First, when it comes to pine and gum I’m not sure, but I know that pine shavings are harmful, though pine pellets for litter are OK. But bunnies do not eat the litter.
Apple tree sticks and bark are safe, but my philosophy is “when in doubt, don’t”. The biggest offender for any tree branch or bark is actually the chemical sprays that are used to ward off bugs and pests being toxic. So few of us live in places now that are not effected by pesticides, even airborne from neighbors, that is make it potentially hazardous.
My suggestion would be to get some willow twigs (chew toys), or apple twigs, and put them with your pile. Give those to the bunnies, but steer them away from the rest. Fibers are also very hard to digest, and ferns may be a hazardous natural plant, so I am concerned without someone stating they are safe. (I just looked on the HRS website but didn’t find it on their list of OK or bad edibles.)