Plant diversity in eastern U.S. forests comes not only from trees, but from the ferns, wildflowers, and other herbaceous plants on the forest floor. Some researchers have found that as much as 90 percent of plant diversity is due to these understory species. “Until recently, not much was known about the role these plants play in ecosystem processes,” says U.S. Forest Service scientist Katherine Elliott.
In 1998, Elliott and her colleagues began studying the understory in a forest at the Southern Research Station Coweeta Hydrologic Laboratory in western North Carolina. They designed the 14-year study, recently published in the journal Ecosystems, to see whether herbaceous plants contributed to carbon and nutrient cycling and whether they limited the amount of light and nutrients reaching tree seedlings. “We also wanted to determine whether the quick turnover of biomass — as the aboveground portions of herbaceous plants die and decompose quickly – helps trees to grow,” says Elliott.
Elliott and her colleagues installed 18 plots in a rich cove forest that was dominated by tulip poplar, American basswood, buckeye, birch, black cherry, and white ash. Rich cove forests tend to have relatively high water availability and rich, deep soils, and often harbor lush understories of native herbaceous plants such as ginseng, black cohosh, trilliums, ferns, Jack-in-the-pulpit, and many other species.
To find out whether these herbaceous plants contributed to carbon and nutrient cycling, Elliott and her colleagues separated the study area into treatment plots and untreated controls. In some plots, herbaceous biomass was added, and in other plots it was removed. Researchers measured mature tree growth, litterfall, and tree seedling recruitment numerous times over the course of the study, and also measured soil processes such as plant available nitrogen (the amount of nitrogen that is converted into forms plants can use for growth), and the amount of carbon dioxide released by plant roots and soil organisms.
The scientists found that plots with added herbaceous biomass had twice as much available nitrogen as control plots. “Our study showed that inputs of herbaceous plant material increases nitrogen in foliage of mature trees and contributes to soil available nitrogen,” says Elliott. Mature trees in these plots also produced more leaves, and despite the doubling of herbaceous plant material, the forest floor was unchanged, suggesting that the added herbaceous plant material decomposed very quickly.
In plots where the herbaceous biomass was removed, tree seedlings were far more likely to survive, suggesting that the herbaceous plants out-compete tree seedlings, most likely by limiting light. Even the red maples, which can tolerate more shade than other tree seedlings, had greater survival in plots where the herbaceous plant community was removed. “Our study shows that herbaceous plants affect tree seedling survival,” says Elliott. “However, they are an important part of the nitrogen cycle, and although rarely studied, play an important role in forest ecosystems.”
For more information, email Katherine Elliott at firstname.lastname@example.org.