How big a wood is has profound effects on the wildlife in it: the ecological literature is full of graphs of species-richness plotted against site-size for example; studies of the interaction of size and isolation effects on species richness, based initially on islands, have been extended to habitat patches of one sort or another. Size was one of the fundamental criteria set out in the Nature Conservation Review for judging which were the most important Sites of Special Scientific Interest. Unlike some of the other criteria size appears to be a straightforward, objective measure, but appearances can be deceptive.
In Britain woods are neatly demarcated on maps as green blobs, to distinguish them from the bulk of the landscape which lacks tree cover. (I found that the reverse was true on my first visit to Finland – the white areas on the map were tree-covered and farmland was coloured.) We could just measure the area within the green boundary and take that as the extent of the wood. However how well does the map boundary reflect the edge of the woodland environment on the ground? Even if it is a hard boundary such as a fence or ditch and the trees are rooted inside, the influence of the tree shade and its roots may extend out beyond the fence, just as conversely the influence of the adjacent field may extend into the area marked as a wood. If the wood boundary is a soft one with tree density gradually declining out from a core patch, the location of the wood’s boundary and hence its extent becomes even more debateable.
How small a patch of trees can count as a wood? For pragmatic reasons during the development of the ancient woodland inventories, only woods marked on 1:25,000 maps that were over 2 ha in extent were looked at further. In the more recent revisions smaller patches have been considered, greatly increasing the number of ancient woods, and, to a much lesser extent, the total area recognised. However very small patches (say <0.5 ha) cannot provide the same range of conditions as a patch of 5 ha which in turn cannot provide the same provisions as does a 50 ha patch (all other factors such as soil being equal). For some purposes therefore it might be more appropriate to exclude the small areas: unless of course the species or process of interest can operate across multiple patches (so 100 tiny patches act the same as 10 medium-sized patches or one large patch). Great spotted woodpeckers has been described as a forest interior species, which is more likely to nest in large woods, but the one drumming on our chimney in suburban Oxford has clearly decided that a couple of streets-worth of garden trees is ‘wood-enough’.
Some species can maintain themselves for a long while in very small areas, but that does not necessarily mean that that only such patches need to be conserved in the long run. A single ancient oak may support many generations of a small beetle population that lives in the decaying timber within it, but perhaps after decades, perhaps after centuries, the beetles will need to move on. Their long-term survival depends on more than one oak tree. The patches of yellow birds-nest in Wytham occupy only a few square metres of beech woodland and have survived at these points at least since the 1950s, and possibly since the early 19th century, but what will happen when the canopy trees above them blow down, as a few of their neighbours do each year? Will that lead to the extirpation of this particular species from the Woods? The longer the time horizon, the bigger the size of the population or the habitat patch likely to be needed to ensure that species will continue to thrive.
Size and quality of habitat do not always go together: more species and more individuals of those species may be maintained in a small but ‘high quality’ patch than in a larger, but lower quality area. The declines in woodland butterflies in the late 20th Century have been as much (if not more) to do with changes in the nature of the woodland cover, as with a decline in its absolute extent – the decline of coppice management meant that there were far fewer recently-cut, open sunny areas that the species or their foodplants needed.
So, size is not everything and the answer to how big your wood is, may be ‘how long is a piece of string’.