Hello,
Insufficient air circulation in there. Wood stacked next to plexiglass wall will retain and even absorb condensation. Most greenhouses are configured for heat and moisture retention not dehydration, even with a draft going through evaporated water is not carried off efficiently. Solution: remove wood from greenhouses and stack it outside, covered, and in a wind exposed location. One could expect Alder to dry out within six months in even less than ideal conditions as it is, similar to poplar or willow a light and soft wood.
Greetings,
Don