Earlier today, I sighted a porcupine snoozing in a cottonwood, in the Rio Grande Bosque in Albuquerque. Here are two pictures.
Combined with my other warm-weather sightings, this encounter reinforces my suspicion that porcupines ascend into the cottonwoods along the Rio Grande in all seasons. They're more commonly seen in the winter months, but that's not surprising: they're much easier to spot when the cottonwoods have shed their leaves.