I'm surprised no ones qouted Hermann Phlepps "Craft of Log and timber Construction" on the subject of time for timber harvest. This book is the holy bible of timber and log joinery. In it he qoutes one old adage " If in Yuletide days your felling , ten fold the lifespan of the dwelling. By Fabian and Sebastian (Jan. 15)sap is on the rise again." I believe they where refering to conifers primaril.Another adage claims "When the larch is cold (and bare) timber feld is true and fair. These guys where definetly European cause they haven't seen the marginal larch we have here in upstate NY. Phlepps indicate that many if not all conifers have multipal sap runs, probaly triggered by warm spells. Or as suggested earlier it never stops just slows with temperture.(conifers only) Up here in NY the easiest time to remove wood from the forest is winter. I'm on Tug Hill though and we get deep snow, or we use to. Any one who atemps summer logging will quickly relise why the old timers did it in the winter, its hard work. A great video was produced by wnpe public tv in Watertown NY on winter logging on the Tug, it's wicked cool!