Ash before Oak - we're in for a soak, Oak before Ash - we're in for a splash
Nowadays we don’t often see Ash trees coming into leaf before Oak trees do. Using data1 from the Nature’s Calendar website we can see that since the year 2000 there have only been two years (2010 & 2013) when Ash trees came into leaf before Oak trees did and there was one year when it was a dead heat (2006) – Graph 1.
In the mid to late 1700’s Robert Marsham (sometimes called the Father of Phenology) recorded the timing of spring events, such as the bud burst of common trees, on his estate in Norfolk. In those days (Graph 2) it was quite common for Ash trees to come into leaf before Oak trees (1751, 1769, 1778, 1783, 1785, 1786 & 1788 shown in orange) or for the two events to be noted on the same day (1760, 1765, 1766 & 1781 shown in green). So what’s changed?
Well one thing that has changed is that, as a result of man-made climate change, springs tend to be warmer now. The average temperature (Jan - Apr) for the years 1751 – 1787 was 4.7°C, for the years 2000 – 2015 it was 6.3°C. The only two recent years when Ash came into leaf earliest stand out as ones with cold, late springs. You might remember the long cold winter of 2010 and how the spring of 2013 didn’t seem to start until late April. So how does temperature affect the timing of bud burst in these two tree species?
We can use data2 from the Met Office’s Hadley Centre to look at this. The timing of bud burst in Oak is very strongly correlated with temperature. In warm springs the Oak leaves will burst much earlier than they do in cold springs. In fact for every extra degree of warmth they come into leaf about 6 days earlier. The relationship between the timing of bud burst of Ash trees and temperature is much weaker. So as springs get warmer the Oaks come into leaf earlier and earlier. Ash trees do too, but not as quickly as do the oaks – they’re being left behind. So now it’s only in a cold spring that we are likely to see “Ash before Oak”. As to the whether we get a “splash” or a “soak” well that’s another story altogether.
Data sources 1http://www.naturescalendar.org.uk 2http://www.metoffice.gov.uk/hadobs/hadcet/data/download.html
Tuesday, March 29, 2016