Frequently Asked Questions (FAQ)

Q:
Are these questions really frequently asked?
A:
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.

Q:
Scube?
A:
Like S3, from the S's in "Stand growth model for Spruce in the SBS". Yes, no imagination.

Q:
What happened to the older version?
A:
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.

Q:
How accurate is this anyway?
A:
Not 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. On 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.

Q:
I don't care about breast-height, I want total age.
A:
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 et al (1994). Short answer: add  7.7 + 111 / Site  to 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  for 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 relevant.

Q:
Can I simulate changes in species composition?
A:
Yes. The spruce% (cell D6) can be changed at any time. But just doing that, everything would be recalculated, so first we need to "freeze" the values for the previous years. Select the relevant rows on the left margin, and copy (ctrl-C, or right-click and choose Copy, or Edit>Copy). Now, before touching anything else, go Edit>Paste Special, choose Values, and click OK, and Esc. That replaces formulas with values, that will not change afterward.
Another way: substitute a function of age in D6.

Q:
What about climate change?
A:
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.

Q:
Why bother?
A:
Good question.