There are several key differences between short-rotation forestry and conventional managed forestry practices (Figure 3). Exceptional growth rates are requisite for short-rotation forestry. Depending on tree species and site, short-rotation forests may grow as much as 10 feet of height or more per year. Conventional forestry species grow slower at 1.5 to 3.0 feet per year for young pines and even less for some young hardwoods. Because short-rotation forests reach a given height and diameter in fewer years, rotation lengths are shorter than conventional forestry species. When grown as biomass for energy feedstocks, short-rotation forests can be harvested at 1-year to 3-year intervals with specialized harvesters. When grown for solid wood products or veneer, rotations are 12 to 20 years. In contrast, conventional forestry requires 25 to 35 years for pine and 60 to 80 years for hardwoods to produce lumber, veneer, and poles for forest products. Biomass feedstock production and phytoremediation applications capitalize on dense plantings of several thousand trees per acre. Willows have been experimentally planted at 13,000 trees per acre for bioenergy. Poplars grown for paper or energy may be planted at ~1,000 to ~2,000 trees per acre, respectively (Figure 4). Conventional forestry utilizes ~ 400 to 650 trees per acre.

Form of planting stock: Some short-rotation-suitable hardwoods, such as poplar, willow, and sycamore, can be planted using cuttings or “sticks” from existing tree branches and stems. Most other hardwoods and softwoods, such as loblolly pine, must be planted as bareroot seedlings. The ability to use cuttings facilitates easy regeneration of trees without additional commercial purchase. Likewise, cuttings allow for clonal forestry wherein desired characteristics can be easily replicated at a site by using the identical genetic material of cuttings from the same trees. Conventional forestry for other hardwoods does not easily support clonal forestry, and clonal forestry for pines is available but expensive.

Coppicing ability: Coppicing is the tree’s ability to produce multiple sprouts from a stump following harvest. Most hardwoods have some coppicing ability, but willows, poplars, and sycamores robustly coppice and can essentially regenerate following harvest. Coppicing is a desirable characteristic if tree form quality is not important; hence coppicing is a great fit for biomass production. For high-value products requiring excellent stem quality, coppiced sprouts require thinning to achieve a single stem per stump to grow the next generation log (Figure 5). Alternatively, a new cutting may be planted adjacent to a harvested tree stump to more easily produce excellent stem quality. The harvested tree stump requires herbicide treatment to stop coppicing re-growth.

Tolerance of weeds and grass: Weeds and grasses represent competition for all newly planted tree plantations. Competition for resources, such as light, soil, water, and soil nutrients, is intense and will reduce tree growth over time. During the first few years of growth, robust competition can cause tree mortality and, for a plantation, may require replanting. Diligent control of competition is absolutely necessary and begins well before trees are planted. In North Carolina and the Southeast, annual precipitation of 40 to 60 inches per year and long growing seasons especially encourage vigorous weed and grass competition. Hardwoods require more help out-competing weeds, vines, and grasses in the first few years of establishment than loblolly pines. Once a plantation reaches closed canopy, the effort expended for weed, grass, and vine control declines significantly. However, invasive plants, such as privet, can be problematic. Densely planted, short-rotation hardwood forests reach closed-canopy quickly within two years. Depending on the site, loblolly pines usually outgrow most competition after five years.

Figure 4. High density planting of hybrid poplar trees at 2 years (left) in the North Carolina coastal plain and 3 years (right) in the North Carolina mountains.