I started thinking about this because of two comments from recent conservation discussions. In the first planting was argued to be less desirable than natural regeneration because ‘unnaturally high densities’ of trees were being planted. In the second natural regeneration was being promoted as better than planting because the much higher numbers of seedlings produced per hectare gave more scope for natural selection to act and the resulting population of young trees would thus be likely to be better suited to local conditions. There was an element of truth in both arguments in the contexts in which they were presented, but nobody put any numbers to the density of planted or naturally regenerated/planted trees that they were thinking about – it was all rather hand-wavey.
A young stand of planted oak versus a young naturally-regenerated ash stand – little difference in density?
During the 1950s it was not uncommon to plant trees at 3 foot spacing, roughly one metre in modern money, so 10,000 trees per hectare. Many of the plantations created in Wytham Woods in the 1950s were established at this density: they have thinned out a lot since. During the 1980s conservation advisors, including me, advocated wider spacing (3 m) between trees e.g. about 1100 planted trees per hectare to allow for natural regeneration to fill the gaps although this did not always happen. Modern schemes tend to fall within these values, at the high end where wood production is a concern, towards the lower end in conservation schemes.
The beech stand in the rear was planted in the 1950s at 1 m spacing; the oaks in the foreground at 3 m spacing.
How does this range of 1100 – 10,000 per ha compare to the densities of seedlings found in natural regeneration? In the permanent plots across Wytham we record potential recruits (seedlings and saplings generally about 10-30 cm tall) in thirteen 0.1 m2 sub-plots (total area 1.3 m2) within a 10 x 10 quadrat. The same system of recording was used at the Warburg Nature Reserve south of Oxford, so we can scale these up to suggest the distribution of recruit densities found in different parts of these two Oxfordshire woods.
Recruit densities (no of stems per ha) at Wytham Woods and the Warburg Reserve; data collected 2019-19.
The level of sampling would not detect regeneration at a density less than 7,000 (1.2 m spacing), since this would be equivalent to less than one recruit across the thirteen sub-plots, so for about a third of the plots at each site the recruit density had to be below 7,000 per ha. Two thirds of the plots then had recruit densities that were equivalent to 1.2 m spacing or less (>7,000/ha). About half the plots in total had densities greater than the 1950s standard of 10,000 per ha, in some case very much greater. At least in these two woods it would be hard to argue that planting densities were ‘unnaturally high’ in themselves, although you could argue for greater variety of densities in different places to mimic natural regeneration patterns. These high densities reminded me of a beech wood in Cumbria where the natural regeneration – at 100 cm tall – was so dense it was almost impossible to move through.
Young ash natural regeneration at high density in Wytham, unfortunately now getting Ash Dieback.
Young oak at high density coming up in a canopy gap in Sherwood Forest.
Of course, in many plots the recruits were seedlings which might not survive more than a year: natural selection does weed a lot out and after 5 years the density of those that survive might be more like (or less than) that of planted stands. Selection during this period may not however be doing a lot for long-term resilience: the surviving recruits are likely to be those that happen to be in favourable seedling microclimates, that escaped being nibbled by voles or deer, and could cope with whatever the weather was like during their first five years of growth. These pressures may or may not have favoured the individuals that we will need later, for example tolerance to tolerate Ash Dieback and long-term climate change. Planting may allow some of the other threats at the seedling stage to be sidestepped so that a greater number of individuals are available to see how they respond to the new challenges facing tall saplings and young trees.
A planted stand (one-metre original spacing), but now the subject of natural selection.
The past focus on planting has meant that we have not considered alternative approaches as often as we might, but that does not mean that they will magically solve all our woodland regeneration problems. Natural regeneration and planting both have a place in the restocking of existing woods and the creation on new woodland.