Weatherlore says that if an oak leafs before an ash then we will have a dry summer:

If the oak before the ash,
Then we’ll only have a splash.
If the ash before the oak,
Then we’ll surely have a soak!

oak before ash
An oak tree (bottom left) in leaf earlier than three ash trees at different leafing stages

I wrote previously about phenology in ash and other tree species.  This photograph illustrates well the natural variation within species, as well as between species.  The oak tree, bottom left, is in full leaf.  The neighbouring ash trees demonstrate three different stages of flushing or leafing; from almost dormant (bottom right) to almost fully leafed (top left), with the centre ash intermediately leafed.  Elsewhere in this woodland there were oak trees that were less advanced in leafing than some ash trees.  This natural genetic variation emphasises the importance of a large data set, such as the observation records collected by the UK Phenology Network, to gain an accurate average leafing date for species.

Every Spring ash appears to leaf before oak ever more rarely, only being earlier than oak four times in the last 50 years.  Oak will leaf one day before ash with every 0.25oC increase in temperature, while ash responds more to day length in Spring.  In warm Springs therefore, oak will have an advantage over ash.  Long-term records, such as the celebrated Marsham record, show that the climate warmed in one local area of Britain over hundreds of years, with observer records showing corresponding earlier oak leafing dates 1.

Next time you hear the well-known “If the Oak before the ash …“, think as well about the ash before the ash before the ash ….

Gabriel Hemery

1 Sparks, T.H., Carey, P.D. (1995).  The responses of species to climate over two centuries: an analysis of the Marsham phenological record, 1736-1947. Journal of Ecology 83, 2: 321–329. doi: 10.2307/2261570.

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