After visiting the BIFoR FACE experiment we were taken by Alex Malkin, Head Forester of Norbury Park, to visit the new plantations on the estate. We drove to the Norbury Estate sawmill and timber stores and from there walked a short distance to a typical example of the mixed-species plantations that are been developed over hundreds of acres of what had been arable farmland. Many tree species are grown together in an ‘intimate mix’, which results in faster growth rates than monocultures, as the trees compete less with each other, and problems due to pests and diseases are reduced. Deer are rarely seen on the estate and there is a rigorous grey squirrel control programme, using modified Kania traps developed on the estate. However, hares have been a nuisance, sometimes going along a row of trees and biting off the leader of each.

In this plantation we saw a larch that seemed amazingly large for its age and many other individual trees had grown noticeably strongly. Because of the rapid growth, a first thinning is required after only ten to fifteen years. Every fourth or fifth row of trees is removed to gain access. “Winners” are then selected, which are the individual trees that have grown most strongly, with good form. Side branches on these are pruned off, to one third of the tree’s height. Conventionally, the non-winners would be removed completely, but the Norbury approach is to pollard them. As a result, they continue to provide light competition which tends to prevent formation of more side-branches on the trunks of the winners. This process of ‘halo pollarding’ will be repeated every five to ten years until there is a crop of harvestable softwood or hardwood timber after only thirty or forty years.

We then walked to another new plantation which is an experiment in the inclusion of nitrogen-fixers into the species mix. With rising carbon dioxide levels, shortage of nitrogen in the soil is likely to limit tree growth in the future, rather than carbon dioxide. Robinia, Laburnum and Alnus are among the species being trialled in the experiment. All of them have a mutualistic relationship with soil bacteria that convert atmospheric nitrogen into ammonium, some of which becomes available to the tree, increasing its growth rate. This fixed nitrogen subsequently becomes available to the other trees, so the whole plantation should grow faster. The experiment will help to decide what proportion of nitrogen-fixers to include in species mixes and whether one species is particularly effective.

As you can tell, we learned a lot in the hour or so spent with Alex, who was an excellent guide. The Norbury Plantations are innovative, impressive and influential. They offer hope at a time when it is easy to take a very bleak view of the harm humans are doing to trees and forests on Earth.