Are 'steigen' and 'steigern' alternative spellings, or is there some difference in meaning?
An aspect not yet mentioned: *steigern" (increase somthing) is transitive, so you necessarily have to specify, what to increase. *Steigen" as in "Die Benzinpreise steigen" (the fuel prices increase) is perfectly happy without additional specification.
They are two totally separate verbs. Steigen means to rise, climb, ascend, soar, etc. whereas steigern means to raise, boost, augment etc.
1. steigen (Verb) The word "steigen" is often used to describe a physical activity for climbing up somewhere.
For example: Den Berg besteigen.
However, there are several other possible usages for different scenarios.
2. steigern (Verb) The most common usage for this Verb is in the sense of increasing or enhancing something. For example: Den Umsatz eines Unternehmens steigern. or Der Sportler steigert das Tempo.
There are other possible usages.
I hope this helps. :)
No, they're not equal in meaning. Off the top of the head I cannot come up with an example where you can replace one for the other without changing the meaning. I'm sure there are some examples, though, since the two words have a tiny overlap. Sometimes you get a similar meaning with just a subtle difference:
Die Gehälter steigern: the act of increasing the salaries
Die Gehälter steigen: the fact that salaries increase
Steigern in general mean: to improve, to get better/more/higher.
Steigen, in contrast, mean: to climb, to get on a higher level.
So, the only overlap is "to get higher" which is covered by the "salary"-example.