Forest Project Looks at Mixing Species

SHORTER, Ala. — Why has a five-acre plot, crisscrossed with plantings of loblolly pine, sweetgum and native broomsedge attracted the attention of forest industry leaders nationally and internationally? Forestry researchers in Auburn University's Alabama Agricultural Experiment Station, who established the test planting, say the simplicity and practicality of the research site and the potential bonanza of useful information are reasons for the interest.

Each year private and commercial landowners in the South plant about 1.5 billion pine trees. Alabamians plant about 150 million pine seedlings annually. From these planting figures, forest industry experts project timber harvests 20 to 30 years into the future. When dealing with millions or billions of trees, even small things make big dollar differences. In Alabama, forestry is a billion dollar per year industry, and trees are the single largest cash crop in the state.

Dean Gjerstad, professor in the School of Forestry at Auburn, says the test planting at the E.V. Smith Research Center in Shorter is typical of millions of acres of land throughout the South. "Planting loblolly and slash pines in areas covered with native broomsedge is a common practice, yet we have little information as to how much competition the broomsedge provides and subsequently how much it slows down pine growth," Gjerstad notes.

"Competition from sweetgum is also a common occurrence throughout the South, yet we have relatively little information as to the amount of pine growth lost solely to this competition," the Auburn researcher said. "The reason we have limited information is because other herbaceous plants or hardwood species have not been eliminated in other tests," Gjerstad explained.

In the Auburn test, researchers eliminated all other herbaceous plants and trees by fumigating the test site with methyl bromide, then hand weeding any escapes. "We did this because our goal was to better understand the impact of competition between these three species without other weeds present, however, methyl bromide and hand weeding are too expensive to apply in practice," Gjerstad explained. In the plots, different densities of broomsedge and sweetgum, from one to 16 per square meter, (one square meter equals about 10 square feet). As expected, pine trees have reacted differently to the different densities of competition.

"We have miniaturized these plots to shorten the growth cycle from 20 years to the five-year length of the project, so the densities with which we are dealing are 20-25 percent that of actual commercial plantings," Gjerstad explained. In the first year of the test, plots containing four loblolly pine seedlings per square meter with no competition were over 10 times larger in both height and stem diameter than those containing 16 broomsedge plants. In plots with four pines and four sweetgums, the pine seedlings were as tall as pine without competition, but the diameter was less than half that of pines with no competition.

"We chose sweetgum and broomsedge because data from a multi-year study at 16 sites across the South indicated that broomsedge was the dominant herbaceous competition and sweetgum the dominant hardwood competition for pines," Gjerstad pointed out. Unfortunately, much of the land planted in pines in the South has broomsedge densities comparable to the highest (16 per square meter) rate in the Auburn test and/or more than four sweetgums per square meter, according to Gjerstad.

In the first year of data from the test, broomsedge densities of two per square meter restricted pine growth. In planted pine plantations, much of the broomsedge will die out in the third or fourth year, probably due to restricted light, but seedling growth will have been reduced often to a great degree. By the fifth or sixth year, sweetgum becomes the dominant competition. As pine seedlings compete for sunlight with sweetgum, pines grow tall, but don't develop sufficient foliage to provide adequate energy for maximum growth. How much growth this robs, and subsequently how much money it cost landowners, is currently only a guess.

"By the time we finish this project in 1994, we hope to have a clearer idea of the competition among sweetgum, broomsedge and pine seedlings. We hope to extend it to other competing species, so that growers will eventually be able to determine exactly when it is economically feasible to remove competition from pine seedlings," Gjerstad concluded.

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Contact Jamie Creamer, 334-844-2783 or jcreamer@auburn.edu
by Roy Roberson

2/17/92