This study identifies the most influential variables in harvesting operations and compares the productivity and costs of two lowinvestment biomass harvesting system for young trees, likely to be used by owners of small and medium-sized forests in south-central Chile. The focus is harvesting the natural regeneration of Acacia melanoxylon, characterized by a high density, around 6,900 trees ha-1, with an average diameter at breast height (DBH) of 5.5 centimeters, under the cover of a Eucalyptus globulus plantation established in 2004 in Valdivia, Los Ríos region, Chile. Whole-tree harvesting was used and included felling and skidding activities. In the motor-manual system, felling was carried out with chainsaws and skidding with oxen. In the semi-mechanized system, two chainsaws equipped with a felling frame and a farm tractor with grapple for skidding were used. Time studies were done, productivity models
were developed and harvesting costs were calculated. The cost of both harvesting systems was relatively similar, for an 80-meter average skidding distance, with an average cost of 24.7 USD Mgw -1 (wet weight) in the motor-manual system and 24.0 USD Mgw -1 in the semi-mechanized system. Understory density and terrain slope had a highly significant impact on productivity and felling costs. For skidding, the hauling distance and skidding capacity significantly affected productivity and costs, while terrain slope, with ranges between 0 and 20 %, had no significant effects on productivity.