The majority, approximately 99 percent, of our raw materials (processed and unprocessed residues and low-grade wood fiber) come from timberland that is then reforested by either direct replanting or natural re-growth. Approximately 20 percent of this amount comes from timberland that is actively replanted via artificial regeneration. The remainder is subject to natural regeneration management. A typical “stand” of artificially regenerated trees is planted with approximately 600 trees per acre (1482 per hectare). Mortality rate in the first year is approximately 10 percent which means that after the first year the stand will contain approximately 540 trees. By year 15, approximately 500 trees will remain. Natural causes of mortality include tree competition for nutrients and sun, weather related events, disease and infestation.
At this point, the tree stand is overstocked and has reached a growth, quality and health plateau. Therefore the lower-quality, distressed and/or diseased trees need to be culled in order to alter forest density and provide adequate space, sunlight and nutrients for the forest stand. The first “thinning” occurs at about year 15, depending on site quality, which takes half the standing trees, leaving 250 in the stand. As thinnings take place, the timberland is not replanted, to ensure the proper amount of light and nutrients reach the existing stand. A second “thinning” occurs between years 20-22, which will take half the standing trees, leaving 125 in the stand. Final Harvest is at year 30, taking all the remaining trees. Once all trees are cut, replanting/reforestation takes up to two years after the final harvest. Because the revenue stream for a timberland owner includes the two thinnings and the final harvest over the course of 30 years, an owner of 100 acres would generally have two to three stands, for which the thinnings would rotate every five to eight years.
An alternative forest management technique used by Enviva suppliers to ensure the health and biodiversity of the forest is continuous selective harvesting or silviculture.