The project, which is supported by the Scottish Parliament, began in April and is led by the FIM investors that acquired the estate in 2006.
Tim Liddon, district manager for UPM Tilhill, will manage the operational delivery of the project.
He said: "Southern Scotland is one of the best places in the world to grow timber. Our Sitka spruce thrives on the mild climate, converting misty sunlight, drizzle and carbon dioxide directly into timber."
Based near Langholm, in the Borders, the trees will be planted across 8.14 square kilometres of land.
Tilhill was established 50 years ago and has a turnover in excess of £80m. It supplies UPM's Caledonian mill, near Glasgow, and some of the paper giant's Finnish mills.
Director of FIM Services Colin Lees-Millais said there were real opportunities for a major expansion of UK forestry.
"Increasing recognition of the versatility of timber is leading to growing international demand and this is driving prices."
UPM Tilhill kicks off massive tree planting project
Forestry company UPM Tilhill has begun planting around 1.2m trees in one of the UK's largest commercial woodland projects for more than a decade.
