Staying rooted in one place is a thing forests are good at, yes.
The forest will not do anything to Masaru before he tries to get through it. However, if he wants more space than he currently has, he will have to make it or find it. (Pro-tip for Masaru: look up.) The plant-creature will react to attempts to do so at it sees fit. (It is expecting creation of said clear space, and won't necessarily react to it. Why, is something I'd actually like for Masaru to think of for a second.)
EDIT:Yeah, well. I expected Momoka to do the explaining. Like I said before, strawberry surprised me by taking a quip I made far more literally than I expected.Maybe I was wrong, but I'd read that statement as being how long you'd spend having someone explain the nature of the training area to Masaru, not "THE TREES ARE ATTACKING!".
Speaking of Momoka, she presumably (apparently?) has the means to step outside the barrier, so she can watch the whole thing fast-forwarded. She also had plenty of time to prepare before this, so I hope she was smart enough to pack some snacks.