Should we revisit conifer-broadleaved mixtures?

There are major government targets for creating new woodland and increasing the benefits that come from existing woods. Different types of woodland are needed to deliver different objectives. There are woods where natural processes should predominate but elsewhere various forms of intervention will be used to ensure that a specific mix of products, such as timber, carbon sequestration, water management and biodiversity, are provided. Some of the new stands formed will be from natural regeneration within existing woods and natural colonisation of open ground, but many other stands are likely to be planted in whole or part.

During the early twentieth century what are now broadleaved stands (both within woods and on open land) were often established using mixtures, with broadleaves such as oak planted with a conifer ‘nurse’. I suspect from looking at 19th Century Ordnance Survey maps, many more mature high forest oak and beech stands were established in this way than we acknowledge, because the conifers have largely gone. There are for example good examples from the Chilterns, Forest of Dean and even Johnny’s Wood in Borrowdale, Cumbria.

Johnny’s Wood Cumbria, NCR Gr.1, SAC, SSSI on the 1897 six-inch map, showing conifer symbols, and in 2021.

There is limited information on how the conifer element in the mixture affect the biodiversity of the stand in the long-term. The generally greater shade of the conifer element will cause local reductions in the ground flora across the rows (although the reverse is true for larch-beech mixtures) – see figure below from  (KIRBY, 1988). However after the conifers have been removed the floral richness that develops may be little different to that of stands that were broadleaved throughout (Kirby et al., 2017, Kirby and Thomas, 2017).

Variations in ground flora richness in transects across alternating groups of rows of oak and spruce

The experience of restoration of plantations on ancient woodland sites is also that recovery of ground flora can be successful. At Dalavich Oakwood (Scotland) one of the first sites where the Forestry Commission took out the conifers starting in 1986 (Kirby and May, 1989) it is now difficult to see where they had been.

Dalavich oakwood in 1986 (left) and in 2014 (right), almost the same spot.

One problem is that in many of the mixtures planted in the 1950s and 1960s the conifers were not thinned out quickly enough, if at all, and the broadleaves were suppressed. However, provided the management can be got right, there is a case for revisiting conifer-broadleaved mixtures for new woodland establishment, even from a biodiversity perspective.

Adjacent stands in Wytham both planted as mixtures in the post-war period, one of which is now pure oak while the other looks from the distance to be mainly conifer (Jones, 1964)

Using a mixture means that a larger total area of woodland can be created with the same number of broadleaved transplants – an important gain if there is concern about the capacity of nurseries to supply British-origin material. The greater suppression by the conifer of the open ground flora might make it easier for woodland flora and shrubs to invade as the faster growing conifer crop is thinned out. The dense foliage of genera such as spruce might provide more cover for small mammals and birds in the early years when the broadleaves are still quite spindly (Trout et al., 2012).

Even if there is a biodiversity disbenefit during the mixed stage, this might not persist beyond the fifty-year mark, by which time most of the conifers should have been removed. Any biodiversity disbenefit might also be off-set by other potential gains if mixed crops result in faster early carbon sequestration and improved economic returns from the conifer thinnings, with a larger extent of broadleaved woodland final crop being created.

The twentieth century mixtures were often pyjama stripe row plantings; aesthetically not very pleasing, but they did mean that nowhere was more than a few metres away from the broadleaved part of the crop . Another alternative for 21st century priorities might be to use a different pattern of mixtures, with blocks of conifers, not so much as a nurse crop, but as a temporary component of the new woodland. When these are removed after a few decades (when it is economic to do so) they would leave an open area in which a new crop of broadleaves might be established (through natural regeneration or planting) providing increased age-class and structural diversity, or the cleared area could be left as open space.

Pyjama stripe planting on Radbrook Common at Wytham in the 1950s.

In the past I have argued against mixed planting and would still do so in ancient and long-established woods, where we should be looking to encourage natural regeneration wherever possible anyway, but in other situations I think we should give conifer-broadleaved mixtures more consideration in future, given that we cannot pretend we are re-creating the woods of the past.

JONES, E. W. 1964. Oak larch and oak thuja mixtures in woods of hazel wytham. Quarterly Journal of Forestry, 58, 197-207.

KIRBY, K. J. 1988. Changes in the Ground Flora under Plantations on Ancient Woodland Sites. Forestry, 61, 317-338.

KIRBY, K. J., GOLDBERG, E. A. & ORCHARD, N. 2017. Long-term changes in the flora of oak forests and of oak:spruce mixtures following removal of conifers. . Forestry 90, 136-147.

KIRBY, K. J. & MAY, J. 1989. The effects of enclosure, conifer planting and the subsequent removal of conifers in Dalavich oakwood (Argyll). Scottish Forestry 43, 280-288.

KIRBY, K. J. & THOMAS, R. C. 2017. Restoration of broadleaved woodland under the 1985 Broadleaves Policy stimulates ground flora recovery at Shabbington Woods, southern England. New Journal of Botany, 7, 125-135.

TROUT, R. C., BROOKS, S. E., RUDLIN, P. & NEIL, J. 2012. The effects of restoring a conifer Plantation on an Ancient Woodland Site (PAWS) in the UK on the habitat and local population of the Hazel Dormouse (Muscardinus avellanarius). European Journal of Wildlife Research, 58, 635-643.

2 thoughts on “Should we revisit conifer-broadleaved mixtures?

  1. Yes, this establishment system should be promoted but as you say, not in ASNW or PAWS. Thinning blocks of conifers mixed with broadleaves is extremely difficult. I suggest the method of softening the harsh appearance of straight-row planting by curving the rows by as little as 2m in any 20m run should apply to ‘pyjama-stripe’ planting too.

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