Some Woodland Dynamics
If I wanted to be a bit controversial, I might claim that there are few ‘natural’ woods in Britain, on the grounds that not many have any ‘room for manoeuvre’, any space for regeneration. I have been surprised, too, how few planting schemes, really do include that space; in the case of light-seeded trees, in particular, that area should be downwind, if possible.
The fact that so many woods are hemmed in, on all sides perhaps, by land that is effectively denied to them, (whether by roads, houses, industry, agriculture or simply water), means that the dynamics that are natural to woodland cannot function, and we don’t often see them in action. But when we do see them in the Highlands, sometimes their results are puzzling, even controversial………..
In the South of Britain it is, I think, generally reckoned that the oldest hedges, perhaps even some of the oldest woods, are the ones with the most species, but in the Highlands that is often turned on its head. The celebrated Caledonian Pinewood is often very poor in species, sometimes almost a monoculture, which might be considered puzzling. It is the result of the simple fact that most broadleaves, (the relevant ones might be birch, hazel, oak occasionally, rowan) do not regenerate well, or even at all, in the deep shade cast by tall-growing conifers. So, through time, a pinewood will lose most of its broadleaves. It is sometimes said, too, that a dense accumulation of fallen needles will render the soil between the trees more acidic; it certainly can discourage further regeneration of other species.
But this is only part of the natural dynamic within a pinewood. Foresters refer to pine as a “ fire climax species “; it burns well, (as do dry needles), but it is the heat of a fire which opens the cones and releases the seed on to the new ashy seedbed with its easily-accessible nutrients. So a fire will burn and kill many of the trees within the pinewood, but it launches the next generation on its way. However, those ‘pioneer’, fast-moving species, the birch and rowan, also benefit from the new seedbed and open skies, flourishing for a period, before the slower-growing but eventually taller pine again shades them out.
This is a natural dynamic, even if the fire only occurred every few hundred years, probably caused by lightning or the occasional human error. And it explains why pinewoods in history and prehistory seem to have had more broadleaves; the fires were allowed to spread and burn themselves out. But now, as our native pinewoods became more islanded and were seen as more precious, every effort (understandably) was made to stamp out such fires. The inevitable result is that we have suppressed the natural dynamic in such woods, which now often look very limited in species, and have rather less natural history interest than they probably should.
(Admitting that no-one is likely to feel that we can let occasional fires rip through our native pines, in order to create some open space where the broadleaves may get going at least for a while, bodies like the RSPB did begin to create small clearings, (for instance, in Abernethy) which was a step in the right direction. I have always felt that bolder action was required, but have never really hoped that my suggestion of bombing such forests would be taken seriously!).
The regeneration of another very common Highland species, the birch, is sometimes equally misunderstood. As hinted above, it regenerates in open spaces (on suitable soil), but initially in very dense belts or ‘waves’; the saplings are likely to be so close together that you would not try to walk through them. As the trees mature, they thin out naturally until eventually you will only see a few substantial stems. Given plenty of space, this means that birch woods are highly mobile and there will be waves of young, middle-aged and senescent birches stretching across the landscape.
(Regeneration in the more open ground under the senescent stems is often poor - generally, as said, it is downwind - and tends to be replaced, sadly, by vigorous growth of bracken).
Such a “wood” will prove rather difficult to delineate, (for estate management or notification purposes, for instance), to protect (for instance from deer), or to categorise: part will be in ‘unfavourable’ condition, part in reasonable state, and part absolutely bursting with health and energy.
If, as is more often now happening in the process folk call “Rewilding”, large herbivores were resident in these woods, bashing, trampling, munching, as the natural dynamics unfold, the result might well be much messier, but also much more biodiverse and wildlife-friendly. We might even not need my bombs…………