Establishment Success and Growth of Southern Pine Plantations on Water and Nutrient-Poor Soils

Linda Benedict  |  6/28/2006 8:49:32 PM

Michael A. Blazier and Terry R. Clason

Forests of Louisiana managed for timber production constitute the state’s top agricultural crop, contributing an estimated $4.554 billion to Louisiana’s economy in 2005. Louisiana’s managed forests, which are often loblolly pine plantations, are among the world’s most productive forests at producing timber and fiber. Typical rates of return for loblolly pine plantation management are between 8 percent and 15 percent.

The potential for good returns on investment makes loblolly pine plantation management attractive to landowners. However, profitably establishing and managing loblolly pine plantations on the sandy, gravelly soils common in the upland areas of Louisiana can be problematic. These soils are poor at holding water and nutrients, so seedling mortality can be high, particularly in a droughty year, and the long-term growth of the surviving trees is often relatively low. Poor tree survival and slow growth reduce timber yields and economic returns of loblolly pine plantations.

The conventional management tactic for overcoming survival and growth problems on droughty sites is to plant extra seedlings, sometimes more than twice the number planted on better sites. The hope is that enough leftovers will remain to produce an adequate plantation. However, seedlings rarely die in an evenly distributed pattern. The resulting plantation often has sections of tightly-spaced, low-value trees and other sections with few trees. Thus, the tendency to plant higher numbers of seedlings on droughty sites negatively affects economic success because it increases planting costs and reduces the value of the surviving trees.

The feasibility of managing loblolly pine plantations on droughty, nutrient-poor soils can be improved by planting the same number of trees optimum for better soils – 300 to 600 trees per acre – and using management practices that increase the distribution of water and nutrients to crop trees. Crop trees can be given a head start by using herbicides near planting time to reduce woody and herbaceous vegetation that compete with trees for water and nutrients.

Containerized Seedlings
 Another tactic that can improve pine survival and growth on sandy and gravelly soils is the planting of containerized seedlings. This type of seedling, available at most nurseries, is planted with the root systems encased in a soil-filled plug (Figure 1A). Compared to the more conventionally planted bareroot seedlings (Figure 1B), which are planted with exposed root systems, the root systems of containerized seedlings are larger and healthier. A healthier root system may increase survival of seedlings during the sensitive first growing season on soils with poor waterand nutrient-holding capacity.

At the LSU AgCenter’s Hill Farm Research Station in northwest Louisiana, a research project was conducted to determine the pine survival and growth rates associated with various herbicides and seedling types on a soil with relatively low water- and nutrient-holding capacity. In 1993, a loblolly pine plantation was planted on a gravelly, fine sandy loam soil (classified as a Darley-Sacul soil). The effects of the herbicides applied near planting time on loblolly pine growth and survival were assessed with the four treatments listed in Table 1.

Seedling type effects were tested by comparing growth and survival of containerized seedlings to that of bareroot seedlings. All containerized and bareroot seedlings were of the same loblolly pine family. The family was selected because of its good growth potential on welldrained soils.

Among the herbicides tested, the HEXSULF treatment produced the highest tree survival rates from 1993 through 2003 (Figure 2). The HEXSULF treatment also promoted the greatest tree growth rates in the 10 years after planting among all the herbicide treatments. Overall, the mixtures of herbicides used in the HEXSULF and IMAZMET treatments produced better growth than the single-herbicide LOHEX and HIHEX treatments (Figure 3). Using mixtures of herbicides controls a wider array of underbrush species, which was particularly beneficial on this water- and nutrient-poor site. The herbicide mixtures used in the HEXSULF and IMAZMET treatments are widely available and used.

Survival Rates
Planting container seedlings had a profound effect on survival on this droughty site (Figure 4). The survival advantage of planting container seedlings was striking in the first year after planting, in which survival of container seedlings was 11 percent higher than that of bareroot seedlings. Survival of the container trees continued to be 10 percent to 15 percent higher than that of bareroot trees up through 2003.

This finding highlights the paramount importance of good root systems on such soils. Containerized seedlings survived well because they had a fully formed root system when planted. Their root systems were able to expand quickly and mine the soil for water and nutrients. By contrast, bareroot systems started with a partial root system. The extra time and energy required to form their root systems can make a difference between life and death on a water- and nutrient-poor soil.

The superior root systems that have likely given a survival advantage to container pines are also apparently helping them grow better than bareroot pines on this soil. The stems of containerized trees have persistently grown at a faster rate than those of bareroot trees (Figure 5). As of age 11, the combination of better tree growth and survival has led to wood volume yields per acre for the containerized trees that are 35 percent greater than those of the bareroot trees.

Although container seedlings cost twice as much as bareroot seedlings, the greater survival and growth of container seedlings can make them more economically advantageous to plant. Based on the estimated yields at this study site, the rate of return for planting the container seedlings may be up to 10 percent higher than for planting bareroot seedlings.

Michael A. Blazier, Assistant Professor, Hill Farm Research Station, LSU AgCenter, Homer, La.; and Terry R. Clason, State Forester, USDA Natural Resources Conservation Service, Alexandria, La. 

(This article was published in the spring 2006 issue of Louisiana Agriculture.)
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