There’s nothing like the possible debunking of ‘a well-known fact’ to get The Ranger’s nose twitching. Recently I went for a visit to the mainland and spent a day enjoying the delights of Lymington. This lovely coastal town is on the edge of the New Forest, and as we walked through the old leafy lanes down to the sea it was no surprise to see the typical New Forest oaks lining the banks. Alongside, as is often found in the oldest of oak woods, was holly.
Holly is able to grow in places where there is not much light, so often does well under old trees, such as in the New Forest. I noticed however that some of these holly trees were remarkably un-spiny. In fact many of the leaves had but one or two spines on them. I remembered the generally-recounted explanation for this: the smoother leaves are higher up the tree, where the spines are not needed for protection against browsing animals. But these holly trees had some smooth leaves at ground level – and some spiny leaves higher up! There seemed to be no connection between spininess and height. What was going on?
This observation brought to mind a niggle that has been lodged at the back of my mind for many years. The common explanation about the browsing animals really doesn’t seem to be very good: this distribution of spiny leaves has never actually seemed to fit with the given theory. At this point it’s worth pointing out that planted garden varieties of holly aren’t really in the running here – obviously if they’ve been bred for ornamental purposes this kind of subtlety will be hard to detect. Indeed most garden holly seems to be spiny all over in my own experience of it. It is only where holly is growing as a native and established plant that these smooth leaves seem to be found. And when they are, as in the hedges of Lymington, they are often found alongside spiny leaves, and at various heights. Certainly there is no clear high-low distinction. So does this mean the browsing theory is wrong? Well, no it doesn’t as it turns out. However to fully explain the erratic spine distribution a little more detail is required. Potter and Kimmerer (1988) looked at both mammals and insects browsing on American holly. They found, perhaps unsurprisingly, that insect predators didn’t care whether the leaves were spiny or not. Perhaps more unexpectedly, the bigger mammals didn’t either. They also compared height of leaves, size of trees, and spininess. They reported:
Leaves on lower branches of mature forest trees were slightly more spinescent than were upper leaves, and juvenile trees were slightly more spinescent than were mature trees. However, there was no relationship between degree of spinescence and feeding damage.
It would probably be worth reading that aloud just to enjoy saying ‘spinescence’. Once over that pleasure, one could conclude that in American forests at least, the theory of height-related spines on holly does not seem to be clearly supported by Potter and Kimmerer’s work. So maybe something else is at work here.
More recently, and working on European holly, Obeso (1997) sheds some light on the issue. Professor Obeso found evidence – contrary to the American study – that spinier leaves did actually deter browsing by large mammals. He also suggested how this spininess was caused. He found that bigger leaves had fewer spines, which he suggested meant that when a plant had its leaves nibbled, the regrowing leaves could be smaller and thus spinier. This theory of leaf age-related spinescence (see, I said it too, and with a straight face) explains the reason for the Lymington holly, and also explains why holly does not actually show a simple distribution of spines relative to leaf height. Holly in the Lymington lanes (where the New Forest ponies don’t go) would rarely if ever be browsed, and so could in time grow big and smooth leaves. But even on unbrowsed trees, leaves will start off small, and so those younger leaves will be spiny, with each individual tree having a range of leaf ages, and so a range of leaf spinescence. See, there was a simple explanation after all. And we got to say ‘spinescence’.