Wessex Silvicultural Group: Field Meeting Notes

2016 Study ‘The Silviculture of Mixed Stands’

Meeting 3: Crichel Down Estate & Cranborne Estate

Continuing this year’s theme of ‘The Silviculture of Mixed Stands’, the third field visit of 2016 took the WSG to Crichel Down and Cranborne Estates. The visit offered WSG members the opportunity to examine 20 to 60 year-old oak-dominated stands where past management has produced increased diameter growth in line with ‘active silviculture’, an approach proposed in ‘Oak: fine timber in 100 years’ by Jean Lemaire, translated by Bede Howell (2014). The collection of stand performance data in mixed broadleaved stands was also addressed, and the application and value of such data explored.

Morning Session

Beginning at Chetterwood, Crichel Down Estate, our host Andy Poore (Consultant Forester at both Crichel Down Estate and Cranborne Estate) introduced the site and explained how the day would focus on the implications of Jean Lemaire’s work (2010; 2014) on stands of oak across the two estates. Crichel Down Estate comprises around 15,000 acres, with much of the wooded area being classified as ancient semi-natural woodland (ASNW). Beginning in the 1940s much of this ASNW, viewed as being ‘low productivity’, had been transformed to conifer/broadleaf mixtures. Good records detailing the past management of the estate’s woodlands exist, and this coupled with the current owner’s interest in the technical aspects of silviculture, has allowed well-established stands to be examined from new perspectives.

Baydon’s Copse (Cpt 13d) was our first stop of the day. Here ASNW had been clear-felled and re-planted in 1954 with oak (Quercus petraea), a smaller component of beech, and conifers in four different mixtures. The 7.3ha compartment was split into quadrants, with the following arrangement at planting (Poore & Deffee, 2016):

Northeast quadrant: NS (3:3 row, NS 6ft x 6ft; SOK 3ft x 4ft)
Southeast quadrant: EL (Matrix of EL at 5ft x 5ft; SOK in groups of 13, groups at 30ft spacing (120/ha)
Southwest quadrant: CP (4:3 row, CP 5ft x 5ft; SOK 3ft x 4ft)
Northwest quadrant: JL (Matrix of JL at 5ft x 5ft; SOK in groups of 13, groups at 30ft spacing)

In 1970 (year 16), ‘Elite’ oaks were selected throughout the four quadrants every 30ft (120/ha) and marked with paint. Oaks selected as being Elite typically displayed superior vigour, were relatively free from epicormic growth and showed reasonable form. Following selection of Elite trees, quadrants were thinned and Elite trees pruned every 3 years to 24ft (7.5m) until all coniferous components had been removed. Conifers were removed gradually, being completely removed by year 25 (SE quadrant), year 28 (NW), year 29 (SW) and year 42 (NE). The site is well suited to oak, with moderately acid soil and good nutrient availability. Following the publication of Lemaire’s work, there was interest in how stands at Crichel Down might compare with Lemaire’s growth models. Subsequently, 12 abbreviated Association Futaie Irrégulière (AFI) plots were established across all quadrants and a range of measurements taken. Evaluating the trial as a whole, and taking new data into account, a number of conclusions have been drawn from the conifer nurse trial (Poore & Deffee, 2016):

Discussion continued amongst the stems of the southeast quadrant (EL nurse), a stand which is now predominantly oak with a mean DBH of 43cm, yield class 8 (equivalent to French YC 1/2) and a basal area of 21. It is evident that the system here has worked very well, and has produced a fine stand of oak.

‘Active silviculture’, an approach proposed by Lemaire (2010; 2014), focuses on the development of large oak crowns - the aim being to maintain the diameter of the green crown close to half the total height of the tree (i.e. a crown diameter to height ratio of 1:2). The data obtained from the measurement of oak stands at Crichel Down Estate, and equivalent stands at Cranborne Estate, appear to be consistent with growth models constructed by Lemaire. One such model indicates how diameter at breast height (1.3m) and age relate to predict French yield class (YCF, which ranges from 1 (high) to 5 (low)) when employing ‘active silviculture’. Lemaire believes that it is possible, on good soil, to achieve 65cm diameter breast height with 6m of clear stem in less than 100 years, as opposed to the traditional 140-150 years. Stands at Crichel Down Estate and Cranborne Estate appear to fit Lemaire’s growth trajectories, falling between YFC 1-2, and indicating that such targets may be achievable. ‘Active silviculture’ assumes complete crown freedom is granted to each Elite oak tree, and aims for a relatively even-aged stand structure (often uniform shelterwood). Stands at Crichel Down and Cranborne Estates differ in that Elite trees have not experienced compete crown freedom (crowns have typically been closer to two thirds open), while management has aimed at achieving an irregular stand structure. This raises the question as to whether targets achieved under ‘active silviculture’ may be emulated using less extreme management prescriptions.

Comparisons were drawn between ideal stocking regimes prescribed by Lemaire, and those employed at Crichel Down and Cranborne Estates. When plotting top height (x-axis) against basal area (y-axis), stocking regimes closely follow those prescribed under ‘active silviculture’, with the BA reduced to around 15 at each thinning intervention above a stand top height of c. 24m (Poore & Deffee, 2016).

Heavy or 'free growth' thinning experiments conducted by Forest Research at Crumbland, Gwent provide useful comparative results (e.g. Kerr, 1996). These experiments looked at various approaches to the thinning of oak stands, aimed at producing mean diameters of 60cm before 100 years of age. The prescription for free growth conditions required that one half to one quarter of the crown width be maintained around each of the selected trees. When stands reached a top height of 12-13m, around 70 trees per hectare were selected as final crop trees, and the stand then thinned regularly until reaching a top height of 18m. With stands now approaching 90 years of age, stems in some areas of the trial are now in excess of 60cm DBH, although results are variable. There have been concerns that growing oak in free growth conditions may increase the proportion of sapwood – this will be an important aspect to investigate further once stems at Crichel Down and Cranborne Estates have been felled. Studies conducted by Forest Research have also highlighted the considerable cost of controlling epicormic shoots which is required when growing oak under free growth conditions. It has been suggested that free growth thinning may be more appropriate to other species such as ash, sycamore and wild cherry (Kerr, 1996).

Analysis of the potential value of oak stems grown under different conditions at Crichel Down and Cranborne Estates allows the management team to determine optimum target dimeters for felling. This analysis indicates that it is economically optimal to fell larger diameter stems in stands that are faster grown (i.e. under a system akin to ‘active silviculture’), while the optimal felling size is lower if stems are slow grown (Poore & Deffee, 2016).

Moving on from the conifer nurse trial, compartment 12a comprised P83 oak (Quercus petraea) with birch and hazel regeneration. Previously ASNW, the stand had been felled and replanted with oak in groups at 39ft x 39ft centres (72/ha), with five oaks planted in shelters in each group (at 3ft spacing). Despite sound maintenance of the stand including vegetation management, formative pruning, and pre-commercial thinning of wolf trees, stems present were much more variable in quality compared with stems in the conifer nurse trials. It was also apparent that stems within groups were leaning away from the centre of the cluster, suggesting an unsuitable initial spacing within groups.

Afternoon Session

The second part of the day was spent at Cranborne Estate, where we began by visiting Boulsbury Wood, Cpt 204. Once a stand of conifers, the compartment was clear-felled and re-planted in 1989 with oak (Quercus robur) and ash at around 700 stems per hectare (4 x 3.6m spacing). There has been prolific natural regeneration of birch throughout the stand which has increased the stocking density. In 2012, with Lemaire’s system in mind, Elite trees were selected at irregular spacing at around 70/ha, pruned and a light pre-commercial thin carried out. The stand is now marked for a further thin, which will focus on the development of Elite trees - the selection now confirmed at 65/ha. Within the next 5 years, birch will be removed, along with non-elite oak, non-hurely stick ash, and potential hurley stick ash if between two Elite oak. The establishment of permanent extractions racks at 28m intervals will help to protect natural regeneration. The site is highly suitable for the growth of oak - this stand has a yield class of 10, equivalent to a French yield class of 1. The mean crown diameter to height ratio of Elite trees here is 1:3 - higher than recommended by Lemaire. With a BA of 16, the stocking density is also slightly higher than the French system would prescribe. The top height of the stand (17m) is now approaching the height at which Lemaire would start thinning at stand level, rather than individual tree level (above 17.5m). Despite the wide initial spacing of the stand, and the presence of a significant understorey, the oak component within this compartment appears to fit Lemaire’s growth trajectory, suggesting that growth targets achieved under ‘active silviculture’ may also be achievable here.

Luckhams Mead (Cpt 22), our final stop, comprises 5.4ha of mixed broadleaves planted on open farmland in 1902. The stand, irregular in structure, contains one of the UK’s three Association Futaie Irrégulière (AFI) plots, which form part of a network of research plots throughout Europe. Measurements within the AFI plot are carried out every five years, and give a snapshot of stand structure. Based on the use of fixed plots, the sampling scheme generates data which can be used to track, for example, increment, stem distributions, volume distributions and level of regeneration. Increment data is used to establish sustainable timber yields, and to indicate the performance of the growing stock. Within this stand, 63% of the growing stock (large trees) contributed 75% of the overall increment during the last 5 years, with beech appearing to perform better than had been anticipated. On a wider scale, it is hoped that data produced from the Europe-wide network of AFI research plots may be used to better inform the management of irregular stands in the future.

Freia Bladon

References/Further Information

Association Futaie Irrégulière (AFI)

Kerr, G. (1996). The effect of heavy or 'free growth' thinning on oak (Quercus petraea and Q. robur). Forestry 69, 303–317

Lemaire, J. (2010) Le chêne autrement: Produire du chêne de qualité en moins de 100 ans. Institut pour le Développement Forestier.

Lemaire, J., translated by Howell, B. (2014) Oak: fine timber in 100 years (English version of Le Chene Autrement). Future Trees Trust.

Poore, A., & Deffee, R. (2016) Wessex Silvicultural Group visit to Crichel Down/Cranborne Estate. Supplementary notes.

Wessex Silvicultural Group: Notes from previous meetings can be found here.