The growth potential of downy birch (Betula pubescens (Ehrh.)) in Ireland

Abstract

Birch in Ireland has long been regarded a timber species of minor importance, and it is noted more often for its invasiveness in young coniferous forests and clearfelled areas than for its potential as a commercial forestry species. While research on other native hardwood species such as oak or ash has been ongoing for a number of years, very little is known about the Irish birch resource. The lack of quantitative data relating to height growth, diameter growth and volume increment of birch in this country is of particular concern. The objective of this study was to examine the growth potential of birch in Ireland. Following a field survey, eight well‐stocked, unthinned birch stands were selected for inclusion in the study. All of the selected stands were determined to be downy birch. Following analysis of sample tree disc sections, two stands were excluded (because of indistinct annual rings) and the study was restricted to the six remaining stands. A total of 100 sample trees were felled at the six sites. Tree ring data were collected from a total of 1333 sample tree disc sections. Using these ring data, the historic patterns of radial growth at breast height, height growth and volume growth of the six stands were reconstructed, examined and analysed in detail. The results showed that for well‐stocked, unthinned, even‐aged stands the period of maximum radial growth, and therefore diameter growth, occurred between the ages of 5 and 20 years. The fastest growing tree achieved a diameter of 25 cm in 32 years. It is suggested that for the stands included in this study, the lack of management, in particular the lack of adequate thinning, will have resulted in excessive crown competition and consequently reduced diameter growth. Maximum height increment occurred before the age of 20 years and fast growing trees achieved a height growth of > 1 m per year during this period. The results showed that a well‐stocked, unthinned downy birch stand can achieve a standing volume (under‐bark) of 200 m 3 ha −1 in 42 years. While some of the stands included in the study had not reached the age of maximum mean annual increment, comparison with the Forestry Commission yield models showed that stands of downy birch in Ireland can achieve a yield class of 8 and, given the correct thinning regime, total recovered volume production could possibly be raised to that equivalent with yield class 10.