I thought this issue was covered in one of the TBB volumes . Yew wood was safe at 8% MC , below that it and it may break . Alberta gets very dry in the winter , many homes have natural gas furnaces which suck the moist air out of the house and very dry air comes in to replace it, hence many homes have humidifiers. Interestingly local native woods are well suited for local humidity conditions for bows . Yew performs well in damp temperate areas. Keeping a yew bow inside a car in Utah for a few days and then shooting it there is asking for disaster ! So if you have a yew bow in a dry climate , keep the moisture level up .
Mongolia and other Central Asian countries also experience very low humidity in the winter , so low that composite bows would delaminate and break . The archers there built cabinets with meat in them where the bows were stored , the meat gave off moisture that the bows absorbed .