SILVICULTURE Figure 2: Tree survival rates


return for loblolly pine plantation management are between 8-15%, according to the Texas Cooperative Extension. The feasibility of managing loblolly pine plantations on droughty, nutrient-poor soils can be improved by planting the same number of trees that is optimum for better soils (740 to 1,240 trees/ha) and using management practices that increase the distribution of water and nutrients to crop trees. Crop trees benefit from application of herbicides near planting time to reduce woody and herbaceous vegetation that compete with trees for water and nutrients. Another tactic that can improve pine survival and growth on sandy and gravelly soils is the planting of containerised 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 bareroot seedlings more conventionally planted in the Southeastern US region (Figure 1B), which are planted with exposed root systems, root systems of containerised seedlings are larger and healthier. A healthier root system may increase survival of seedlings during the sensitive first growing season on soils with poor water- and nutrient-holding capacity. At the LSU AgCenter’s Hill Farm


Research Station in Northwest Louisiana, a research project was completed 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 that was classified as a US Department of Agriculture Natural Resource Conservation Service Darley-Sacul series. The effects of 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


containerised seedlings to that of bareroot seedlings. All containerised and bareroot


seedlings were of the same loblolly pine family; the family was selected due to its good growth potential on well-drained soils.


Herbicides


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 suppressed 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 in forest management in the Southeastern US


Figure 3: Tree volumes


Table 1. Herbicide treatments applied to a loblolly pine plantation near the time of planting on a droughty soil in northern Louisiana US


Treatment


LOHEX HIHEX


HEXSULF Herbicides*


hexazinone hexazinone hexazinone,


sulfometuron methyl


Application rate (kg active ingredient per hectare)


2.24 4.48 2.24


0.10


IMAZMET** imazapyr, 0.84 metsulfuron methyl


0.04


Using mixtures of herbicides suppressed a wider array of underbrush species, which was particularly beneficial on this water and nutrient-poor site


Seedling type


Planting container seedlings had a profound effect on survival on this droughty site (Figure 4). The survival advantage of planting


58 International Forest Industries | AUGUST/SEPTEMBER 2011


container seedlings was striking in the first year after planting, in which survival of container seedlings was 11% higher than that of bareroot seedlings.


Survival of the container trees continued to be between 10-15% higher than that of bareroot trees up through 2003. This finding highlights the paramount importance of good root systems on such soils. Containerised seedlings had relatively high initial survival and growth 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 apparently made a difference between life and death on this 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 containerised trees persistently grew at a faster rate than those of bareroot trees (Figure 5). As of age 10, the


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