Where in England do ancient and veteran native oaks occur? The two species involved, pedunculate oak (Quercus robur) and sessile oak (Q. petraea) are among the most common trees in Britain and are almost ubiquitous. Going west and north, sessile oaks become more common and pedunculate oaks less so, but taken together, as we do here, they can be said to grow in virtually every acre of woodland and every 100 meters of hedgerow with trees plus uncountable solitary trees in fields up and down the country. However, the distribution of truly old oaks is much less evenly spread.
Left: Distribution and density map of 3,300 oaks in England with >=6.00 m girth; Right: Distribution of 3,300 oaks in England according to five size classes (range 6.00 m to 12.00+ m). Map production: Justin Moat, GIS Unit, Royal Botanic Gardens, Kew.
To demonstrate this, the first map shows the distribution of all 3,300 oaks recorded in England with a stem circumference of >= 6.00 m. This distribution is indicated with black dots. Due to the scale of the map many dots overlap, but the pattern is visible regardless. To show where the concentrations really are, i.e. accounting for the hidden dots as well, a so-called heat map is calculated, presenting in coloured fields where concentrations occur. The colours change from low (green) to high (red) density while the area of the coloured fields gives additional indication of numbers. The ancient oaks occur from a single tree on a farm to many trees in an ancient deer park too small to separate the dots on this map. Now a clear pattern emerges. The north of England has far fewer ancient and veteran oaks than the south and only a few small concentrations. They are even scarcer in the midlands and in Cornwall and the western half of Devon. The greatest concentration is in the three counties bordering Wales: Shropshire, Herefordshire and Gloucestershire plus Worcestershire and Warwickshire. A close second are the Home counties and Sussex but not Kent, while East Anglia comes third. There are other concentrations in Hampshire, Dorset and Somerset, as well as more isolated hotspots in e.g. Nottinghamshire, Oxfordshire and Wiltshire.
We can also investigate whether there is an additional pattern according to size. Are the sizes distributed evenly or unevenly? We can divide the measured girths of all 3,300 oaks into five size classes (the range is 6.00 m to 12.00+ m) and present the distribution of these in the second map. Broadly described, the two patterns are similar. The most striking difference is found between East Anglia and the border counties with Wales: ancient oaks are significantly smaller in the east than in the west. The north of England also has relatively few very large oaks.
These distribution patterns require explanations. My research has focused on the historical context: what happened to the landscape in which the oaks grew? To each of the 3,300 oaks I have assigned a land use category looking at the past before 1600. These are Deer Park (Medieval or Tudor), Royal Forest, Chase, Common and Manor. About 500 oaks remain "unknown", but with this information the distribution of ancient oaks in England can be analysed in more detail.