. As a citizen science project, it also involves volunteers contributing their observations. It is run and coordinated by the Woodland Trust and funded by the People’s Postcode Lottery, Postcode Green Trust. Here we use their data from 1998 onwards. The number of records varies from year to year, but the current scheme is substantially bigger than the earlier one.

Since locations reflect the distribution of volunteer recorders they are not evenly spread across the UK, but rather reflect the density of human population, with a greater number of recorders in the South-East and fewer in the North.

The reliability of the data is dependent on the frequent and timely observations of the natural world by the volunteer recorders. The data show such a strong relationship with temperature that we have confidence in their reliability.

Changes to the method in 2019

• Previously, the Spring Index was calculated using only data from locations for which all four Spring Index species have been recorded. Recorders gave a postcode and could record anywhere within 10 km of that postcode.
• However, in 2017 (when Nature Calendar’s website was launched), recording location changed to plotting records on a map and each recording location is now the area of a circle with only a 20m radius.
• This change means that it is much less likely that a single recorder can record all four Spring Index species in a single location.
• The new method (as applied to data utilised since the 2020 publications) takes into account all the individual sightings submitted.
• Consequently, the new method increases the amount of data that can be used in the calculation. It also allows an additional year of data (1998) to be included in the dataset.

Development plan

Since our previous publication we have adapted the language and visualisations used in this indicator. We are keen to hear from our users about these changes, as well as our published development plan, please email us.