Seldom does it girdle a tree low enough down to kill it. But it will kill leaders high up, topping some young trees entirely, and ruining the lumber potential of others. The only consistently effective animal predator is the fisher, an arboreal relative of the weasel that is able to attack porcupines in the trees. On the ground, a porkie will ball up under its extended spines, flailing its spine-covered tail and twisting around in a circle as it flails. Bears and coyotes have been found starved to death with mouths and throats full of porcupine spines.
Intentional control is difficult.
Strichnine-laced salt blocks have been tried, but they killed all the deer and everything else in the woods but the target animals. Porcupines are apparently immune. Leghold trapping has been tried, but porcupines are notorious for chewing off the caught limb and waddling away, apparently unfazed. The only bait known to attract them is decaying porcupine flesh. Odd, but apparently true.