{"id":817,"date":"2016-05-10T14:55:21","date_gmt":"2016-05-10T14:55:21","guid":{"rendered":"http:\/\/blogs.napier.ac.uk\/cwst\/?p=817"},"modified":"2016-05-11T17:01:17","modified_gmt":"2016-05-11T17:01:17","slug":"growth-rate-and-wood-density","status":"publish","type":"post","link":"https:\/\/blogs.napier.ac.uk\/cwst\/growth-rate-and-wood-density\/","title":{"rendered":"Growth rate and wood density"},"content":{"rendered":"

In this post I am\u00a0building upon\u00a0Dan’s earlier discussion <\/a>of the misconceptions surrounding growth rate and wood density. This post extends the discussion to include virtually all wood species.<\/p>\n

“Rate of growth” can be expressed as either\u00a0changes in ring width or the speed of growth of the tree.\u00a0\u00a0In this\u00a0blog post\u00a0we are\u00a0only\u00a0concerned with ring width.<\/p>\n

In most cases there is little or no correlation between\u00a0ring width\u00a0and wood density. The only species where a relationship might – repeat might – exist are softwoods such as spruce and hardwoods such as the deciduous oaks. Confusingly, however, the relationship is not consistent between these two groups. In the former group fast growth sometimes produces low density timber whereas the opposite relationship can occur\u00a0in the latter group.<\/p>\n

Confused? Don’t worry you are not alone. As we\u00a0shall see, many commonly held assumptions about growth rate and density are incorrect.<\/p>\n

Most tree species form their wood tissue in growth increments which are correlated with annual or seasonal climatic changes. Wood formed at the beginning of the growth period\u00a0is termed \u2018earlywood\u2019, while\u00a0wood formed later\u00a0is known as \u2018latewood\u2019. Earlywood is usually paler in colour and less dense than latewood due to differences in the type and thickness of conducting cells.<\/p>\n

In tree species that form distinct growth rings there are three fundamental patterns of earlywood and latewood within the growth ring:<\/p>\n

    \n
  1. No change in cell type across the growth ring<\/li>\n
  2. A gradual reduction of the inner diameter of conducting cells from the earlywood to the latewood<\/li>\n
  3. An abrupt and distinct change in the inner diameter of conducting cells from the earlywood to the latewood<\/li>\n<\/ol>\n

    These patterns appear in both softwood and hardwood species although there is a difference in how they are manifested due to the contrasting anatomical structures of these two botanical categories.<\/p>\n

    Consequently, wood species can be divided into six groups (three softwood and three hardwood) on the basis of their growth ring characteristics when seen in a transverse cross section. The six groups\u00a0can be illustrated as follows and\u00a0are summarized in the table below.<\/p>\n

    \"6<\/a><\/p>\n

    (From Chapter 3 of\u00a0USDA Wood handbook: Wood as an engineering material<\/a>)<\/p>\n

     <\/p>\n

    Table. Growth rate and wood density relationships<\/strong><\/p>\n

    \n

     <\/p>\n

      \n
    1. Softwood species with no visible growth rings<\/strong> (e.g. some tropical and southern temperate timbers\u00a0including\u00a0most species in the Podocarpaceae, Cupressaceae and Andcupressaceae families). In these species there is no correlation between growth rate and wood density.
      \n
      \n<\/li>\n
    2. Softwood species where the growth rings have a gradual transition from earlywood to latewood<\/strong> (e.g. the spruces and the firs). In these species growth of the denser latewood part of the growth ring is relatively\u00a0constant irrespective of climate,\u00a0whereas the earlywood part varies from year to year. This means that that a wider ring has more of the less dense earlywood and so density might decrease with increasing growth rate. The relationship is not consistent, however, as the density of a given ring width is affected by the genetic makeup and growing conditions of a tree, and can also be influenced by its stage of development. Therefore, particularly for timber of mixed origins, ring width alone is not an accurate predictor of wood density.
      \n
      \n<\/li>\n
    3. Softwood species where the growth rings have an abrupt transition from earlywood to latewood<\/strong> (e.g. the larches and the hard pines). In these species there is no correlation between growth rate and wood density.
      \n
      \n<\/li>\n
    4. Hardwood species where the growth rings have fairly evenly distributed vessels meaning that there is little difference between earlywood and latewood<\/strong>. (These species are termed \u2018diffuse porous hardwoods\u2019 and include most tropical hardwoods along with temperate species such as the birches and the evergreen oaks.) In these species there is no correlation between growth rate and wood density.
      \n
      \n<\/li>\n
    5. Hardwood species where the growth ring characteristics are intermediate between ring porous and diffuse porous<\/strong>. (These species are termed \u2018semi-diffuse porous hardwoods\u2019 and include European walnut.) In these species there is little correlation between growth rate and wood density.
      \n
      \n<\/li>\n
    6. Hardwood species where the growth rings have earlywood cells of greater diameter than those formed later in the growing season<\/strong>. (These species are termed \u2018ring porous hardwoods\u2019; they\u00a0are always deciduous and include European oak and European ash.) In these species, the width of the low density earlywood part of the growth ring\u00a0fairly consistent, irrespective of the growing conditions, whereas the width of the denser latewood varies from year to year. This means that fast growth tends to produce a higher proportion of high density latewood. Within limits, fast growth produces relatively dense, strong timber and vice versa.<\/li>\n<\/ol>\n
      \n

       <\/p>\n

      Not all timber species neatly fit into the six\u00a0categories listed above. Teak and hickory, for example, can be either semi-diffuse porous or ring porous depending upon the growth conditions.<\/p>\n

      If you want to find out more about these relationships, have a look at the following textbooks which should be stocked in most university wood science libraries:<\/p>\n