pine flooring gaps
A long time ago the pine that was used was from virgin forests and much more stable than what is now available but they still suffered the same problem. I suggest that you do what they did. There is a product called Butcher wax for floors. It gives a nice warm amber glowing colour that compliments pine nicely. It will fill the cracks and buff out and harden up enough to stay put. It will also help slow the movement of moisture in the wood and decrease the movement it causes.
As it does swell again gradually in the summer, the wax will be squeezed out but normal foot traffic will wear it off so you will not notice it goingt off.
Then, it will be necessary to renew it again next winter.