Learn Spanish (Latin America): Cuando hace mucho calor, la escasez afecta el bosque. - When it is very hot, the shortage affects the forest.

While cuando (when) shows a time relationship, this sentence clearly implies that the heat causes the problem. To use a formal causal connector with the same verb phrase, you could say: Debido a que hace mucho calor, la escasez afecta el bosque (Due to the fact that it is very hot...). If you wanted to use a noun instead of a full clause, you could use a causa de: A causa del intenso calor... (Because of the intense heat...).

Why is it mucho calor instead of muy calor?

In Spanish, weather expressions usually use the verb hacer (to do/make) combined with a noun, like calor (heat) or frío (cold). Because calor is a noun, you must use the adjective mucho (a lot of) to describe it. Muy (very) is only used before adjectives and adverbs.

Doesn't cuando usually trigger the subjunctive? Why is hace in the present indicative here?

Cuando only triggers the subjunctive when you are talking about a specific future event that hasn't happened yet (for example, Cuando haga mucho calor mañana...). When describing general facts or habitual actions that happen regularly, as in this sentence, cuando is always followed by the indicative.

Why isn't there a 'personal a' before el bosque?