Abstract: | The frequency of lines of denser wood within theannual ring in larch (Larix decidua Mill.) was compared withmonthly records of rainfall and tables of droughts and rain-spellsin the Bristol region. Moderate correlations were found withJune rainfall (inverse), days in summer droughts, and a ratiogiving a measure of the maximum dry-wet contrast between successivemonths from May to August. This contrast ratioshowed a high degree of parallel variation with linefrequency. A study of individual years suggested that the formationof a pronounced line is associated with the occurrenceof a spell of dry weather followed by a wet spell during thelater part of the summer. Mid-lines were foundto be associated with a June drought breaking in July; latelines with a dry-wet contrast in July or August,but in some trees these may be suppressed by the stoppage ofgrowth. Vaguer lines of lower frequency were associatedin certain years with wet spells not preceded by noticeabledrought. Individual lines can thus be identifiedand roughly dated by some weather feature, so that their positionsheds some light on the length of the growing season. It issuggested that lines of denser wood may be occasionedby the killing or inactivating of mycorrhizal rootlets eitherby drought or by waterlogging, followed by partial recoverywhen the weather changes. This would accord with the theorythat summer wood formation is normally associated with a developingwater-deficit in the wood. |