I have wood about 4 years old. That wood sitting on the ground sure does absorb moisture. We had some good rain the week before last. Some of my wood wasn't covered with a tarp. I split this wood. The outside was slimy from sitting on the wet ground, but the inside felt dry. Hey, I figured. Big log. Water won't soak through. Wrong! It felt dry inside, cold but dry. I put it in the stove. Stove wasn't heating up. I looked at my wood. There were boiling bubbles at the end of the wood. Water had been sucked into the wood. The wood couldn't heat the stove if it's heat was used to boil off water.
Birch and poplar are really bad for sucking up water. They also rot quick if you don't burn it.
Tarps are a worthwhile investment.