In AD&D, many spells have effects with areas that are a specified amount “per level”. From my understanding, this is the caster’s level. My question, is whether or not a spellcaster could choose to cast a weaker spell, to have a smaller area for example (and limit the destruction)?
Wild mages could cast a different caster level, but could not choose.
tl;dr Wild mages were the closest published mechanic that could cast spells at a level different than their caster level, but it was a random effect.
In the Tome of Magic one of the features of wild mages is "Level Variation":
...affects all spells that have a level variable for range, duration, area of effect, or damage. Each time a wild mage uses a spell with a level variable, they randomly determine the resulting casting level of the spell. The spell may function at a lesser, equal, or greater effect than normal.
In practice this involved rolling a d20 to do determine a modifier to apply to the mage's true level and end up with the effective spell level for that casting. The range was +/- 5 levels of the caster's true level, and also involved a chance for a different mechanic, wild surge, as well.
If we read the spell descriptions, we can see that the wording for some of them specifically tells us that the caster level dependent features are upper limits. For example:
Mount: The type of mount gained by this spell depends on the level of the caster; of course, a caster can choose a lesser mount if desired.
Dimension Door: the wizard instantly transfers himself up to 30 yards distance per level of experience
Minor Creation: The volume of the item created cannot exceed one cubic foot per level of the spellcaster.
Moreover, if we read the Spell Descriptions section of Appendix 2, we read:
Duration: ... Some spells have a variable duration. The caster cannot choose the duration of spells, in most cases.
These can be considered circumstantial evidence that the caster cannot downcast unless a specific exception is written in a given spell's description.
Not all spells are written with the same care, particularly those in various campaign expansions can be quite problematic. For example, in our own gaming table, we had serious discussions about the Elves of Evermeet - it was a nice flavourful book, but introduced many mechanics that we found to be unbalanced. So in general, you should really discuss with your DM. For handling spells from such sourcebooks as a DM, I strongly recommend that you look for a similar spell in Player's Handbook and adjudicate based on that. (I would definitely rule that that the Construction spell should work similar to Minor Creation, allowing smaller structures to be built.)
Yes. In AD&D, including 2e, the general guideline is that unless something was forbidden it was allowed. When adjudicating the fine points of the rules, the writers assumed that the common sense of their readers would fill in the details, even if that common sense might vary from reader to reader.
At conventions, in letters, and over the phone I’m often asked for the instant answer to a fine point of the game rules. More often than not, I come back with a question—what do you feel is right? And the people asking the questions discover that not only can they create an answer, but that their answer is as good as anyone else’s. The rules are only guidelines.—David “Zeb” Cook, Foreword, AD&D2e Dungeon Master’s Guide.
What do you “feel” is right? Does it make more sense that a caster can cast at lower levels of power, or more sense that as the caster becomes more adept at spell-casting they are locked into higher levels of power?
For me, the first answer feels right, but I can understand that others might feel differently.
There is textual evidence for the notion that casters can adjust the level of their spellcasting. For example, the wizard spell “Mount” on page 136. The text says that “of course, a caster can choose a lesser mount if desired”. The use of “of course” doesn’t make sense if the ability to use a spell at a reduced power is limited only to this spell. If only certain spells were able to be cast at a lower level, then it wouldn’t be an “of course” that this is one of those spells.
“Pass Without Trace”, on page 201, has a similar note, that “of course, intelligent tracking techniques, such as using a spiral search pattern, can result in the trackers picking up the trail at a point where the spell has worn off.”
If that note were not in the spell description, the fact that searchers can pick up the trail where the spell is not in effect would still be a fact. “Of course” is used to preface an example that is true even if the “of course” went unmentioned.
Similarly, from the opposite end, the Priest spell “Speak With Dead”, on page 214, mentions that “Of course, the priest must be able to converse in the language that the dead creature once used.” The phrase is again used to note something obvious, that applies to any such spell. If the spell gave the Priest the ability to converse in unknown languages, that would be an effect of the spell. It’s not mentioned as an effect, so “of course” the spell does not confer that ability.
The description of how spell scrolls are created, on page 145 of the Dungeon Master’s Guide, also indicates that spells can be cast at lower levels.
All scroll spells are written to make use as quick and easy as possible for the writer. The level of the spell and its characteristics (range, duration, area of effect, etc.) are typically one level higher than that required to cast the spell, but never below 6th level of experience.
Thus, a 6th-level wizard spell is written at 13th level of ability, a 7th-level spell at 15th level, etc. The DM can make scroll spells more powerful by increasing the level at which they are written.
The wizard must be able to cast the spell before creating a scroll for that spell (page 85). And the wizard is specifically using their spell book to create the spell (page 86).
Thus, it is a given in the game that wizards can use their spells at lower levels at least when creating scrolls from those spells.
The description of how memorization works, on page 81, could be used to argue that if a spell is to be cast at a lower level, it must be memorized with that use in mind.
These patterns are very complicated and alien to normal thought, so they don’t register in the mind as normal learning. To shape these patterns, the wizard must spend time memorizing the spell, twisting his thoughts and recasting the energy patterns each time to account for subtle changes—planetary motions, seasons, time of day, and more.
The “and more” could be argued to include the level the spell is to be cast at.
The note about duration on page 129 of the Dungeon Master’s Guide (the same text is on page 87 of the Player’s Handbook) might seem to be a counterfactual, but if you read the full note, it’s really contrasting spells with a duration against spells that can be ended at will.
Notice that there is no similar statement in the “Area of effect” description on the same page. This description is clearly more about the inability to use cubes smaller than 10 feet on a side than about the actual level of casting.
It can certainly be inferred, if wished, from the discussion about the 12 ten-foot cubes that a 12th level caster has to account for, but the constant mention of what an nth-level caster can do can also be taken as a term of art to make the text easier to understand. Otherwise the examples would all be of the form “a 12th level caster casting at the 12th level of effect”.
It comes down to, what feels right for your game? That’s the correct answer.