Oak before Ash
We’ll have a splash
Ash before Oak
We’re in for a soak
This quant old country saying is still widely known today, and whilst not a scientific way to predict summer rainfall, it does demonstrate a long-standing interest in the variability of seasons from year to year and how trees respond differently to it.
Ash tree at Preston Park on 17 April showing black buds and emergence of male caktins
From my casual annual observations, Ash is even more of a sleepy head than Oak. It always seems last in the spring sprint for leaf growth, and as it loses its leaves early in the autumn too, it amazes me that the species is so widespread and succesful. So is Ash ever out first and if so has this changed in historical times?
Well there is quite a bit of data around to explore! Nature’s Calender, coordinated by the Woodland Trust, is a fantastic citizen science project where thousands of people throughout the UK monitor signs of Spring.
But the study of phenology (nature’s reponses to seasonal changes) has been recorded by naturalists for a fair few centuries. Whilst their data isn’t as extensive, these nature diaries are valuable observations with which to compare more recent data.
Robert Marsham is considered to be the father of phenology. He recorded 27 signs of spring, starting in 1736 and continuing for over 60 years on his English estate. Successive generations of his family added to his work until well into the 20th century
So back to our original question about Ash and Oak. Using phenological data from Nature’s Calendere there has only been a handful of times in the last 50 years when Ash has beaten Oak, whereas in the 18th Century this was much more frequent.
The Field Studies Council analysed data on bud burst of the two species between 2000-2016 and compared it to meteorological data and in doing so they found a correlation between years when Oak came into leaf first with those which experienced a warmer springs.
So the increasing occurrence of Oak coming into leaf before Ash appears to be result of warmer spring temperatures. Kate Lewthwaite of the Woodland Trust says “For every 1°C rise in temperature Oak gains a four-day advantage over Ash.”
But why and how do the two species differ in their response to warming temperatures? I can’t seem to find much on this apart from the FSC blog: “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 is Oak a climate change winner? Or does it make any ecological difference? Many sycamore trees are now in full leaf whilst my oaks are just starting to unfurl their first leaves, and the black buds of ash are still tightly shut. Does this early bud burst in Sycamore give them a competitive advantage? As with most things in nature I expect there is no simple answer, but one that depends on a more complicated and unrelated set of factors…
Sycamore Guisborough 26 April
And what difference could it make to the many species of insects that feed on oak? I imagine this may have particular significance for species such as the winter moth larva that emerge with the first flush of Oak leaves.
I uncovered some research that does suggest changes are afoot with work by Marcel Visser et al indicating that warmer springs disrupt the synchronicity of oak and winter moth phenology. This is a strong indication that rapid changes in temperature patterns may affect ecosystem interactions more strongly than changes in mean temperature…
Frustratingly, as I am neither a University academic or student I can’t access scientific publications easily and can just read tantalising summary paragraphs online.
I will be keeping a look out for winter moths and the oak leaf roller moth larva as the leaves of my oak trees unfurl this Spring.