RESEARCH SUMMARY: Regionwide Trials | Basic Research | Software | Future Direction

The foundation of our research continues to be field studies examining the effects of silvicultural treatments applied during stand establishment (e.g., soil tillage, vegetation management, fertilization) and in established stands (e.g., thinning, fertilization, woody vegetation control) on stand growth and nutrition. Since 1970, FNC members have established over 350 installations of 19 “regionwide” studies in the southeast US, Argentina, Chile, and Colombia and almost 100 are still active today. Studies are focused on a few important questions that are more effectively answered by members cooperating rather than competing. Most studies examine several silvicultural treatments providing an opportunity to better understand interactions among treatments. All studies include multiple installations across a range of soil/site types where treatments are replicated at least twice on each site. Data from these studies provide the basis for estimating responses to silvicultural treatments, developing prescription guidelines, and parameterizing growth models. Up-to-date results are available on the members website. STUDIES AT PLANTATION ESTABLISHMENT The Regionwide 7 study examines the influence of site preparation, fertilization, weed control, and their interactions, on pine survival, growth, and nutrition. Trials of this study series have been established with loblolly pine (Pinus taeda) in the Southeast USA (late 1970’s) and Argentina (2002) and with Pinus radiata in Chile (2000). Each trial is a replicated split-plot design with two levels of site preparation (typically with and without soil tillage) as main plot effects and a weed control (none and weed control for the first two years) and fertilizer (none and 250 lbs/acre of diammonium phosphate (DAP) at time of planting) factorial as subplot effects. Measurements have been completed on loblolly pine trials in the Southeast US providing stand growth data for 20 or more years. Trials in Argentina and Chile are still active. Key Regionwide 7 results: Early cultural treatments dramatically accelerate the early growth of individual trees and, consequently, shorten the time required to achieve full site occupancy on most sites. Stand uniformity is significantly improved and tree mortality is significantly reduced with intensive culture. For loblolly pine trials in the Southeast US, dominant height responses to early cultural treatment ranged up to 6.5 feet at the end of the rotation. Volume growth gains averaged 860 ft3/acre and ranged up to 1740 ft3/acre. Gains due to P fertilization continued to increase throughout the rotation on many sites; however, gains due to weed control generally began to diminish after year 10 but were still evident at 20 years. Strong responses to P additions at two trials suggested that well-drained Gulf Coastal Plain sites may also be P deficient. On the more fertile and well structured soils in Argentina and Chile, significant growth responses have only been observed with weed control. To better understand the strong P responses found on two Gulf Coastal Plain Regionwide 7 studies, the Regionwide 14 study was developed to identify upland P deficient sites in the Southeast US and to examine the need for K additions on these sites. Trials were established at time of planting or in young plantations. The study design consisted of six treatments including a control and several combinations of N, P, and K and included different material sources for each nutrient. All treatment plots received operational first-year vegetation control treatment. A total of 21 trials were established principally in the Gulf Coastal Plain. Key Regionwide 14 results: Strong responses to early P fertilization have been observed on many sites on the Citronelle and associated terraces in the Gulf Coastal Plain. Gains averaging over 500 ft3/acre are typical. With weed control, DAP proved to be the best P source with responses to DAP equal to, or exceeding, responses to triple super phosphate (TSP) on all sites. Responses to K additions were minimal on most sites. A slash pine stand in the flatwoods of Georgia did show a strong additional response to K when added with DAP. In the 1990´s, the use of subsoiling and combination plowing on well drained sites dramatically increased in response to highly visual improvements in survival and early growth of pine plantations on several sites, however, very little data were available to support the wide spread use of tillage on upland sites. To fill this data gap, the Regionwide 16 study was developed to examine the effects of surface and subsurface tillage on pine survival, pine growth, and soil physical properties, and to identify what well drained sites types would be candidates for tillage treatments. The study’s experimental design consisted of a replicated factorial treatment matrix of surface and subsurface tillage with all plots receiving weed control and fertilization. Key Regionwide 16 results: The FNC six-year height performance standard for loblolly pine in the Southeast US (22 feet) was met or exceeded on weed and feed no-till controls in four of seven trials. Six-year height averaged 21 feet with a maximum of 29 feet. Six-year height response to subsoiling averaged less than 0.5 feet. The maximum response to subsoiling was 2.0 feet. Other growth parameters had similar patterns of response. Six-year height growth responses to combination plowing were equal to or better than the other tillage treatments on the three sites where it was tested. The maximum response to combination plowing was 3.0 feet, and occurred on a sandy clay loam with siliceous mineralogy. Height growth with tillage followed a Type B response, with the maximum response occurring during the first two years. Little further gain is expected beyond six years. ESTABLISHED STAND STUDIES In the mid 1980´s, the Cooperative established a large N x P factorial fertilizer study (Regionwide 13) to reduce the uncertainty concerning the value of fertilizing midrotation loblolly pine plantations in the Southeast US. Over 40 installations were established in moderately stocked, midrotation, site prepared loblolly pine plantations. The results from these trials were instrumental in quantifying the relationships among growth, nutrition, and fertilizer response in midrotation loblolly pine plantations. Since early results from the Regionwide 13 were reported in 1988, fertilization of established pine stands in the Southeast US increased from 65,000 to 1.2 million acres per year. N and P are not the only nutrient elements limiting plantation productivity. Since the mid 1990´s, the Cooperative has installed field trials (Regionwide 13 and 15) to investigate what other nutrients in addition to N and P may limit growth in midrotation plantations. Trials have been installed in plantations of loblolly pine in the Southeast US, Pinus radiata in Chile, and Eucalyptus grandis in Colombia. Three or four replicates of a core set of treatments including a control, and combinations of N, P, K, and micronutrients have been installed at each location. Key Regionwide 13 and 15 results: Pine plantations in the flatwoods of SE Georgia and NE Florida have responded dramatically to additions of K and/or micronutrient when they were added with N and P. Although boron is suspected as the principal micronutrient limiting growth on most sites that have been responsive to micronutrient additions, additional work is underway to ascertain what micronutrient(s) are actually needed. Early results from Chile indicate that thinned midrotation Pinus radiata stands on granitic and metamorphic soils will be responsive to additions of N, P, and B when the fertilizer is applied in the fall at the beginning of the rainy season. Midrotation pine stands typically have varying levels of hardwood and brush vegetation. Fertilization and/or vegetation control are often used to increase resource availability in midrotation stands but few data are available that quantify the growth responses to one-time applications of vegetation control alone or in combination with fertilization. The Cooperative’s Regionwide 17 study was developed to address several questions related to vegetation control and fertilization including: 1) what is the magnitude and duration of response to vegetation control, 2) what quantity and composition of competing vegetation limit pine production, 3) to what degree does competing vegetation limit pine response to fertilization, 4) do vegetation control and fertilization substitute for one another, and 5) how much does the competing vegetation benefit from fertilization? The core design consists of four replicates of a 2 x 2 factorial of vegetation control (none and one-time application) and fertilization (none and one-time application). Detailed assessments are made of growth and nutrition for pine and the composition and quantity of other vegetation. Thirteen trials have been installed in the Southeast US, ten in loblolly pine plantations with hardwoods as the primary competing vegetation and three in slash pine (P. elliottii), with evergreen shrubs as competing vegetation. Key Regionwide 17 results: Pine volume growth has been significantly increased by both fertilization and vegetation control on most sites. Growth responses to fertilization occurred immediately whereas responses to vegetation control were not evident on most sites until the fourth year after treatment. Vegetation control provided 70% or greater reduction in the dominant competing vegetation (hardwoods and/or shrubs) on most sites; however, after 6 years woody shrubs and brush have begun to reoccupy many of the vegetation control plots. Response to fertilization was slightly reduced with increasing levels of hardwood basal area. Fertilization typically did not significantly increase the levels of competing vegetation and fertilization did significantly reduce specific types pf competing vegetation through shading at some sites. Given the strong responses of many midrotation plantations to fertilization and our observations that significant nutrient limitations may develop by canopy closure, fertilization of young stands may be an attractive investment opportunity. The Cooperative’s Regionwide 18 study was designed to quantify the rates and frequencies of nutrient application at which growth and fertilizer use efficiency are optimized throughout a rotation. The experimental design includes an incomplete factorial of nutrient dose (0 to 240 lbs/acre of N) and frequency (.5 1, 2, 4, and 6 years depending on species) applied in young plantations. P, K, and B additions are also included as a fixed proportion of the nitrogen dose. A total of 37 trials have been established, 27 in the Southeast US and two in Argentina with loblolly pine, and eight with Eucalyptus grandis in Colombia. Key Regionwide 18 results: Fertilization significantly increased growth at the majority of sites with responses exceeding 100% at several locations where N, P, K, and B were added. Volume growth gains in loblolly pine plantations in the Southeast US averaged 200 ft3/acre with a maximum response of over 600 ft3/acre during the first four years after fertilization. The volume responses found on the juvenile loblolly pine Regionwide 18 stands were similar to responses found in our midrotation loblolly pine studies. Although N and P were added at the same relative rates, foliar P concentrations typically did not increase to the same extent as foliar N concentrations. However after four years, the N:P ratios did not exceed 12 (except for one site) even at the highest N addition rates. Four-year volume growth response was linear to the 180 rate with modest incremental response at higher rates. Optimum nutrient addition rates will depend on the desired magnitude and duration of response. One-half of the studies showed significant frequency or frequency x dose effects on four-year volume growth; however, no clear trends were apparent as some sites exhibited higher responses for the two year application frequency whereas others had better response to the four year application frequency.