The effect of heavy or ‘free growth’ thinning on oak (Quercus petraea and Q. robur)
Free growth is a type of heavy thinning which aims to maximize diameter increment and produce valuable timber on a relatively short rotation. An experiment is described which investigated the application of free growth to a stand of oak Quercus petraea and Q. robur) planted in 1930. At age 58 free growth thinning had resulted in a mean diameter at breast height of 39.0 cm with an estimated mean tree volume of 0.98 m3, compared with 29.3 cm and 0.52 m3 for equivalent crown thinned trees. Discounted cash flow calculations showed that free growth thinning of oak could be justified using a 3 per cent discount rate assuming that the increased intensity of pruning results in a large proportion of veneer quality timber; an independent assessment indicated this may be possible. Free growth thinning of oak is not a common practice in British broadleaved silviculture probably because of the cost of controlling epicormic shoots. It is suggested that it may be more appropriate to other species such as ash, sycamore and wild cherry.