Wood drying (not to be confused with "seasoning") may be described as
the art of ensuring that gross dimensional changes through shrinkage
are confined to the drying process. Ideally, wood is dried to that
equilibrium moisture content as will later (in service) be attained by
the wood. Thus, further dimensional change will be kept to a minimum.
It
is probably impossible to completely eliminate movement in wood, but
this may be approximated by chemical modification. This is the
treatment of wood with chemicals to replace the hydroxyl groups with
other hydrophobic functional groups of modifying agents (Stamm, 1964).
Among all the existing processes, wood modification with acetic
anhydride has considerable promise due to the high anti-shrink or
anti-swell efficiency (ASE) attainable without damaging the wood
properties. However, acetylation of wood has been slow to be
commercialised due to the cost, corrosion and the entrapment of the
acetic acid in wood. There is extensive literature relating to the
chemical modification of wood (Rowell, 1983, 1991; Kumar, 1994; Haque,
1997).
Drying timber is one approach for adding value to sawn
products from the primary wood processing industries. According to the
Australian Forest and Wood Products Research and Development
Corporation (FWPRDC), green sawn hardwood, which is sold at about $350
per cubic metre or less, increases in value to $2,000 per cubic metre
or more with drying and processing. However, currently-used
conventional drying processes often result in significant quality
problems from cracks, both externally and internally, reducing the
value of the product. As an example, in Queensland alone (Anon, 1997),
assuming that 10% of the dried softwood is devalued by $200 per cubic
metre because of drying defects, sawmillers are losing about $5 million
per year in that State alone. Australia wide this could be $40 million
per year for softwood and an equal or higher amount for hardwood. Thus
proper drying under controlled conditions (prior to use) is of great
importance in timber utilisation in any country, where climatic
conditions vary considerably at different times of the year.
Drying,
if carried out promptly after the felling of trees, also protects
timber against primary decay, fungal stain and attack by certain kinds
of insects. Organisms, which cause decay and stain, generally cannot
thrive in timber with a moisture content below 20%. Several, though not
all, insect pests can live only in green timber. Dried wood is less
susceptible to decay than green wood (above 20% moisture content).
Apart
from the above important advantages of drying timber, the following
points are also significant (Walker et al., 1993; Desch and Dinwoodie,
1996):
Dried timber is lighter, and hence the transportation and handling costs are reduced. Dried timber is stronger than green timber in most strength properties. Timbers
for impregnation with preservatives have to be properly dried if proper
penetration is to be accomplished, particularly in the case of oil-type
preservatives. In the field of chemical modification of wood and
wood products, the material should be dried to a certain moisture
content for the appropriate reactions to occur. Dry wood works, machines, finishes and glues better than green timber. Paints and finishes last longer on dry timber. The electrical and thermal insulation properties of wood are improved by drying. Prompt
drying of wood immediately after felling therefore results in
significant upgrading of, and value adding to, the raw timber. Drying
enables substantial long term economy in timber utilisation by
rationalising the utilisation of timber resources. The drying of wood
is thus an area for research and development, which concerns many
researchers and timber companies around the world.
Originally used to model the evolution of the first macromolecules on earth , the quasispecies concept has been applied to populations of a virus within its host [1]. The quasispecies model is deemed to be relevant to RNA viruses because they have high mutation rates
in the order of one per round of replication [2], and viral
populations, while not infinite, are extremely large. Thus the
practical conditions for quasispecies formation are thought to exist.
The significance of the quasispecies model for virology is that, if the mutation rate is sufficiently high, selection
acts on clouds of mutants rather than individual sequences [3].
Therefore, the evolutionary trajectory of the viral infection cannot be
predicted solely from the characteristics of the fittest sequence.
The importance of quasispecies concepts in virology has been the
subject of some discussion [4][5]. Significantly, it has been shown
that there is no necessary conflict between a quasispecies model of intra-host evolution and traditional population genetics [3]. Instead, viral quasispecies can be considered as cases of coupled mutation-selection balance models for haploid organisms.
It may be useful to understand the etymology of the term. Quasispecies are clouds of related elements that behave almost (quasi) like a single type of molecule (species). There is no suggestion that a viral quasispecies resembles a traditional biological species.
This may be defined as “The inability of a genetic element to be maintained in a population as the fidelity of its replication machinery decreases beyond a certain threshold value” [6].
In theory, if the mutation rate was sufficiently high, the viral
population would not be able to maintain the genotype with the highest
fitness, and therefore the ability of the population to adapt to its
environment would be compromised. A practical application of this
dynamic is in antiviral drugs employing lethal mutagenesis. For
example, increased doses of the mutagen Ribavirin reduces the
infectivity of Poliovirus [7].
However, these models assume that only the mutations that occur in the fittest sequence are deleterious,
and furthermore that they are non-lethal (fig. 1). It has been argued
that, if we take into account the deleterious effect of mutations on
the population of variants and the fact that that many mutations are
lethal (fig. 2), then the Error Threshold disappears, i.e. the fittest
sequence always maintains itself [3] [6] [8]. Empirical data on the
effect of mutations in viruses is rare, but appears to correspond more
closely to fig. 2 [9].
Mutational Robustness.
The long-term evolution of the virus may be influenced in that it may be a better ESS (Evolutionarily Stable Strategy) to generate a broad quasispecies with members of approximately equal fitness than to have a sharply defined 'most fit' single genotype
(with mutational neighbours substantially less fit). This has been
called 'survival of the flattest' - referring to the fitness profiles
of the two strategies respectively
Over the long-term, a flatter fitness profile might better allow a quasispecies to exploit changes in selection pressure, analogous to the way sexual organisms use recombination
to preserve diversity in a population. At least in simulations, a
slower replicator can be shown to be able to outcompete a faster one in
cases where it is more robust and the mutation rate is high
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