Supersized land anemones
Meet the swimming sea anemone:
It's desperately avoiding the grasp of a nearby sea star, which is interesting but not what we're interested in. Rather, notice how it bends its ring of tentacles nearly all the way to the ground.
What if this wasn't a defensive mechanism, but rather an offensive one? Our supersized land anemones would remain quietly waiting until they detect
prey pesky teenagers, either by touch or through the network of symbionts surrounding them. Once triggered, the entire crown of the tree, full of tentacles and the anemone's nematocysts, would smash down upon the prey.
It'll take a some handwaving to make this happen, but that's mainly to do with the transition of the anemone from sea to land. Anemones are supported by water pressure, so our land versions would only be found in the dampest areas such as swamps.
It's quite possible to imagine an anemone that ends up looking like a tree, especially if it's a strong evolutionary pressure. The stalk of the anemone might become tough, leathery and brown simply by drying out, giving a very bark-like texture. On land, zooxanthellae can be much more helpful given the higher intensity of sunlight, so they'd proliferate wildly. However, these symbionts are brown algae, so the tree canopy would remain a brownish golden color all year long
As for the prey size requirements, the anemone is unlikely to notice small organisms relative to tree height, and would therefore only trigger once a certain size threshold is exceeded. That just happens to be the size difference between small animals and humans.
Heliozoans are a class of aquatic predators that look like underwater spikeballs:
They act almost like underwater spiders, as they use those long, thin spines to catch prey, which are nearly invisible underwater. They're coated in a sticky substance that may have some paralyzing effects. They sit and wait for a
prey item pesky teenagers to bump into a spine, at which point the spines actually retract with incredible speed- up to 2 body lengths per second. Prey Teenagers are then brought near to the cell body and digested.
Fortunately for us, some species already come with trunks!
Again, these are aquatic creatures that don't like being suspended in the air. Fortunately, they also have a sporelike function that they'll use when food is scarce or the streams dry up. It's possible that such a heliozoan, once left in a dry streambed for long enough, will nonetheless try to feed. Realizing the vast amounts of food available, they'll be selected for size and eventually (with a tiny bit of handwaving) become the size of trees!
The same size-requirements apply here. The tree wouldn't worry about small things touching its bark or leaves, but instead would use its energy to deter larger creatures that might actually threaten it. The spines would be set on a large-size trigger where a certain length must be touched, or perhaps multiple spines in a short time window, akin to the Venus flytrap triggers.