<p>This is just to add to what has already been said…</p>
<p>The adjective clause "in which Sequoia trees grow’ is restrictive (because it is essential in order to understand which habitat the author is referring to) and is not set off by commas. The next construction, “extends…California,” acts as an adjective modifying the noun ‘habitat’. The verb form ‘extends’ would turn that modifying phrase into the predicate of habitat, thus turning the sentence into one with a compound predicate which would require deleting a comma and inserting a conjunction ‘and’ in a place where one is not given the option to make changes. (between ‘California,’ and ‘is’) Since that option is not allowed, the only other choice is to change the word ‘extends’ into a present participle ‘extending’ and making the participial phrase 'extending…California, into an adjective modifying the subject of the sentence and the rest of the sentence (is kept moist, etc.) into a single predicate. </p>
<p>The difference between essential and nonessential clauses is determined by whether or not the information the clause contains is essential to understand the writer’s meaning. Subordinate clauses that act as adjectives are the ones that require this distinction. An example:</p>
<p>Situation one: I am talking about a man who is standing in a group of men.</p>
<p>“The man who is wearing a red hat is my uncle.” This clause is essential because one needs the information in order to understand which of the men I am referring to.</p>
<p>Situation two: Now I am talking about a man who is standing alone.</p>
<p>“The man, who is wearing a red hat, is my uncle.” This clause is nonessential because the information only tells you that I am amazed by his taste in hats, but it is not essential to know which man I am referring to.</p>
<p>It may be that your problem is not really about essential and nonessential clauses, but about when to use the relative pronouns ‘which’, ‘that’ and ‘who’. ‘Who’ only refers to people, ‘which’ only refers to things, and ‘that’ can refer to either.</p>
<p>FWIW The term "historical present’ is used to describe the tense used when describing the action of novels and movies. The idea is that the book exists in the present, so what happens in the book is spoken of as if it is happening in the present.</p>