The most important issue with cracks is to "cleat" them. Small patches (on the inside) near the ends of a crack will prevent any further split. If the crack developed unexpectedly try to determine what caused such dryness and correct that. Humidify your music room in the dry season. Small guitar humidifiers can cause wide swings in humidity which often do more harm than good. Few of them do anything to keep the fretboard from shrinking. The good news: your guitar has solid wood, plywood never cracks.