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Formative shaping of broadleaves

Broadleaved species may deteriorate in quality when planted out in open fields due to late spring frosts, exposure to cold, desiccating winds and insects and diseases. Studies on the growth of broadleaf tree species at the Teagasc Research Centre in Kinsealy found that the rate of this deterioration in quality was so great that remedial action was necessary.

Implications

  • There is a considerable decline in stem quality due to the effects of climate, disease or inherent defects in the tree in the years following establishment. Formative shaping is essential to maintain a sufficient number of quality trees from which to select a final crop. Formative shaping will ensure a small defect core thereby allowing smaller diameter thinnings to be used and maximising the amount of defect-free timber in the final crop.
  • Oaks tend to bush and develop a twiggy habit in the initial years and it is four or five years before they develop discrete leading shoots. Therefore shaping is best delayed until this time. The precise timing is still unclear and requires further research. Ash and sycamore are two of the most important species in our current planting programme. Both benefit significantly from formative shaping and many Quality Category 3 and 4 trees can be transformed into Quality Category 1 and 2 trees.
  • Apart from oak, formative shaping should begin when the trees are in the second year after planting out, or between 0.8 to 1.5 metres in height, when the shoots are young and relatively plastic. If formative shaping is delayed and defects become lignified they will not straighten following later pruning. In most cases formative shaping will need to be done a number of times. A second shaping should be done not later than the start of the third growing season after the first shaping to ensure 3 metres of straight stem in the crop.
  • It is not necessary to shape all trees in a crop. After first shaping there should be at least 60% of the stems in Quality Categories 1 and 2. Following second shaping at least 50% of the stems should be in Quality Categories 1 and 2.
  • Ash, cherry and sycamore are relatively fast growing. Early shaping is particularly important in these species. Shaping increases the height of trees in Quality Categories 1 and 2.
  • Removing large side branches or clusters of co-dominant shoots improves tree form but can have a negative effect on diameter growth. However if Quality Categories 1 and 2 only are examined in sycamore, formative shaping has a positive effect on diameter growth as all growth is concentrated on a single stem. Even where diameter growth is slightly reduced, when offset against the improvement in form and quality and the reduction in the defect core of the tree, the overall value of the crop is improved.

Background

With limited information available on pruning and shaping young broadleaf trees a trial was initiated at Kinsealy to assist in the development of a programme to promote leader growth and improve stem form. A number of species were assessed, including ash, beech, cherry, pedunculate oak, sessile oak, sweet chestnut, sycamore and walnut, to establish if formative shaping could halt or control the decline of tree stem quality by ensuring the presence of an apically dominant leading shoot.

A quality ranking system was devised to assess tree form and, in particular, stem quality. The 5 point ranking scale ranged from category 1, very good quality, to category 5, very poor quality.
Formative shaping involved removing forks, dead leaders, co-dominant shoots and disproportionately large side branches. In the case of forks, the weaker, least straight or poorer quality shoots were removed. Small side branches were not removed. Shaping began when the trees were in their second year after planting, measuring between 0.8 and 1.0 metres in height and continued, as necessary, on an annual basis. The aim was to produce a straight stem of at least 3 metres using simple tools and the least time and physical effort.

Shaping was carried out in June/July in one half of each row in the plantation. The other half row remained unshaped as a control. Branches were removed using good quality (bypass) secateurs and loppers. Care was taken not to damage the branch collar or the branch bark ridge. These are ridges that develop where the branch meets the main stem of the tree. Shaping cuts were made just outside the branch collar. Each year tree height and stem diameter (at 20 cm above ground level) were measured and quality category was assessed for each tree.

Findings

  • During the first growing season there was a dramatic overall decline in stem quality. After seven growing seasons approximately 47% of the unshaped trees were in Quality Categories 4 and 5 while 60% of the shaped trees were in Quality Categories 1 and 2. After four years, formative shaping had a significant effect on the quality of every species except sessile oak. However in later years there was also a positive effect on sessile oak.
  • There was a significant increase in the mean height of shaped ash, sweet chestnut and sycamore after four years. The growth was concentrated in one leader while in unshaped trees the energy was dissipated over a number of competing forks or co-dominants. Overall, apart from oak, formative shaping had a gradual cumulative positive effect on tree height over the seven-year period. Ash, cherry and sycamore had the greatest height growth, in that order.
  • Formative shaping had a small, but significant negative effect on the diameter at 20 cm above ground level after four years of ash, cherry, sycamore and walnut. However when measured at 1.3 metres above ground level after seven years growth, with the exception of sweet chestnut and sessile oak, trees in the shaped treatment had greater diameters. This is because many of the unshaped trees were forked below 1.3 metres and only the strongest of the leading shoots were measured.

Based on End of Project Report:

Studies in the Planning, Location and Management of Farm Forestry: Formative Shaping of Broadleaves: Armis No. 4125.  Authors: M. Bulfin and T. Radford, Kinsealy Research Centre. Compiled by: Nuala Ni Fhlatharta, Teagasc, Athenry, Co. Galway; Tel. 091 845200