Bark on many species of trees comes off easier right in the spring when the sap starts to rise. But it gets extra hard to strip bark during the winter.
Wood is often best harvested in winter when the sap is down. It takes less time to dry out, and there usually is less checking/cracking in it as it dries because of the less moisture.
But so much also depends upon the environment around you and where you collect/store/work the wood. Hot, cold, damp, dry, sunny, shady. They all affect the wood.
Of course, these are pretty simple generalizations. Do a web search for traditional woodworking, or colonial woodworking. It should lead you to some better info. Also check out the books and videos for The Woodwrights Shop by Roy Underhill - a PBS series. He covers the various trees, and harvesting them for specific projects. Some are worked green, some require dry seasoned wood.
You might also check out the Society for Primitive Technology. They have a lot of articles about aboriginal methods of working with wood and cordage/fibers.
Harvesting plants for their fiber happens after the plant is mature, but best done before it dies completely and dries out too far. If the plant is too dead/dry, the fibers might break too easily.
Examples.
Flax is cut before the grain head is mature, and then dried in the field for storage. It is then "retted" (soaked in water to partially rot it). This breaks down the husk of the plant leaving the linen/tow fibers to be pulled out. Linen is the fine fibers, tow is the coarser fibers. You do need to be careful not to rett" the flax too much - as that weakens the final linen thread spun from the fibers.
The same type of threads in nettles and hemp are also worked the same way.
The inner bark of basswood can be twisted into wonderful cordage - and strong. But you have to harvest it during the spring/summer/fall when the sap is in it, and it is flexible. You can work it at other times, but it is harder to harvest from the tree - and to split the inner bark from the outer layers.
Just a few humble thoughts to share. Take them as such.
Mikey - yee ol' grumpy German blacksmith out in the Hinterlands (who occasionally butchers some wood)
