A STUDY OF GROWTH RINGS IN TREES

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Abstract
The frequency of ‘lines’ of denser wood within the annual ring in larch ( Larix decidua Mill.) was compared with monthly records of rainfall and tables of droughts and rain-spells in the Bristol region. Moderate correlations were found with June rainfall (inverse), days in summer droughts, and a ratio giving a measure of the maximum dry-wet contrast between successive months from May to August. This ‘contrast ratio’ showed a high degree of parallel variation with ‘line’ frequency. A study of individual years suggested that the formation of a pronounced ‘line’ is associated with the occurrence of a spell of dry weather followed by a wet spell during the later part of the summer. ‘Mid-lines’ were found to be associated with a June drought breaking in July; late ‘lines’ with a dry-wet contrast in July or August, but in some trees these may be suppressed by the stoppage of growth. Vaguer ‘lines’ of lower frequency were associated in certain years with wet spells not preceded by noticeable drought. Individual ‘lines’ can thus be identified and roughly dated by some weather feature, so that their position sheds some light on the length of the growing season. It is suggested that ‘lines’ of denser wood may be occasioned by the killing or inactivating of mycorrhizal rootlets either by drought or by waterlogging, followed by partial recovery when the weather changes. This would accord with the theory that summer wood formation is normally associated with a developing water-deficit in the wood.

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