A tree harvester, suitable for harvesting small trees in row(s) as a source of biomass, has a harvesting head with a chipper drum and complementary anvil. A pick-up front has at least one spiral and guide member, such as a guide rail or counter-rotating spiral, to maintain a tree in a substantially vertical position when, and after, its stem is cut by a base cutter. The pick-up front has a support plate or conveyor, which co-operates with the at least one spiral or guide member pair, to convey the tree to a location about the chipper drum, for engagement by at least one pair of nip rollers above the chipper drum, while maintaining the tree in the substantially vertical position. The, or each, pair of nip rollers feeds the tree downwardly (and rearwardly) to the chipper drum, while still maintaining the tree in the substantially vertical position, for conversion to biomass, such as woodchips.The harvester may have two-or-more of the harvester heads arranged side-by-side to enable trees in adjacent rows to be harvested.