© 1965 by Institute of Chartered Foresters
The Summerwood: Springwood Ratio in Conifers
How it arises, varies, and affects end use for timber, board, and paper
Forestry Commission
A theoretical discussion of variation in summerwood and springwood contents among conifers. Three methods of measuring these, by cross-sectional area of logs, by dryweight fibre percentage, and by nominal specific gravity, are compared.
High-summerwood tissues have a nominal specific gravity of 0·45 or over; low-summerwood tissues have a specific gravity below 0·45. All conifers have a low summerwood content in their juvenile wood or core wood formed towards their stem-tips. In Pinus, Larix, and Pseudotsuga the summerwood percentage increases markedly with age, but only in the outer layers towards the butt; in Picea, Abies, and Tsuga similar increases occur, but high-summerwood tissues are seldom found. Within any species, and for any particular growth rate, low-summerwood tissues are characteristic of regions of high relative humidity where summer water deficits rarely occur, such as high altitudes, far northern latitudes, and the maritime climate of western Britain. High-summerwood tissues develop where summer temperatures are high, with associated strong sunshine, low relative humidity, and frequent summer water deficits.
High-summerwood tissues have greater strength in compression and bending, relative to volume; but low-summerwood tissues have greater strength relative to their weight. Low-summerwood material has proved satisfactory for most structural purposes, and for the manufacture of several kinds of artificial board. Its technical properties for the making of many types of paper are excellent, provided high resistance to tearing is not required. Likely summerwood content should guide planting programmes and marketing policies.