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Spring Index 2016

We have data back from 19 locations, 15 centres and 4 individuals. Some locations had missing dates for some of the indicators and I’ve put averages in (shown in red in the table below) in place of these.

The average index across all our locations for this year is 26th April, the same as in 2015. Preliminary data from the Nature’s Calendar2 website gives exactly the same date for the overall 2016 index based on sightings submitted to them.

We were a day later for Horse Chestnut flowering (5th May) and for Orange – tip butterfly sighting (1st May), 5 days later for Hawthorn flowering but 7days earlier for our first sighting of Swallow. The fact we got the same average suggests some robustness in the way the index measures spring.

The Nature’s Calendar Spring Index for 2015 was 23rd April, three days earlier, so the official UK Spring Index suggests 2016 had a slightly later spring than in 2015. Given that the overall range in dates of Spring Index recorded since the 1900’s is something like 30 days this isn’t indicative of any great trend.

Measured against recent years, and like last year, it seemed to be a cold, late spring but when compared against longer term trends then a Spring Index of 26th April would have been a pretty early date in the early 1900’s.

Once again it raises the idea of a “shifting baseline”. It seem slate because we constantly re-calibrate and compare against recent years rather than seeing it as part of the long term trend for spring to get earlier as the climate warms up as a result of anthropogenic increases in CO2 levels.

In terms of differences in the timings of individual events then the biggest difference between 2015 and 2016 was in the date of first sighting of Orange - tip butterflies. In 2015 the average date we recorded was 19th April, in 2016 it was 12 days later on 1st May. The Nature’s Calendar site records a similar discrepancy, with average dates of 21st April in 2015 and 30th April in 2016. The story is a little more complicated, though, than 2016 just being a later year for Orange - tip butterflies.






There was a little flurry of records in mid April then things went quiet with the bulk of the sightings being after 1st May. This corresponded to similar changes in temperatures. The drop - off in the number of locations recording Orange - tip butterflies in 2016 occurring during the later half of April when there was a pronounced cold snap with numbers of first sightings of Orange - tip butterflies only recovering as temperatures started to climb again at the very end of April.

Spring 2016 was notable for an incredible show of Hawthorn flowers. Our first sightings ranged from some very early ones in the first week of April with the bulk of them towards the end of April and into early May. By late May and into early June hedgerows were covered in Hawthorn blossom https://www.theguardian.com/environment/2016/jun/07/country-diary-hawthorn-may-blossom-june.

Perhaps the cold period between the 15th and 29th April held most of the Hawthorn back from flowering and then the sudden and rapid rise in temperature at the beginning of May triggered most of the Hawthorn into a spectacular, co-ordinated flowering.

In summary Hawthorn came into flower about 8 days earlier in 2016 according to our Spring Index data (Nature’s Calendar has it about 5 days earlier), Horse Chestnut in contrast was slightly later in 2016, by 2 (FSC data) to 4 (Nature’s Calendar) days, and we recorded Swallows about 5 days earlier in 2016. The precise reasons why somethings were later (e.g. Orange - tip butterflies) and others (e.g. Hawthorn) were earlier is difficult to discern especially when you consider that;

  • Swallows will be affected by conditions on-route to the UK not just by what the spring is like here,
  • Orange - tip butterflies overwinter as pupa (chrysalis) and are reportedly able to delay their maturation into adults by up to two years if conditions are poor,
  • and the flowering of Hawthorn depends partially on how heavily they flowered in the previous year.

Variations between centres were appreciable and similar to those seen in 2015. The earliest Spring Index date of 14th April (Flatford & Nettlecombe) was nearly a month ahead of the latest, Malham (13th May). The comparable figures for 2015 were 15th April (Epping) and 15th May (Malham). Centres in the South and West tend to have earlier spring index values than do those in the North or those higher up, and if the centre is both in the North and high up, Spring can be very late.