Oh my, how you have grown!

All kids have to put up with this sort of comment when aunts and uncles visit, but it is equally a common reaction from ecologists who study long-term changes in the tree layers of British woodland. During the first and then the second world war many of our broadleaved woods were cut-over, so most of the trees in our woods are less than 100 years old. Differences in size, form and species composition in such young stands are more apparent over periods of a few decades than when comparing the difference between trees 200 and 250 years-old. In the lowlands of Britain two other events that shape our impressions of change are the pulse of regeneration that followed the decline of rabbits from myxomatosis in the mid-1950s and subsequent elimination of much of the regeneration layer as deer numbers climbed from the 1970s onward.

In the early 2000s I was part of a British Ecological Society group visiting Wytham Woods. As we walked down the hill Kitty Paviour-Smith mentioned that she had in 1968 plotted the trees and regeneration over a patch of about a hectare; and she still had the data in notebooks in her attic.

A page from one of Kitty’s notebooks.

In 2007 a Hungarian MSc student worked with Kitty to re-find the area and look at what had happened to the 105 canopy trees that Kitty had measured (Mihók et al., 2009).

This year (2021) – with the help of an Israeli visitor – we did another census, looking not just at the what had happened to the original canopy trees but also what else had grown up from the dense regeneration that Kitty had painstakingly recorded. We had only been able to do a partial recording of this in 2007.

The results confirm the trends found by Barbara Mihok in 2007. The original trees where they had survived had grown! Just under two thirds of the 1968 canopy trees were still present, sometimes with their original labels. Most of the losses were of sycamore that had been felled prior to 2007 to widen the ride that runs along the eastern edge of the plot and along another ride that crosses the plot east to west. A quarter of the oaks had gone, although these have mostly fallen naturally.

A bigger change was in the number and species composition of the canopy. A strong cohort of ash and sycamore saplings, had been noted in the 1959 management plan for the Woods (probably getting away following the death of the rabbits), and this formed the bulk of the 1968 understorey records. Many of these trees have now grown up into the canopy. There has been no recruitment of oak to the canopy, so its contribution has declined in both relative and absolute terms. The increased deer browsing since the 1970s has also reduced the current understorey density.The different dynamics of the species are seen in the distribution of their size classes. Oak shows a bell-shaped curve with a peak in the larger size classes, whereas both ash and sycamore show much larger peaks in the smaller size classes, typical of expanding populatons.

Size classes (girth in cm) of different species in 1968 and 2021

The area was originally chosen for study because it was relatively rich in large oaks, indeed it would probably have been described as oak woodland at the time. Their relative abundance prior to 1968 almost certainly reflects the favouring of oak under 19th century management of the woods. Oak seems likely to decline further as every few years one or two more fall over. The canopy is still generally too dense for sapling establishment.

Oaks are still the commonest large tree, but since 1968 ash has shown the strongest growth and biggest increase in density compared to 1968. Ash dieback is now present however, and, even if it does not kill all the ash, is likely to reduce their growth. Sycamore’s is likely to expand further into the gaps which we are accepting as part of the future natural state of the woodland here. Both ash and sycamore are capable of over-topping the remnant oaks. All this means that ecologists starting their researches in this area will be working in a very different environment to that which existed only a few decades ago.

I like this story as an example of serendipity (I happened to be talking to Kitty as we passed the area), the value and vulnerability of long-term data, and of collaboration across generations of ecologists and across nationalities. Now I just have to make sure these records get into the Wytham database, so that I am not responsible for a repeat of the ‘data in the attic‘ story ten years down the line.

MIHÓK, B., KENDERES, K., KIRBY, K. J., PAVIOUR-SMITH, K. & ELBOURN, C. A. 2009. Forty-year changes in the canopy and the understorey in Wytham Woods. Forestry, 82, 515-527.

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