Short rotation woody crops such as willow and poplar, along with C4 grasses, have been the primary feedstocks studied for bioenergy. Expanding upon those requires obtaining knowledge of the ecophysiological requirements of other potential feedstocks. John S. King, Department of Forestry and Environmental Resources at NC State, has studied a variety of tree species for biomass potential, and American sycamore (Platanus occidentalis) is an above-average candidate.
“The changing climate and economics are everyday concerns, and that’s context within which I think about our bioenergy research,” King said in a recent webinar. He explained that the ecophysiology of the crop – the environment where it grows, it’s water use efficiency, carbon capture capacity and other characteristics which impact how low-input the crop really is, as well as the relative amount of bioenergy it can produce – are all parts of the equation.
“Energy is a commodity…and we can’t be spending a lot of money to culture it,” King said. The goal is to “start using ecophysiology as an applied science, to design bioenergy systems that are climatically robust and appropriate for the areas where they’ll be deployed.”
The American Sycamore
The American sycamore is a promising candidate for bioenergy production in the U.S. It is a native species with a wide range of growth habitats. Seed is readily available at low cost and produced abundantly, and improvements to the species can be readily made by selecting for desired properties, such as salt tolerance, productivity and disease resistance. It has a high tolerance for both biotic and abiotic stressors, with few pest issues and an ability to withstand both drought and flood conditions, all of which are occurring with increased frequency and intensity.
It can be coppiced, so cutting down the primary crop still allows extensive secondary growth to occur, and the tree does not have to be replanted after every harvest. It is able to out-compete weeds. It also has the ability to improve soil biology, chemistry and physical properties, so it can be used as a rotation crop before planting fields back to field crops. It grows well in a low input system, requiring minimal resources.
On the energy side, American sycamore is a uniform producer, and its wood has a medium-value energy content. It dries well, which is important for bioenergy use, and its thin bark does not produce much ash.
In NC State cropping system research trials, a variety of tree species, including American sycamore, were planted in mixed and individual species blocks at varying densities. They tested how water input, with 20% reduction in rainfall from ambient precipitation via a gutter system which removed rainfall, impacted growth. Trees were planted directly into a subsoiler trench. Within the tree rows, weeds were left standing.
Almost every tree except American sycamore died two years in a row. By the end of the fourth growing season, the sycamores – even in the reduced rainfall plot – were uniform, productive and ready to harvest. Sprouting began 60 – 90 days after harvest. The same stand was harvested five years later, with no health issues. Because it coppices readily, the growth cycle can continue for at least three harvests prior to reduction in output.
Biomass data from the study show that the 5,000 and 10,000 trees per hectare density provide the most productivity, with little difference between them. Planting American sycamore at 5,000 trees/acre is the most cost-effective and recommended rate.
When compared with other biomass studies, American sycamore is shown to be as productive as other species grown under high input conditions. The energy value of American sycamore is about 12 megagrams/hectare/year when grown with no fertilizer, no irrigation and no herbicides.
“Just sticking the trees in the ground and walking away,” King said, produced very good bioenergy results.
Future studies in the research plot are being designed to look at the tree’s root density biomass at different planting densities. These studies are showing differences in soil properties due to the American sycamore crop. Plant-available water, drainable porosity and permanent wilting point are all being impacted already in a positive manner. Researchers are examining if the trees can mine legacy nutrients, bringing them to the surface and dropping them in their leaf litter, making them available again in the topsoil. Even over a period of several years, changes have been seen in the soil to indicate this is a good probability.
Carbon and water cycling are being examined in a newly established plot planted in 2021. The trees were planted in a weedy hay field via subsoiler trenches. Even in weedy areas, the trees were out-competing weeds and reducing the weed population significantly within a few seasons.
American sycamore can help to integrate food and fuel production via intercropping, King said. This would diversify local economies, improve the environmental footprint and enhance soil health. Decreases in weed seeds or nematodes might also be a benefit.
For the first three or four years, grasses growing between rows of American sycamore could be harvested and would not interfere with tree establishment. After that, the trees would produce too much shade, unless row spacing was wider. Potential for grazing exists, as long as the trees are not newly planted and can withstand some animal rubbing or browse.
Growing a tree crop on agricultural lands can add carbon to the soil and change the soil aeration via the tree root system. This type of cropping would provide a biofuel crop while improving the climate profile via carbon sequestration, fossil fuel offsets and “bio-physical feedbacks to the climate system,” such as water cycling, King said.
Just like a fallow field, the American sycamore crop could be used to enhance soil properties. But it also would be providing a new product stream, resulting in economic benefits to the farmer. “I call it an economic fallow,” King said.
More research is needed to understand the multiple benefits which can result when American sycamore is planted as a biofuel crop. When used wisely, the food vs. fuel argument does not need to come into play.
As climate change continues, finding the best alternatives to fuel our world while reducing greenhouse gases is an ongoing goal. American sycamore is a prime candidate for an efficient and effective low-input woody biomass crop.
by Tamara Scully