North American lumber prices crossed back above the $1,000 level on Thursday as seasonal demand begins to pick up.  Wholesale and retail lumber inventories have been conservative in recent months, according to one lumber industry executive.  "Heading into what is the typical seasonal strength of the lumber market, a substantial buying round is currently upon us," Sherwood Lumber's Kyle Little said.  North American lumber prices have reversed some of their March losses and crossed back above the crucial $1,000 per thousand board feet level.  According to one industry executive ...