"Good God!" exclaimed St. Aubert, "you surely will not destroy that noble chestnut, which has flourished for centuries, the glory of the estate!" (1.1.13)
St. Aubert doesn't want Quesnel to go all George Washington on his tree because his identity is completely wrapped up in its existence. St. Aubert is nothing without this symbol of the natural world.
It was a still and beautiful night, the sky was unobscured by any cloud, and scarce a leaf of the woods trembled in the air. (1.6.42)
Em gets all spiritual when contemplating nature because she can't handle how grand it all is. Poetry is one way she can express All the Feelings about something totally incomprehensible.
"How delightful," said she, "to live amidst the coral bowers and crystal caverns of the ocean, with my sister nymphs, and listen to the sounding waters above, and to the soft shells of the tritons!" (2.2.27)
Hold up, so Em has sister nymphs now? In times like these, it really seems like Em thinks of herself as a woodland spirit. She has a fantastic imagination, that's for sure.
Another gate delivered them into the second court, grass-grown, and more wild than the first […] (2.5.28)
Udolpho is pretty overrun—it's almost like it's a part of the wilderness that surrounds it.
But, as Montoni would not suffer her to pass the gates of the castle, she tried to be contented with the romantic views she beheld from the walls. (2.7.90)
Gazing upon nature is like breathing for Em. If she can't be out in the woods and communing with nature, at least she can make do with a spectacular view.
She remembered how often she had gazed on [the planets] with her dear father, how often he had pointed out their way in the heavens, and explained their laws […]. (2.11.28)
Ah, nostalgia. Em had the pretty rare chance to learn about astrology—if this really was the sixteenth century, that kind of education would not be a given.
Emily remained at her casement, till the vivid lightning, that now, every instant, revealed the wide horizon and the landscape below, made it no longer safe to do so […]. (3.4.14)
There's a real obsession with dangerous elements of nature going on here. Is it totally healthy? Better than ghost-chasing, we guess.
Emily now had a full view of Udolpho, with its gray walls, towers, and terraces, high over-topping the precipices and the dark woods. (3.6.76)
Usually, we get the opposite view—looking over the walls of Udolpho at the surrounding wilderness. Outside the walls, Em feels absorbed back into nature.
Meanwhile, the Countess, reflecting, with regret, upon the gay parties she had left at Paris, surveyed, with disgust, what she thought the gloomy woods and solitary wilderness of the scene […]. (3.10.11)
Point taken: not everyone's into nature stuff. But enthusiasm for nature is a great way to tell who's going to be a main character. The Countess really gets the short shrift after she pooh-poohs the gloomy woods.
O! How joyful it is to tell of happiness, such as that of Valancourt and Emily; to relate that […] they were, at length, restored to each other—to the beloved landscapes of their native country […]. (4.19.8)
Em and Valancourt are almost as excited to have their "beloved landscapes" as they are to have each other, which tells us something about the importance of keeping land in the family.