Frequently Asked Questions
- Are these questions really frequently asked?
- Of course not. As in most
FAQ, the majority are rhetorical. They address bugs, er...,
undocumented features, or details that the author has been too lazy to
spell-out in the main documentation.
- Like S3, from the S's in "Stand
growth model for Spruce in the SBS". Yes, no imagination.
- What happened to the older version?
- There have been three main versions of Scube (four or five,
counting stillborns). All shared the site index and height growth
sub-model developed by Zhengjun Hu. A first attempt at the mortality
and basal area part is in Zheng's thesis. The relationships
were changed for the second version, presented at a G&Y
Meeting in Prince George in 2005, and previously available
here as Scube 0.9 to 0.93 (beta). Predictions were close
to those of the current model within the range of the data, but
sometimes had an ugly "kink" when extrapolated to old ages, and kept
going up for ever. Scube 2 changed that to something more
plausible, and uses some new equations that are more esthetically
pleasing (to me).
Update: Scube 2.1 simplified some of the relationships of Scube 2.0, with little effect on the predictions.
- How accurate is this anyway?
very (although better than what we had). For many stand conditions we
do not have any data, and
there these are best (?) guesses, based on biology and extrapolation.
Where there is data, natural variability is high.
average, for a prediction from bare land your favourite sample plot
will be off by about 2% in height, 7% in number of trees, 9% in basal
area, and 10% in total volume (for breast-height ages above 25 years, Table 5 in the publication) .
Provided it is not hit by pests, storms, or other calamities;
those risks must be considered separately. Hopefully errors
cancel out, and predictions are better at the stand or forest level.
- I don't care about breast-height, I want total age.
- Years to breast height (YTBH) vary widely, and in planted
stands are sensitive to site preparation, nursery and planting
techniques, vegetation control, etc. In Site Tools,
MoF recommends Equation (11b) of Goudie
(1984) to estimate YTBH from germination for stands of natural
origin, apparently reducing that by 4 years for planted stands, also
age from seed. There are some figures also in Table
5 of Coates
(1994). Short answer: add 7.7 + 111 / Site
breast-height age for age from seed in natural stands.
You might want to add the years from disturbance to
germination, somewhere in Coates
et al there is some info on that, I think. For
plantations, add 1.7 + 111 / Site
age from planting, with two-year planting stock. See the carbon
example. By the way, age is not used in the model (other than
to estimate site index from age and height), only time differences are
- Can I simulate changes in species composition?
- Yes. The spruce% (cell D6) can be changed at
time. But just doing that, everything would be recalculated,
first we need to "freeze" the values for the previous years.
Select the relevant rows on the left margin, and copy (ctrl-C,
right-click and choose Copy,
Now, before touching anything else, go Edit>Paste
and click OK,
That replaces formulas with values, that
will not change afterward.
Another way: substitute a function
of age in D6.
- What about climate change?
- One could use the tricks of the previous answer to change
site index over time (once scientists make up their minds about if
growth will increase or decrease, and by how much). But it is probably
better to handle it through distorting the time scale (left as an
exercise). Same for fertilizing.
- Why bother?
- Good question.