Spring Valley EcoFarms is a non-profit organization focusing on education, research, and outreach to promote more ecologically sustainable agriculture. Its seat is Spring Valley Farm, 100 acres in the Georgia Piedmont. The vision is to reduce reliance on external subsidies in agricultural systems through incorporating free services of nature. The goal is to develop a model for conservation of biological diversity and to provide a laboratory where ecological science and theory are put to a real-world test.
Natural forests usually consist of many tree species of all age classes. In contrast, most forest plantations are monocultures. All the trees are of the same species and of the same age. From an ecological point of view, even-aged monocultures are generally undesirable. They use environmental resources inefficiently. The canopy of all the individuals has the same shape and is at the same height, resulting in competition for light. The roots are all at the same depth, competing with each other at the same time for water. All the individuals have the same nutrient requirements, with the result that some nutrients in the soil are scarce, while others are underutilized. And in this age where biodiversity is recognized as an important quality of ecosystems, monocultures support a very depauperate fauna.
Fig. 1. Loropetalum (pink flower) planted under Paulownia fortunei (white flower). Loblolly Pine (Pinus taeda) was planted in the outside row of Paulownia.
Co-existing to Efficiently Leverage Resources
In mixed species forests, each species has slightly different physiological requirements. Light filters through the irregularly shaped canopy down to where understory trees can utilize it. Some species are deep rooted and take up water from depth, others are shallow rooted and absorb rain water more readily. One species may do well during a dry summer, another during a wet summer. One species may put on most of its growth in the early spring, another in the summer. The species may complement each other in their nutrient requirements. One may require lots of calcium, another may require more phosphorus. As a result, when they co-exist in a forest, they exploit more efficiently the environmental resources.
The Problem with Monocultures
Given the ecological superiority of uneven-aged, mixed species forests, why do many landowners plant even-aged monocultures? Because it is easier and cheaper to plant and harvest a plantation of one species than a forest of many species. However, there may be combinations of species that use environmental resources efficiently, and at the same time can be planted and economically utilized as efficiently as when in a monoculture.
Experimenting with Paulownia and Loropetalum
A long-running project at Spring Valley EcoFarms has been to experiment with combinations of species that might have such potential. One effort involves the Princess Tree ( Paulownia fortunei), a fast growing pioneer tree that casts a light shade and loropetalum (Loropetalum chinense), an ornamental shrub that is shade tolerant. We are interested in Paulownia because we feel it has a great but unrealized potential (see “Princess Tree” under “Trees and Plants” in this website). We used the loropetalum plantation as a source of green cuttings for a horticulture operation. The Paulownia did well in combination with lorapetalum (Fig. 1). The combination is reminiscent of traditional coffee plantations in the tropics, where coffee trees grow in the shade of overstory trees, whose leaf litter supplies nutrients for the coffee trees.
Experimenting with Varieties: Fast, Medium and Slow Growing Trees
A second experiment involves combinations of fast, medium, and slow growing species: white oak (Quercus alba) (a slow growing species) , Paulownia fortunei (a fast growing species) , green ash (Fraxinus pennsylvanica) (medium growing) , white poplar (Populus tomentosa) (fast growing), and tulip poplar (Liriodendron tulipfera)(moderately fast). The best combination was Paulownia and white oak. The Paulownia grew quickly and towered over the oak, which remained small for three or four years while its root systems developed. When the Paulownia is harvested, the cut acts as a release for the oak, which then takes advantage of full sunlight. Green ash grew more quickly than the oak, but it also was a good combination. Tulip poplar grew as quickly as the Paulownia, and proved to be a competitor. White poplar was difficult to establish, but once going, it became very competitive, and crowded out the Paulownia. It also forms underground root networks, and between rows it sends up sprouts that must be continually cut back. Because of the low quality of white poplar wood, we do not plant it anymore.
The best combination is oak and Paulownia. In another trial, we used three species of oak (white oak, pin oak, willow oak) and two species of Paulownia (Paulownia tomentosa, Paulownia fortunei). Not only are the two genera (Quercus, Paulownia) ecologically compatible, they offer a nice economic combination. Ordinarily, the time for oaks to reach harvestable size is too long to make oak plantations economically attractive. But when Paulownia produces a harvest at 12-15 years, it can be the main source of income, and the oaks can represent an additional benefit for the future.
Paulownia Feed?
After about 6 years, we allowed our horses to graze on the fescue grass that was abundant under the Paulownia. When we returned from Christmas vacation, we found that the horses had girdled all the Paulownia. There apparently is something in the bark of Paulownia that is very attractive to horses (also cattle and deer). Fig. 2 shows the girdled Paulownia marked with yellow tape for cutting. Between the Paulownia, the smaller oaks are evident. However, the girdled Paulownia were not wasted. We split the trunks and made them into fence rails (Fig. 3).
Fig. 2. Girdled Paulownia marked with yellow tape.The oak saplings between the Paulownia were “released” when the Paulownia were cut.
Fig. 3. Split rail fence made from Paulownia logs.
There is another factor to consider regarding monoculture plantations. The seedlings are always planted close together, often less than five feet. The reason is so that the individuals provide “side shading” for each other. When trees are growing close together, light can reach the trees only from above. Consequently, side branches die, and trees grow quickly in height but slowly in diameter. The result is a long, regular bole that is desirable for lumber. The problem with this is that stagnation of growth ensues as the trees get larger. The trees are too crowded. The solution is thinning, that is, cutting out fifty percent or more of the trees, an economically expensive and often wasteful activity. The Paulownia/oak combination eliminates this problem to some extent. Initially, the Paulownia gives some side shading to the oak. The Paulownia however, must be pruned to ensure a good, straight bole. After the first cutting of Paulownia, when the oaks have begun to form a canopy, a Paulownia sprout from the stump shoots up straight toward the gap in the canopy, and little if any pruning is required to get a long, straight bole.
There are many other species that could be incorporated into forest plantations, so that ecological process mimic those in natural ecosystems. On the degraded soils of the Piedmont, typified by those at Spring Valley EcoFarms, trees that form bacterial associations that fix nitrogen could be the first species planted on an abandoned field, enriching the soil so that later, other more economically attractive species can grow well. Black locust is an example. There is a small plantation of thornless black locust at Spring Valley, and it has done well on the poor soil.
While mixed species are desirable ecologically, economic considerations often outweigh ecological ones. The major challenge for research in mixed species plantations is to devise combinations that are economically attractive. It is a slow process, not readily amenable to rapidly changing grant agency research priorities. It can only be done in places like Spring Valley EcoFarms.
