Regardless of how accurate 'predictive' science is, there will be some
outcome whether it be climatic or biological.

Models are 'representations of a process' [Fowler, In: Brady and Whysong,
1999]. Other definitions include those of Jeffers [1988] who has that a
model is the 'formal expression of the essential elements of some problem in
either physical or mathematical terms.'

One model which I have used has been JABOWA which represents how stands of
trees grow over time. This model is a 'dynamic' gap model. It is not capable
of modelling song bird populations in reponse to stand age and structure.
Forest stand dynamics is often complex and thus 'models...are not general
representations of a system' and they cannot address more than 'a rather
limited...set of questions.' [Brady and Whysong, 1999].

One important model that is used to represent a large forest for commercial
uses is the model which is used to determine 'timber supply' in British
Columbia. Since the forest in BC is 95% owned by the public, the task
therefore is to set a timber supply which is sustainable over a long period
of time. Unlike many forest dependent regions, BC has a planned natural
resource based economy. Many communities depend on a 'sustained yield'
of timber to provide an economic base, and it would be foolish and
improvident to cut all the timber at once or over a short period of time.

The rationale for timber supply modelling is derived from the concept of
sustained yield. Sustained yield could be 'non-declining' or 'declining'.
In the case of most BC forest dependent communities the sustained
yield is expected to 'decline' and in many cases it already has declined.
The timber supply where it has declined or is expected to decline is
determined based on several assumptions which
arise from 'wood science' and growth and yield predictions.

The main reason why there is an expected or existing 'fall down' in the
timber supply is due to the belief that old forests are 'decaying' and
therefore not growing wood. This is a simplification of reality because the
wood in old forests varies in quality and in species. Some forests which are
old are composed of very old trees that are often decaying. There are other
forests which are also old but not in a state of decay.

The belief that the trees in a forest each reach their maximum rate of
growth sometime between the ages of 72 and 140 years is also one assumption
that affects the 'rotation age' [the age at which forests are cut for
commercial reasons]. A Douglas-fir growing on a good site in the interior
will reach the 'culmination age of the mean annual increment' at about 72
years. A yellow cedar or lodgepole pine growing on a submesic to subxeric
site will perhaps reach culmination age at about 140 or so.

To model a forest on the belief that there is a 'maximum sustained yield'
therefore is to assume that there are several working techniques in which
trees and species can contribute in a 'maximally' dependent fashion. To
maximize yield would mean setting the harvest date to the culmination age,
the age at which trees on a given site will reach maximum growth. For each
site the species which contributes the maximum volume would have to be
established. Whole forests of commercially valuable trees would have to be
established such as Douglas-fir on the good temperate sites, lodgepole on
the medium mid-elevation sites and spruce on the mid-elevation and high
elevation good sites.

In general the strategy to replace native forests possessing up to a dozen
species of trees with a few species is expected to result in a maximum
yield. The trees are established at optimum spacing to ensure that maximum
commercial volumes are obtained.

To meet other 'non-commercial' uses of the forest several other maximizing
assumptions are included in the AAC [annual allowable cut] such as 'visual
green-up', 'hydrological green-up' and 'silvicultural green up'. These
assumptions are termed 'constraints' by the commercial forest users since
green-up limits the size of the clearcut. These 'constraints limit to some
extent the capacity of the forest industry in maximizing yield perhaps as
much as 2 to 5 % depending on the forest, whether it is in a community
watershed, a viewscape, or slow growing alpine forest.

Other constraints that are modelled into the 'timber supply model' are
forest health factors. These factors are 'stand level' factors rather than
forest level factors. To obtain the best results forest scientists are
attempting to culture genetic strains of tree species that are resistant to
various forest health factors such as white pine blister rusts, et cetera.

Only certified seed is used for the establishment of some tree species such
as spruce. All spruce that is used in plantations has to be certified and
obtained from selected phenotypes that are tested in seed orchards. The
assumption based on short term height growth trials is that these certified
seed sources will increase yield by as much as 10%. In the US national
forests, the assumption is that the new forests will grow 10% more wood than
the same forest that nature established to to the use of  'plus tree'
selection.

While some practices are meant to counteract 'constraints' perhaps the most
constraining practice is to conserve biodiversity. In BC the assumption that
is used to model timber supply is that up to 4% of the commercial yield of
timber can be impacted by measures to conserve biodiversity. Contrary to the
other constraints, and opportunities, there is no scientific 'rationale' for
setting the limit to 4%.

Other constraints are protected areas which were limited to no more than 12%
of the province [now met in 2001], identified wildlife [no more than a 1 %
constraint].

Perhaps the most significant opportunity that the modellers have is to
encorporate the results of tree growth models from managed forests. The
method results in some increases to the timber supply that are significantly
greater than existing 'un-managed' growth and yield predictions. The method
simply works by measuring trees as young as 3 years-old. You take the
heights and ages at breast height and then look up the 'site index' in a
table for the species and plug this into the 'inventory labels'. This is
work that I have done...many times over the last decade. In almost all cases
the Site Index is much greater than the Site Index based on ecological
correlations. The method assumes that I have taken the 'largest diameter'
tree in a 5.69 meter radius plot.

Well the assumptions may be biased or unbiased, but the effect of the
assumptions will have a determined effect on forest dependent communities
for many decades and even centuries.


The question is should there be some built into this timber supply model
some level of uncertainty? Are the assumptions regarding the Site Indices
only capable of prediction in young, immature stands?

Moreover maximum sustained yield appears to be a 'oxymoronic' statement. How
can yield be maximized and sustained? I had always observed 'variations' and
'fluctuations' in yield whether it be berries, or powder snow.

still chuckling

john









----- Original Message -----
From: Steven Bissell <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, July 24, 2001 6:29 AM
Subject: climate change and models


> Here is an article about NASA's new super computer. It suggest that issues
> about climate change will be resolved through the use of models. I post
this
> to raise the EE issue of the use of models in environmental science. I've
> always been dubious about the use of models by ecologists in place of
actual
> evidence. I think that the use of models in ecology is similar to the use
of
> correlation in economics. Both are substitutes for "real" science in the
> sense that they rely on a certain type of faith. Anyway, things have been
> quite, so I thought I'd stir the pot.
>
>
http://www.enn.com/news/enn-stories/2001/07/07232001/supercomputer_44384.asp
> ?site=email
>
> Steven
>
>  "Our human ecology is that of a rare species of mammal in a social,
> omnivorous niche. Our demography is one of a slow-breeding, large,
> intelligent primate. To shatter our population structure, to become
abundant
> in the way of rodents, not only destroys our ecological relations with the
> rest of nature, it sets the stage for our mass insanity."
>         Paul Shepard