Dani was lost. With twilight rapidly closing in on the Ozark hills, it was either find a way home or spend the night in a tree being eaten by mosquitoes.
Not that any of the trees were worth spending the night in, she thought as she looked around at the spindly trunks, slapping bugs away. They'd break if she leaned too hard on them, and the branches on the bigger ones were too high to reach. Spending the night on the ground was absolutely out of the question.
The air in the trees was close and muggy, beading her brow with sweat which she aggressively wiped away. The long pants she wore were protecting her from thorns and insects, but they weren't doing much for the mid-July heat. Aside from changing, she hadn't done much to prepare for her unintended hike.

I don't have any idea why spending the night on the ground in the Ozarks would be out of the question. If Dani knows--if, for example, she's seen a dragon out there--then we should know, too.

And I don't know why she took this "unintended hike".

These aren't hooks. They're annoying, because you're withholding information.

At a guess, without knowing the story, I'd say you started in the wrong place. If Dani took this hike because she saw something strange from her campsite and followed it, as an example, then that's where the story should probably start.

That, and we need a reason to care about Dani. For that, we probably need a glimpse of who she is. Just being lost isn't doing it for me.

A few observations

I would never think about spending a night in a tree. Is that Ozarkian practice? Why would it be out of the question?

'the air in the trees' She isn't in the trees, she's walking in a forest.

'which she aggressively wiped away' I have never seen anybody aggressively wipe sweat away before.

'aside from changing, she was unprepared for her unintended hike' If the hike was unintended then why did she change for it?

A few things to think about. I'm not in the story. You haven't really said anything except Dani is lost and not looking forward to an uncomfortable night. That's not particularly interesting at this point.

I suspect her reason for staying off the ground is bugs/plants/wildlife, but I agree it should be spelled out. If you can make it sound a little more perilous, I will root for her all the more.

You don’t need to mention that she changed. Maybe she could be frustrated about not having any of her usual gear for this unintended hike, something like that.

I assume the next paragraph will explain the reason for the hike, and how this capable person ended up lost.

I guess I assumed the reasoning behind spending the night in a tree over the ground was obvious: bugs, spiders, snakes, bears, etc. I started the story at the exact moment Dani realizes she’s lost.

Here's my second try (I took all of this from later in the chapter):
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Dani was lost. With twilight rapidly closing in on the Ozark hills, it was either find a way home or spend the night in a tree being eaten by mosquitoes.

Not that any of the trees were worth spending the night in, she thought as she looked around at the spindly trunks, slapping bugs away; they'd break if she leaned too hard on them. Poisonous snakes, spiders, and the local black bear population didn’t make sleeping on the ground an attractive option, either.

She grew angrier with herself for her decision to enter the forest in the first place. She'd been coming here for years and had never done so before; the woods had been taboo to her since childhood because of mom’s “If You Go In There You’ll Die” speech.
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In the next line you find out that she left to take a walk in the woods because her parents are fighting and she's sick of it. Her half-planned walk turned into an unintended hike when she got lost.

There are no elements of fantasy at this point in my story. She's a normal girl in the real world. The fantasy part comes in when she finds an old root cellar to shelter in and wakes up in a completely different forest.