In the last lesson, we introduced biorenewable resources as an alternative to non-renewables, and covered some of the things that can be made from them. But how do we generate all of this biomass? Unfortunately, the lesson has to start with some definitions that are needed as you go through the lecture.

Sources of biomass

Biorenewable resources generally come from one of three sources: forest resources, dedicated crops, or wastes. We briefly discussed waste as a resource in the last lesson, and will revisit it in a later lesson. For now, let's focus on the biomass we grow: trees and crops.

Forest resources

Forests are areas managed for the production of timber (lumber) and other forest products or maintained as wood vegetation for such indirect benefits as protection of watershed areas or recreation.

Roundwood on truck Resources from the forest include wood for products or for energy. Forest resources used for traditional wood products include sawlogs, pulpwood, and roundwood. Defined by the Glossary of Forest Engineering as:

Sawlogs - Logs meeting minimum regional standards of diameter, length, and defect. Logs that are suitable for lumber.
Sawtimber - Trees suitable for production of sawlogs
Lumber (often referred to as timber) is the wood that has been processed into beams or planks and used in construction.
Pulpwood - wood suitable for making into pulp for making paper
Roundwood - Generally any wood in its cut but natural state having a round cross-section. More specifically, roundwood is used for pulp or left as small logs. It is typically taken from near the tops of trees and used for furniture. (Photo is roundwood on a truck.)
Whole Trees - All components of a tree, except the stump.
Logging Residue - Unused portions of pole timber and sawtimber trees killed by land clearing, cultural operations, or timber harvesting.

Forest resources used for energy include whole trees, and the residues that are left over after harvesting or after the lumber/paper making process. Figure 1 below provides a visual of the different types of forest resources.

forest resources

Figure 1. Types of forest resources.

Dedicated crops

Dedicated crops (sometimes referred to as dedicated energy crops) are plants grown as a source of organic matter specifically for production of bioproducts or bioenergy. That is, crops grown for purposes other than food. The term “dedicated energy crops” was originally used to describe plants grown to be burned for electricity or turned into fuel. However, not all dedicated crops are grown for fuels and energy; they might be used for production of chemicals or natural fibers.

Figure 2 shows a breakdown of some crops that are common globally. Crops “for energy, fuels, chemicals, etc.” are those grown to make some of the things we discussed in the last lecture like ethanol, biodiesel, adhesives, and sufactants. It is interesting to note that there are really no dedicated crops used for paper production. Paper comes from managed forests but the wood from those forests is used for multiple purposes.

Herbaceous crops are what typically comes to mind when we think of agriculture - plants like corn and soybeans that have little or no woody tissue and only live for one growing season. But agriculture can produce woody crops too! These are referred to as short rotation woody crops and will be further discussed at the end of this lesson.

Crops grown “for fiber” are used to make clothes, rope, rugs, and more. As with other resources, different species are popular in different area of the world; for example, kenaf (used for making rope, rugs, etc.) is most commonly grown in Africa and Asia, while the U.S. is a huge producer of cotton.

Figure 2. Breakdown of dedicated crops - types and examples.

Biomass production systems

There are four main ways in which we grow the different kinds of biomass described above, which are called production systems. They include: conventional agriculture, organic agriculture, natural forests, and plantation forests.

The following video provides an overview of these four systems. (As always, you can watch the video at faster speeds.)

In the video, Katie Fernholz (guest lecturer from Dovetail Partners) described how forestry is ecosystem-based and relies on natural processes. It is a more “hands-off” production system than agriculture, which are on an annual cycle and require a great deal of attention from farmers.

That is not to say that forests aren't managed. There are certain silvicultural practices that foresters use to make the wood that we get from the forest more valuable. Silviculture is the growing and cultivation of trees, and is part of forest management. The difference between forest and farm is the timescale; the practices that we're talking about for silviculture are done 4-5 times over 50 years, most within the first 15-20 years. Some common silvicultural practices include:

Thinning - some of the young trees are removed in order to reduce competition and allow the remaining trees to grow bigger, thereby increasing wood production. Chemicals can be used to thin trees in the first 2-3 years.
Pruning - some of the lower branches are removed to increase amount of wood that's free of knots. But if pruning is done too extensively, it may decrease wood production.
Genetic selection - the trees in the forest with the best growth or form are identified and propagated (bred) to new generations so more trees have the desired characteristics.

The following video provides an interesting introduction to silviculture. It was made in Canada, but the same ideas apply to American forests.

Plantation Forests

Even more intensely managed are plantation forests, which are planted and managed with the intent of maximizing the production of wood (whether it be for lumber, paper, or energy). About 10% of forests in the United States are plantation forests, the vast majority of which are pine forests in the South. In addition to being planted, plantation forests may be fertilized, watered, and sprayed with pesticides, so they have more in common with cropping systems than natural forests do. These intensive practices also mean that plantation forests are much more productive than natural forests. Typical yields from natural forests are between 1-6 cubic meters of wood per hectare per year (m³/ha/yr). Note: a hectare is about the size of the grassy area inside of a 400 m running track (like the ones at all high schools). In contrast, plantations produce 10 times that much wood per hectare (between 10-25 m³/ha/yr, or even up to 45-70 m³/ha/yr in some species). In 2000, plantations produced about 27% of the wood harvested globally, even though they covered less than 3% of the of the forested land area worldwide (Shmulsky & Jones, 2011). Plantations will clearly play a significant, if not dominant, role in meeting global wood demand in the future.

To help you think about the different levels of management intensity in forestry, look at the spectrum below. On the far left side of the spectrum are the "hands-off" natural forests. In the middle are the semi-natural forests, where silvicultural practices are implemented to help the forest grow or regenerate. On the right side are the planted and intensely-managed plantation forests. Note: if we were to place agricultural systems (including herbaceous, woody, and fiber crops) on this spectrum, they would fall on the far right side. Farmers are implementing management practices (e.g. fertilizing, watering, applying pesticides) multiple times per year, making cropping systems far more intensely managed than even plantation forests.

Increasing forest management intensity →

Short Rotation Wood Crops

One production system that Katie Fernholz did not cover in the video is short-rotation woody crops (SRWC). Although this is a way in which we grow woody species, it is classified as a cropping system as opposed to a forest system, as shown back in Figure 1 (it is the 'woody' category under dedicated crops). This is because the intensity of management and the short period between harvests. The following description is taken from Genera Energy’s website:

Short rotation woody crops are tree species that have been bred and selected to have extremely high rates of growth, allowing them to be harvested after only 3-7 years. Short rotation trees can be utilized for the production of liquid fuels, such as ethanol, diesel, and jet fuels. They can also be used for the production of heat, steam, and/or electricity. Due to their rapid growth, their wood is valued for fiber and not strong lumber or other products.

This is a key difference between plantation forests and short rotation woody crops. While the wood from plantation trees can be used for lumber, paper, energy and more, wood from short rotation trees is almost exclusively used for energy.

Image result for coppiced willow field

Figure 3. Willow crop. You can see that these "trees" really look more like shrubs.

Hybrid poplar (Populus spp.) and willow (Salix spp.) are the two species most commonly adapted to short rotation management. Poplars are typically grown in the mid-Atlantic, Southeast, and Northwestern portions of the US. Willow is a more cold-tolerant species and is the species of choice in the upper Midwest and Northeastern parts of the country. Both types of hardwoods are coppice species. Coppicing occurs when the tree is harvested and a new sprouts grow from the existing root system. Having trees that exhibit this trait allows for multiple harvests following a single planting.

Figure 4. Illustration of the coppicing process.

We will revisit short-rotation woody crops later in the course when we look into bioenergy. We will also spend more time on herbaceous production systems in Unit 4.

In the next lesson, we will take a deeper dive into how plants grow.