Effect of cleaning and thinning on height growth and girth increment in holm oak coppices (Quercus ilex L.)

Abstract

This article studies the influence of cleaning or thinning intensity and of coppice age at the time of silvicultural treatment, on the growth of shoots remaining on the stool, in Quercus ilex coppices. Two experimental designs were used to answer these questions. The first studied the effect of two cleaning intensities on 4, 8, 15, 20, and 25 year old coppices. The second studied the effect of 4 thinning intensities (26 %, 42 %, 58 %, or 78 % removal of the initial basal area) on 43 and 57 year old coppices. In both cases, control plots undergoing no silvicultural treatment were maintained for each age class. The results, 4 and 5 years after silvicultural treatment, concerned girth increment and height growth of coppice shoots. Cleaning or thinning consistently had a positive influence on girth increment. In young coppices, less than 25 years old, mean annual girth increment was approximately 6 mm in the controls and double that under heavy cleaning. In older coppices, girth increment was lower in controls (3 mm per year) but the effect of thinning was very positive and, under very heavy thinning, growth was more than 11 mm per year. Cleaning and thinning often modify height growth. This effect was found to be either positive or negative and no general trend could be defined which was valid for all the age classes. Silvicultural treatments also had a positive effect on the appearance and growth of epicormic shoots on the holm oak trunks. The results were also analysed at the stool level. The number or percentage of shoots removed had a positive effect on the individual growth of selected shoots.