The Old Man of Wytham

Autumn is here and the heatwave of the summer a distant memory, although in Wytham the ground is still quite dry and ponds and streams have not yet filled-up again. Time to start looking at results from the Dawkins Plots surveys, to see what they might suggest for where the Woods are going now that Ash Dieback, caused by the fungus Hymenoscphus fraxineus/Chalara fraxinea, is affecting the trees.

About half the plots in 2018 had some ash showing signs of dieback in the canopy or in the immediate surroundings. Not all of this may be due to Chalara infection, but it suggests that the disease is widespread. This is unfortunate as it was the tree that has shown the best growth and regeneration of the four main broadleaves in recent decades.

From what we know of the disease elsewhere, regeneration and young trees are more vulnerable than older trees. Major dieback is therefore to be expected at Wytham given the size and hence likely age distribution of the ash in the woods. Much of the ash seems to have regenerated in the post-war period. The management plans of the time speak of a cohort of ash and sycamore getting away in the late 1950s after the decline of rabbits through myxomatosis, before the rise in deer pressure from about 1970 onward.

Diameter distribution from 2012 data for ash and one of the groves at risk.

Ash is not spread uniformly across the Woods, so some areas will be affected more than others. The variations in canopy cover between different parts of the Woods are likely to increase. The overall openness of the wood may however still be less than immediately after the second world war when there had been extensive felling in Wytham as in woods elsewhere across the country.

At the plot scale the low shrub cover should increase as bramble, young trees other than ash, and shrubs take advantage of the increases in light and nutrient availability following ash death. Plant-richness in the gaps is likely to increase as well. More shrub cover may reduce the predation risk for small mammals, so bank voles may increase. Birds feeding or foraging in the lower levels of he wood should do better as well. On the other hand badgers appear not to thrive in areas with very dense bramble so there may be changes in the patterns of sett occupancy. There is likely to be a sudden pulse of dead wood added to the forest floor as the trees die.

Distribution of plots with high and low levels of ash in the canopy; growth of bramble is likely to be favoured.

Over much of the Woods ash is present in mixtures with other woody species and these will spread out into the gaps where ash declines. Some of the old oaks, currently being overtopped by young ash may get a  temporary reprieve. The main replacement trees – oak, beech, sycamore – have different characteristics to ash; from the point of view of the ground flora the differences in the shade that they cast and the time it takes for their litter to break down are most important. Other differences such as the roughness and acidity of their bark will affect what mosses, liverworts and lichens can grow on them. There will thus be a slower second phase of changes as the plants, fungi and animals of the Woods adjust to the new patterns in the tree layer. There will also need to be acceptance of sycamore (non-native, often considered invasive) as one of the replacement trees – to try to stop it would lead to more disturbance to the Woods.

Oak and beech could benefit from ash decline, although they have their own diseases as well.

There will be interactions with other pressures on the Woods. For example will Acute Oak Decline spread to Wytham and off-set any benefits to old oak growth through reduced competition from ash? Will any potential regeneration of alternative trees and shrubs be checked by rising deer numbers?

Tree diseases such as Ash Dieback and pest outbreaks are not unusual and new ones are arriving in the UK with depressing regularity. Despite this it is difficult to predict their overall impacts other than in the sort of generalised way noted above because past outbreaks have not been followed through. Comparisons have been drawn between the potential impact of Ash Dieback and the millions of trees in woods and hedges lost to Dutch Elm Disease in the 1970s. Yet there is surprisingly little published work on the ecological  consequences of Dutch Elm Disease that we can draw on.

Elm plots occur in Wytham, but are too few to give a clear picture of what happened when Dutch Elm Disease struck.

We need to do better in documenting the impact of Ash Dieback. Wytham Woods are potentially an ideal place to carry out such a coordinated study because of the background information available from a range of long-term studies on different animal groups, as well as the historical vegetation records. Yet a common theme at a recent British Ecological Society meeting held in Wytham in October, was that grants for maintaining long-term studies were hard to come by, because the studies, almost by definition, do not involve novel techniques and the data may in the short-term not be that different to previous records.

Without long-term studies we lack the baseline data from which to judge the impact of new disturbances, such as tree dieback. It is not clear yet if it will be possible to establish at Wytham a fully integrated study, looking at Ash Dieback impacts across a range of trophic levels, or whether we will have to continue piecemeal, with individual studies doing what they can and the occasional piece of integrated work. If we end up with the latter, we should not then be surprised if we are still unable to predict the effects of the next big tree disease to hit us.