To Douglas Marshall and others,
> I have a general question for the group regarding tree growth and
>density. At our school (Auburn Univ., Alabama, USA) there is an argument
>regarding tree density and whether or not it is significantly affected by
>tree growth.
This is an old question and I believe the answer is different for different
species, hence some confusion and some debate. As I understand it, the
reason why species differ is that some conifers have very pronounced
differences between their late wood and early bands. Others such as radiata
pine, the species I know best,have a more gradual gradation between the two
so that although the two types of wood exist, the boundary is not a sharp
one.
For species where the bands are distinct, factors which cause an increase
in growth such as thinning or fertilising largely cause that increase in
the early growth or lower desity band. This alters the ratio between lower
density early wood and higher density latewood and thus alters mean density
-- downwards. For species like radiata pine, silviculture which causes an
increase in growth can cause an increase in both late and early wood bands
and thus the effect is less and is hard to detect. Analysis of experiments
where growth has been increased by thinning or fertilisation has however
found small, temporary, decreases in wood density. To the best of my
knowledge Douglas fir is a species with pronounced early/late wood
boundaries and for which an acceleration in growth is liable to result in a
reduction in density. (However I claim no great expertise in this species,
and I would be interested in comment from those who know more.)
The situation is complicated by the fact that density does vary in a
pronounced way according to the physiological age of the wood as it is laid
down. Young and therefore less dense wood is usually laid down in a period
of faster growth and it therefore appears that growth rate is the major
influence when it is not.
For radiata pine a good reference is "Properties and uses of NZ Radiata
Pine, Volume one, Wood Properties, 1991: J. A. Kininmonth and L. J.
Whitehouse, published by the NZ Ministry of Forestry, NZ Forest Research
Institute.
Some quotes from this book may be of interest (pp 6-11 and on). "Density
variations within individual growth layers of radiata pine are small in
comparison with ..... Pinus elliotii, Pinus taeda and Pinus caribaea and
very small in comparison with ..... Douglas fir. Of all the factors
causing variability in wood properties of radiata pine ...... the age of
the cambium at the time each annual growth layer is formed (is the most
important). It is possible to analyse the relationship between wood density
and growth within defined increments and to make comparisons between trees
from the same site. Cown and McConchie performed 168 such analyses of which
41 proved to be statistically significant at the 5% level. ...There is an
overall tendancy towards an inverse relationship between ring width and
wood density. ..... rapid growth per se cannot be used as an indicator of
low wood density"
Ryde James
___________________________________________________________________________
Ryde James | Forestry/Science
Tel: (61+6)249-4330 | Australian National University
Fax: (61+6)249-0746 | Canberra, ACT 0200
|
___________________________________________________________________________
email: Ryde.james@anu.edu.au
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