L8. Plant Growth
In the last lesson, we discussed where biomass comes from and the production systems through which we grow it. Here, will being exploring the biology of biomass, learning about how the plants themselves grow.
To start off, let’s talk about the fundamental process that makes biomass a uniquely renewable resource: photosynthesis. In this process, plants use the sun’s power to convert carbon dioxide and water into biomass. The following animation provides a refresher on photosynthesis. Feel free to skip this video if you are already familiar with the process (note that there will be one quiz question on photosynthesis).
In this way, plants are sort of like solar panels – converting energy from the sun into something useful. Solar panels convert the energy into electricity, plants convert the energy into carbohydrates used to make plant cells.
(An interesting aside: solar panels are actually way more efficient than plants. The typical plant is only able to convert 0.1 – 2% of the sun's energy into carbohydrate energy. This is for a variety of reasons: first, more than half of the incoming sunlight is comprised of wavelengths too long to be absorbed and some of the remainder is reflected; and second, some of the energy is used up during the creation of carbohydrates and during respiration. Photovoltaic cells on solar panels are able to turn 10% of incoming sunlight into usable energy - making them around 10 times more efficient than plants.)
Since biomass growth is solar powered, the production of bioproducts requires far less energy inputs than the production of petroleum or mineral-based products. This is what makes biorenewable resources so special; through photosynthesis, plants are able to create the biomass we use out of thin air (along with some water and nutrients). Remember, some plants require more inputs to grow than others. For example, the amount of water and energy that go into growing crops via conventional agriculture is far greater that the resources required for natural forests to grow.
In addition to creating biomass used for bioproducts and bioenergy, photosynthesis benefits us by removing carbon dioxide from the atmosphere. In Lesson 4 we discussed how plants’ ability to trap and store carbon makes them them an integral part of the carbon cycle and an important control on climate change.
Plant Growth
Now we know photosynthesis is the basic process by which plants create the carbohydrate energy they need to grow… but how do they actually go about this? How do plants grow?
To answer we are going to use trees as a case example. This is for two reasons: 1) trees are by far the most widely used biomass resource, and 2) they are huge – so it is easy to see and understand their growth patterns. In the following video, on which there will be quiz questions, Dr. Howe explains everything you need to know about how trees grow. (Dr. Howe is a past instructor for this course and we still use some of his videos because of his strong expertise.)
Hardwood versus Softwood
In the video, Dr. Howe mentioned that where different cells are and what they look varies among tree species. This is especially true when talking about hardwood versus softwood species. Hardwoods are deciduous trees that lose their leaves annually and reproduce via flowers. Softwoods are evergreen conifers which means they reproduce by forming cones. While evergreens do tend to be less dense, or "softer", than deciduous trees, this is not always the case. For example, balsa is a super light and soft craft wood, but comes from a hardwood tree. Therefore, it's best to remember that difference between hardwoods and softwoods is reproduction method - not the hardness (density) of the wood.
Typically, softwoods are more uniform in structure, while hardwoods are more complex. The xylem of softwoods is composed of only a few cell types, with long cells called longitudinal tracheids making up 90-95% of softwood volume. Hardwood xylem is made up of four major kinds of cells, one of which is called a vessel element and is unique to hardwoods. A third difference is the ray cells - which Dr. Howe described as both a sprinkler system and a structural brace - make up 17% of the volume of hardwoods on average, but only 5-7% of the volume of softwoods. Lastly, hardwood fibers are usually less than 1 mm long, while softwood fibers average 3-4 mm in length.
Why do we care?
We care about how plants grow, particularly trees, because the growth patterns ultimately determine what products we can make. We talked above about how the species of tree affects the structure and organization of the cells. Due to their longer fiber length, softwood species are preferred over hardwood species as a raw material for making paper (longer fibers produce stronger paper). But if you'll remember from Dr. Howe's video, properties of the wood vary within a single tree as well. That is, you can make furniture from the inner area of the tree, but not from the bark. The following (required) video describes how the variations in wood across the trunk of a tree are ultimately important to the production and use of timber products.
You may be familiar with cedar wood - it's known for its distinct cedar scent and it's weather resistance. It's used in saunas, decks, shingles, clothing chests, and outdoor furniture. This is because the extractives in the heartwood make cedar naturally resistant to moisture, decay, and insect infestation.
Knots are another example of a growth pattern affecting the end product - when trees grow branches, the wood becomes less desirable for use down the road because it's less homogeneous. The following (required) video provides additional examples of how events in a tree's life and the way it grows affect what wood products can be made from it.
We now have an more in-depth understanding of how the biology and growth patterns of biomass affect the end products. In the next lesson, we will zoom in further, and discuss how the production of bioproducts is influenced by the composition of biomass at a microscopic level.