25. Date of Leaf Emergence of Trees in Spring
TH Sparks

Spring date on which oaks were observed to be coming into leaf in Ashtead, Surrey, shown in relation to the average temperature in Central England in January to March

Specific Indicator
There are many long-term records of the dates of first leafing or flowering of plants in the UK. The Royal Meteorological Society maintained country-wide phenological records from 1875 to 1947. Unfortunately, most records have ended or concern garden cultivars. However, some tree leafing records have been maintained privately and tree leafing dates are widely recorded in continental Europe. A record has been maintained since 1947 of the leafing dates of trees at Ashtead, Surrey. This record is used as a specific indicator, namely, the date of leafing of oak trees (Quercus robur) in spring in Surrey.
[Source: TH Sparks, Institute of Terrestrial Ecology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire PE17 2LS; courtesy of Mrs J Combes, Ashtead, Surrey]

Relevance
The leafing of trees in spring makes a dramatic visual impact on the landscape. Oak has been chosen because it is so well known, is widespread throughout the UK and is important ecologically and historically.

Sensitivity to climatic and other factors
Trees tend to leaf out early in warm springs. However, some species have shown a trend towards earliness that cannot be explained by spring temperature alone. Winter chilling, daylength, conditions the previous autumn and soil moisture may also affect the time of leafing but, except in extreme conditions, these play a secondary role to spring temperatures for most trees in the UK.

Records at Ashtead, from 1947 to 2002, suggested that mean temperatures in January-March in Central England could explain over 55% of the variation in the date of first leafing of oak - the warmer the temperature, the earlier leafing occurred.

Change over time
The Central England records show a general trend towards warmer temperatures in early spring and the records at Ashstead show correspondingly earlier dates of oak leafing. The warm springs of 1957, 1972, 1990, 1998 and 2002 were associated with early leafing and the cold springs of 1969, 1979 and 1985 were associated with late leafing.

The relationship between leafing dates and early spring temperature suggests that a 1?C increase in temperature is associated with a 7-day advancement of leafing. The current climate change scenarios suggest that we might see more first leafing dates occurring in the month of March unless chilling and other controls on leafing exert an effect.