Tree Nutrition and Forest Fertilization of Pine Plantations in the ...

Tree Nutrition and Forest Fertilization of Pine Plantations in the Southern United States

ABSTRACT

Thomas R. Fox, H. Lee Allen, Timothy J. Albaugh, Rafael Rubilar, and Colleen A. Carlson The growth of many pine plantations in the southern United States is limited by soil nutrient availability. Therefore, forest fertilization is a common silvicultural practice throughout the South. Approximately 1.2 million ac of pine plantations were fertilized in 2004. In the last 10 years, considerable advances have been made in identifying the ecophysiological basis for stand growth and the response to fertilizer additions. Nitrogen (N) and phosphorus (P) are the nutrients that most commonly limit growth of southern pine. On wet clay soils in the lower Coastal Plain and on some well-drained soil in the upper Coastal Plain, severe P deficiencies exist. On these soils, P fertilization with 25–50 lb of P per acre at the time of planting produces a large and sustained growth response, on the order of 50 ft3 ac⫺1 yr⫺1 (1.5 tn ac⫺1 yr⫺1) throughout the rotation. On most other soils in the South, chronic deficiencies of both N and P exist. On these sites, soil nutrient availability often is adequate early in the rotation when tree demand is small. However, around the time of crown closure, N and P frequently become limiting. Fertilization with both N and P in these intermediate aged stands typically increases growth for 8 –10 years. The growth response to a combination of 25 lb of P per acre plus 200 lb of N per acre averages around 55 ft3 ac⫺1 yr⫺1 (1.6 tn ac⫺1 yr⫺1) for an 8-year period. The amount of leaf area in the stand is the main factor determining the current growth rate of the stand and the potential growth response after fertilization. When stand leaf area index is less than 3.5, light capture by the stand is restricted and growth is negatively affected. In many of these stands, fertilization will increase leaf area because of increased soil nutrient availability and thus increase growth. The financial return after fertilization depends on the growth response that occurs, the cost of the fertilizer treatment, and the stumpage value of the timber produced. Using a growth response of 55 ft3 ac⫺1 yr⫺1 over 8 years, a fertilizer cost of $90 ac⫺1, and stumpage values from the first quarter of 2006, the internal rate of return from midrotation fertilization of a loblolly pine plantation with N and P would be approximately 16%. Keywords: loblolly pine, slash pine, forest productivity, leaf area, nitrogen, phosphorus

T

he southern United States produces more industrial timber than any other region of the world from a forest base that now comprises almost one-half of the world’s industrial forest plantations (Prestemon and Abt 2002). Currently, there are 32 million ac of pine plantations in the South (Wear and Greis 2002), predominantly comprised of loblolly pine (Pinus taeda L.) and, to a lesser extent, slash pine (Pinus elliottii Englemn.) Historically, forest landowners in the South have focused on minimizing per acre costs associated with plantation establishment and tending. This approach has resulted in millions of acres of pine plantations in the southern United States with growth rates averaging less than 166 ft3 ac⫺1 yr⫺1 (5 tn ac⫺1 yr⫺1). These growth rates are substantially lower than plantations in many other parts of the world (Yin and Sedjo 2001). However, theoretical models (Sampson and Allen 1999, Landsberg et al. 2001), empirical field trials (Allen and Lein 1998, Martin et al. 1999, Amateis et al. 2000, Jokela et al. 2000, Borders and Bailey 2001), and operational experience show that these growth rates are well below what is possible in the southern United States. With investment in appropriate intensive plantation silvicultural systems, growth rates exceeding 360 ft3 ac⫺1 yr⫺1 (10.8 tn ac⫺1 yr⫺1) are biologically possible, financially profitable, and environmentally sustainable for a broad range of site

types (Yin et al. 1998, Fox 2000). The greater inputs typically associated with intensive silviculture may increase per acre costs, but the resultant increases in production can substantially reduce production costs per ton of wood when variable (treatment) and fixed (land and annual management) costs are considered (Allen et al. 2005). Forest fertilization typically must be included in silvicultural regimes that are designed to increase plantation growth in the South (Allen 1987). Considerable research in tree nutrition and forest fertilization has been conducted in recent years. Results from this research have improved our understanding of the growth response that occurs after fertilization and the ecophysiological basis for this increased growth. This review provides a summary and synthesis of the current state of knowledge of tree nutrition and forest fertilization in the southern United States.

Modern Concepts of Ecophysiology and Tree Nutrition Much of the variation in biomass and wood production in forest plantations is caused by variation in light interception (Linder 1987, Cannell 1989, Landsberg and Gower 1997). Light interception is principally a function of the amount of leaf area in the stand (Figure 1). Differences in duration of leaf area display, individual tree crown

Received July 6, 2006; accepted September 29, 2006. Thomas R. Fox ([email protected]), Forest Nutrition Cooperative, Department of Forestry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. H. Lee Allen ([email protected]), Forest Nutrition Cooperative, Department of Forestry and Environmental Resources, North Carolina State University, Raleigh, NC 27695. Timothy J. Albaugh ([email protected]), Forest Nutrition Cooperative, Department of Forestry and Environmental Resources, North Carolina State University, Raleigh, NC 27695. Rafael Rubilar ([email protected]), Forest Nutrition Cooperative, Department of Forest Science, University of Concepcio´n, Concepcio´n, Chile. Colleen A. Carlson ([email protected]), Forest Nutrition Cooperative, Department of Forestry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. Copyright © 2007 by the Society of American Foresters.

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Figure 1. Relationship between annual volume growth and leaf area in southern pine plantations in the southeastern United States.

architecture, and stand canopy structure also can affect light interception. Empirical data from field studies with loblolly and slash pine (Vose and Allen 1988, Colbert et al. 1990, Albaugh et al. 1998) have shown that leaf area and, consequently, wood production are below optimum levels in most of the South (Figure 1). Low nutrient availability is a principal factor causing suboptimal levels of leaf area in many areas (Linder 1987, Vose and Allen 1988, Colbert et al. 1990, Albaugh et al. 1998, 2004). Low soil water availability, high vapor pressure deficits, and high temperatures also have been shown to adversely affect leaf area production and retention (Teskey et al. 1987, Benson et al. 1992, Hennessey et al. 1992, Pereira et al. 1994). Variation in growth efficiency, the stemwood produced per unit of leaf area, also can contribute to the variation in production (Figure 1). Growth efficiency can vary because of differences in photosynthetic efficiency, respiration, and partitioning to various biomass components. Much of the difference in growth efficiency in plantations is thought to be caused by differences in genetics of the trees planted (Li et al. 1991a, 1991b, McKeand et al. 2000). Climatic and soil factors that affect water and nutrient availability also alter growth efficiency (Rojas 2005). Improved nutrient and water availability has been shown to increase photosynthetic efficiency (Linder 1987, Murthy et al. 1996), and aboveground productivity proportionally more than belowground productivity in stand level studies (Gower et al. 1992, Haynes and Gower 1995, Albaugh et al. 1998, 2004). On an individual site basis, variation in growth efficiency generally does not affect productivity as much as changes in leaf area. However, when species such as loblolly pine are planted across large areas with substantial differences in precipitation and temperature, regional variation in growth efficiency may be substantial (Sampson and Allen 1999). What are the limitations to pine production in the southeastern United States? In many stands leaf area is low enough to limit the capture of light, resulting in poor productivity (Vose and Allen 1988, Albaugh et al. 1998, 2004, Jokela and Martin 2000). From a resource availability perspective, water availability, whether too little or too much, has been considered historically the principal resource limiting pine productivity in the South. This may be true for recently planted pine seedlings on many sites (Dougherty and Gresham 1988) and for specific soil types, such as very wet or very dry soils, throughout the rotation. However, recent analyses suggest that 6


Figure 2. Hypothesized relationship between soil N supply and a stand’s potential and actual use of N as related to stand development (age).

chronically low levels of available soil nutrients, principally nitrogen (N) and phosphorus (P), limit growth more in established stands than water (Albaugh et al. 1998, 2004, Sampson and Allen 1999, Jokela and Martin 2000). Low soil nutrient supplies reduce leaf area development and growth efficiency and, consequently, stand production. Empirical results from fertilization trials in loblolly and slash pine indicate that most nutrient limitations can be ameliorated easily and cost effectively with fertilization. Forest managers are now recognizing that intensive plantation silviculture is like agronomy; both the plant and the soil need to be actively managed to optimize production (Fox et al. 2004). Financially profitable options exist to increase production through the management of both genetic and site resources. The key to increasing leaf area and subsequent growth, and thereby achieving optimum value from plantation management, is to develop and implement site-specific silvicultural prescriptions. These prescriptions must account for the specific soil and site properties and local market conditions. Silvicultural prescriptions should consider planting high-quality seedlings from the best families of the desired species. Site preparation treatments that insure adequate survival and early growth of planted seedlings are essential. Fertilization usually is needed several times during the rotation to increase soil nutrient availability on most sites. Competing vegetation should be controlled throughout the rotation to maintain optimal resource availability. Thinning is used to manage stand density and provide adequate growing space for the desired crop trees. Improving stand nutrient supply through fertilization is a viable silvicultural option because of the widespread nutrient limitations in the South. Why are nutrient limitations so common? Nutrient limitations develop when a stand’s potential nutrient use cannot be met by soil nutrient supply (Allen et al. 1990). There is a large disparity between the levels of available N in the soil over a typical rotation and the potential uptake of N by loblolly pine stands (Figure 2). Soil N availability is high after harvesting and site preparation because these disturbances provide suitable conditions for rapid decomposition and release of N from the accumulated forest floor and slash material (Vitousek and Matson, 1985, Fox et al. 1986). For example, N mineralization rates of 98 lb ac⫺1 yr⫺1 were observed after harvest and site preparation in the Piedmont compared with mineralization rates of around 38 lb ac⫺1 yr⫺1 in a mature, uncut stand (Fox et al. 1986). Over time, soil N availability declines as the large pool of organic matter is decomposed and mineralized. Vitousek et

al. (1992) found that by age 5 years, N mineralization rates in a site-prepared Piedmont plantation had declined to the point where they were no different from those in a mature stand. These temporal dynamics of N mineralization produce a relatively short-lived pulse of available N in young stands (Figure 2). However, uptake N by loblolly pine typically follows a sigmoid-shaped pattern (Switzer and Nelson 1972, Wells and Jorgenson 1975), where N demand is out of phase with soil N availability (Figure 2). Uptake of N by loblolly pine seedlings is low because of their small size. As stand growth accelerates and tree size increases, use of nutrients increases rapidly. The N taken up from the soil in developing stands is sequestered within the accumulating forest floor and tree biomass (Miller 1981). Around the time of canopy closure, the environmental conditions conducive to high nutrient availability are no longer present (Piatek and Allen 1999, 2001). At this point in the rotation, a stand’s N requirement for maximum growth generally exceeds soil supply. Retranslocation of N from senescing foliage can supply only a portion of the N needed for continued stand growth (Wells and Jorgensen 1975), and soil N availability begins to limit leaf area production and growth. Similar temporal patterns of soil supply and tree demand exist with P on most soils in the South (Wells and Jorgenson 1975). Based on this pattern of tree nutrient demand and soil supply, N and P fertilization will be needed to sustain rapid growth on all but the most fertile sites. The majority of field trials in southern pine stands older than 5–10 years have shown strong responses following N and P fertilization (Martin et al. 1999, Amateis et al. 2000). An exception to the foregoing pattern of nutrient supply occurs on some soils in the South where severe P deficiencies exist (Jokela et al. 1991a). This occurs on many sites with poorly drained, clayey soils in the lower Coastal Plain (Gent et al. 1986) and on a limited number of sites with well-drained soils in the upper Gulf Coastal Plain (Leggett and Kelting 2006). Phosphorus availability in these soils is less than tree demand throughout the rotation and P deficiencies develop soon after the seedlings are planted. Large and sustained responses to P fertilization applied at the time of planting occur on these soils (Pritchett and Comerford 1982).

Current Fertilization Practices in the South Management of pine plantations in the southern United States is changing (Yin and Sedjo 2001). Intensive silvicultural regimes are now being implemented that include deployment of genetically improved planting stock, tillage, herbaceous and woody vegetation control, and multiple fertilizer applications (Allen 2001, Fox et al. 2004). The area being fertilized annually (Figure 3) is indicative of this change. In 1990, about 200,000 ac of pine plantations were fertilized whereas over 1.2 million ac were fertilized in 2004 (Forest Nutrition Cooperative 2005). P Fertilization at Time of Planting The benefits of P fertilization on poorly drained, clayey soils in the lower Coastal Plain have long been recognized (Pritchett el al. 1961). Volume growth gains averaging 40 –50 ft3 ac⫺1 yr⫺1 (1.2–1.5 tn ac⫺1 yr⫺1) throughout the rotation are typical on these P-deficient sites. The duration of response to a single application of 50 lb of P per acre P may last for 20 years or longer (Pritchett and Comerford 1982). Phosphorus fertilization on these very deficient sites may increase volume by more than 100% at the end of the rotation (Jokela et al. 1991a). Site index gains of 6 –10 ft are typical

Figure 3. Area of southern pine plantations annually fertilized in the southeastern United States.

when P is applied at or near time of planting on these soils (Gent et al. 1986). More recently, results from several trials have shown that large areas of well-drained soils in the upper Gulf Coastal Plain are also P deficient (Allen and Lein 1998). Growth response to P fertilization on these upland P-deficient sites is also large (Leggett and Kelting 2006). In intermediate-aged stands, typically, little response is observed to additions of P alone, except on the P-deficient sites described previously that were not fertilized with P at the time of planting. Midrotation stands on these P-deficient sites typically have very low foliar P concentrations and very low leaf areas. The sources of P fertilizer that typically are used include diammonium phosphate (DAP), triple superphosphate, and rock phosphate. DAP is now the most widely used source of P for fertilization at time of planting. Rates of application vary from 25 to 50 lb of P per acre, which is equivalent to 125–250 lb of DAP per acre. In 2004, about 200,000 ac were fertilized with P at or near time of planting (Figure 3). Applications of 200 lb of DAP per acre typically are used as a remedial treatment in midrotation stands on Pdeficient soils that were not fertilized with P at establishment. Fertilization of these midrotation stands is labeled as P in established stands in Figure 3. Identification of stands in need of early P fertilization can be based on landscape position, geology, and soil type. In the lower Coastal Plain of the Atlantic and Gulf Coasts, poorly drained, clayey Ultisols tend to be severely P deficient. These soils are classified as A group soils using the soil classification system developed by Fisher and Garbett (1980). Along the Gulf Coast, well-drained clayey-toloamy soils on the Citronelle and associated geological formations also have been found to be P deficient (Allen 1990, Allen and Lein 1998). Foliar and soil analysis also can be used to help identify P-responsive stands and sites (Wells et al. 1986). The critical value for soil P below which a fertilizer response is expected is 6 ppm based on the Mehlich-3 extraction procedure. Critical values for foliar P concentrations vary by species and range from 0.09% for slash pine to 0.11% for loblolly pine (Allen 1987, Jokela et al. 1991a). N and P Fertilization in Midrotation Stands A plantation’s potential to use N and P typically is greater than the available soil supply by age 5– 8 years, resulting in restricted leaf area development and growth (Allen et al. 1990). In stands growing on most of the soils in the South, both N and P usually become deficient around the time of crown closure. Consequently, the growth response after additions of N and P are much greater than SOUTH. J. APPL. FOR. 31(1) 2007

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appropriate timing and rates of fertilization. The projected leaf area of a fully stocked stand with basal area greater than 100 ft2 ac⫺1 should be 3.5 or greater. If the leaf area of a midrotation stand is less that this, the stand probably is in need of N and P fertilization, unless other obvious problems have altered leaf area (e.g., fire, ice damage, insect attack, or disease). The probability and magnitude of response are greater at lower leaf areas. Remote sensing techniques using Landsat satellite imagery have been developed that can accurately determine leaf area in southern pine stands (Flores et al. 2006).

Figure 4. Eight-year volume growth response to N and P fertilization in loblolly pine plantations aged 9 –16 years in the southern United States (Source: Adapted from Fox et al. 2004.)

the response to either N or P applied alone (Hynynen et al. 1998). Volume growth responses vary depending on the rates of N and P applied (Amateis et al. 2000) (Figure 4). Results from an extensive series of intermediate-aged fertilizer trials in loblolly pine stands established throughout the South indicate that growth gains averaging 50 ft3 ac⫺1 yr⫺1 (1.5 tn ac⫺1 yr⫺1) over an 8-year period occur after a one-time application of 200 lb of N per acre and 25 lb of P per acre (Hynynen et al. 1998, Amateis et al. 2000). Over 85% of the midrotation fertilizer trials established by the Forest Nutrition Cooperative in the South responded to additions of N and P during midrotation (Rojas 2005). However, growth responses vary among sites, ranging from over 100 ft3 ac⫺1 yr⫺1 (3 tn ac⫺1 yr⫺1) on some sites to less than 10 ft3 ac⫺1 yr⫺1 (0.3 tn ac⫺1 yr⫺1) on other sites. The large increase in the area fertilized annually in the South has been caused by primarily the large increase in fertilization of intermediate-aged stands (Figure 3). A prescription of between 150 and 200 lb of N per acre plus 25 lb of P per acre is used for loblolly and slash pine on most sites. Typically, this treatment is applied as a mixture of DAP to supply P at the desired rate plus urea to provide the additional N required to reach the desired N rate. Because the response to midrotation N and P fertilization is relatively shortlived, lasting only 8 –10 years, repeated applications of N and P fertilizer are needed to maintain optimal leaf area and stand growth. The size and duration of the growth response to these repeated N plus P treatments is similar to that from the first midrotation application (Amateis et al. 2000). The diagnostic techniques for identifying intermediate-aged stands that will be biologically responsive to fertilization recently have undergone substantial revision. In the past, stand attributes such as basal area and site index (Duzan et al. 1982), foliar concentrations (Colbert and Allen 1996), and experience were used together to identify responsive stands and prescribe the appropriate elements and rates to apply (Allen 1994). Critical values for foliar N of 1.2% for loblolly pine and 1.1% for slash pine were used frequently to guide fertilizer decisions (Comerford and Fisher 1984). However, the ability of these diagnostic tools to accurately predict the responsiveness of an individual stand is limited. Recently, this situation has changed with the application of research that quantifies the linkages among stand productivity, leaf area, and nutrient availability (Vose and Allen 1988, Albaugh et al. 1998). Differences between a stand’s current leaf area and its potential leaf area now can be used to estimate responsiveness to nutrient additions and the 8


Nutrient Deficiencies Other than N and P There are sites in the South were nutrients other than N and P limit growth. On these sites, there may be little response to N and P fertilization even when leaf area is low. Deficiencies of nutrients other than N and P have been reported at scattered locations in the South and appear to be restricted to certain soil types that occur in specific geographic locales. Deficiencies of potassium (K), calcium (Ca), boron (B), copper (Cu), and manganese (Mn) appear to occur more frequently on highly weathered, sandy soils in the Coastal Plain. Copper deficiencies in slash pine recently have been observed on a sandy Spodosol in Georgia (South et al. 2004). Jokela et al. (1991b) observed significant growth response after fertilization with Mn on a Spodosol in Florida. Kyle et al. (2005) recently documented a long-term growth response to Ca in loblolly pine on a site in the Coastal Plain of Virginia. Based on the agronomic model of plant nutrition, it is likely that once the deficiencies of N and P are corrected, growth will be limited by other nutrients (Marschner 1986). For example, in a loblolly pine stand on site in southeast Georgia, the 8-year growth response after application of N and P was around 50 ft3 ac⫺1 yr⫺1 (1.5 tn ac⫺1 yr⫺1). However, an application of N and P combined with K and micronutrients resulted in a much larger growth response over the same period, averaging around 100 ft3 ac⫺1 yr⫺1 (3.3 tn ac⫺1 yr⫺1). It is likely that deficiencies of nutrients such as K, B, Cu, and other micronutrients will occur more frequently in the future as elite genotypes are more widely deployed and increased management intensity increases the growth rate and, consequently, the nutrient demand of pine plantations (Allen et al. 2005). Additional research will be needed to identifying sites where nutrient deficiencies other than N and P exist and then to develop the appropriate prescriptions to ameliorate these nutrient limitations. Fertilization Interactions with Other Silvicultural Treatments The growth potential of southern pines planted in the southeastern United States is much higher than commonly thought just a few years ago (Borders and Bailey 2001). The challenge now is to develop and implement the appropriate silvicultural systems to realize this potential in a cost-effective and environmentally sustainable manner. Several opportunities for improving plantation growth and value through the management of site resources are available to foresters in the South including genetic improvement, intensive site preparation, weed control, and fertilization (Colbert et al. 1990, McKeand et al. 1997, Allen and Lein 1998, Jokela et al. 2000). The ability to acquire and use nutrients differs among species with slash pine having higher nutrient use efficiency than loblolly pine (Colbert et al. 1990). Consequently, loblolly pine generally is more responsive to intensive culture than slash pine. Genotype x silviculture interactions are not common in southern pine. Better genetic

families grow better families than poorer families at all levels of silvicultural input, although the differences between good and poor families are greater when stands are managed intensively (McKeand et al. 1997). Weed control and fertilization can dramatically increase the growth of loblolly and slash pine (Jokela et al. 2000). Where two silvicultural treatments affect nutrient availability and the allocation of nutrients to the crop trees in a similar manner, less than additive effects may be observed. For example, where both intensive vegetation control and fertilization result in greater nutrient supply, a less than additive response has been observed with both slash pine on wet sandy sites and with loblolly pine on well-drained upland sites (Albaugh et al. 2003). Without control of competing hardwoods, the growth response of pine crop trees may be less after fertilization because the hardwoods also respond to fertilizer, resulting in increased competition for light and water (Amishev and Fox 2006). Diameter growth typically increases after thinning (Amateis et al. 1989). However, because leaf area development in many loblolly pine stands is more limited by nutrient availability than by light (Vose el al. 1994, Albaugh et al. 2004), the growth response after thinning without fertilization may be relatively small. Fertilization can increase nutrient availability and the amount of leaf area of the crop trees and lead to faster growth rates in the thinned stand. Therefore, fertilization if combined with thinning may increase the growth response of the high-value crop trees (Sword-Sayer et al. 2004). Another reason to combine thinning with fertilization is that stand level response to N and P fertilization is typically small in stands when basal area is greater than 140 ft2 ac⫺1 because there is little room for crown expansion and increased leaf area production (Duzan et al. 1982). Fertilization in stands with high basal area actually can increase mortality of the intermediate and suppressed trees, which decreases net volume growth in the stand. Thinning reduces the stand basal area, providing more growing space to the residual crop trees so that they can respond to the increased nutrient availability after fertilization.

Financial Returns from Forest Fertilization Although most plantations in the South respond to fertilization with some combination of N, P, or other nutrients, financial returns depend on magnitude of the growth response obtained, the product mix in the stand, stumpage prices, cost of fertilization, and the length of time before harvest. Discounted cash-flow analysis can be used to determine the financial returns from fertilization (Clutter et al. 1983). Net present value (NPV) is a commonly used financial criterion that is defined as ⫺ cost 冘 revenue (1⫹i)

n⫽t

NPV ⫽

t

n⫽1

where i is the discount rate, and t is the time of the investment. When the NPV is set to 0 and the equation is rearranged, it can be solved iteratively for i, which is defined as the internal rate of return (IRR) of the investment, another commonly used financial criterion to evaluate silvicultural investments (Clutter et al. 1983). The effect of fertilization on financial returns can be calculated in two ways. The fertilizer costs and the additional revenue from that treatment can be isolated and analyzed on a marginal basis or all costs and revenues that occur throughout the rotation can be included and analyzed in an integrated manner. Because fertilization increases stand growth, it also can decrease the length of a rotation, which can

significantly effect financial returns, particularly when higher discount rates are used. A marginal analysis that isolated the fertilizer costs and returns was conducted to illustrate the financial returns that typically occur after fertilization. Fertilizer costs were obtained from The Market, a fertilizer industry newsletter (2006). In May 2006 the cost of urea was approximately $225 tn⫺1 and DAP was approximately $270 tn⫺1. Therefore, the cost of a fertilization treatment adding 200 lb of N per acre plus 25 lb of P per acre (125 lb ac⫺1 DAP ⫹ 385 lb ac⫺1 urea) would be approximately $90 ac⫺1 including an application cost of $30 ac⫺1. We assumed a fertilizer response of 50 ft3 ac⫺1 yr⫺1 (1.5 tn ac⫺1 yr⫺1) over an 8-year response period. The growthand-yield model FASTLOB (Amateis et al. 2001) was used to model the effect of fertilization on the volume growth and diameter distribution of an unthinned loblolly pine stand with a site index of 65 ft (base age, 25 years) and a planting density of 550 trees ac⫺1. Yield at age 22 years in the unfertilized stand was predicted to be 64 tn ac⫺1 of pulpwood, 38 tn ac⫺1 of chip-n-saw, and 14 tn ac⫺1 of sawtimber. Yield at age 22 years in the stand fertilized at age 14 years was predicted to be 66 tn ac⫺1 of pulpwood, 40 tn ac⫺1 of chip-n-saw, and 22 tn ac⫺1 of sawtimber. Using southwide average stumpage prices for the first quarter of 2006 obtained from TimberMart South (2006) of $7 tn⫺1 for pulpwood, $25 tn⫺1 for chip-n-saw, and $40 tn⫺1 for sawtimber, the total stumpage value of the unfertilized stand was $2,325 ac⫺1 and the stumpage value of the fertilized stand was $2,712 ac⫺1. Thus, fertilization increased revenue by $387 ac⫺1 and provided an IRR of 19%. The silvicultural regime used in a stand, including factors such as the genotype planted, site preparation treatments used, weed control applied, and thinning regime used, will influence the growth response after fertilization. Changes in the cost of fertilizer and the stumpage value of the timber produced also will alter the financial returns. Fertilizer costs are determined by worldwide supply and demand. The cost of urea in the southern United States has varied from a low of around $ 90 tn⫺1 in the summer of 1999 to a high of around $275 tn⫺1 in the spring of 2005. The price of DAP has varied to a similar degree over this time frame. Stumpage prices also vary depending on local market conditions. Consequently, financial returns from fertilization will vary regionally over time in the South. Table 1 summarizes the effect of changes in fertilizer cost, growth response, and pine stumpage value on the rate of return from an investment in forest fertilization. When the growth response after fertilization is less than average (less than 40 ft3 ac⫺1 yr⫺1 or 1.25 tn ac⫺1 yr⫺1), and stumpage price of the wood produced is low (less than $20 ton⫺1), the IRR after a $90 ac⫺1 investment in forest fertilization is negative. In contrast, the rate of return is 16% and greater after the same investment when an average growth response (50 ft3 ac⫺1 yr⫺1 or 1.50 tn ac⫺1 yr⫺1) occurs and mean stumpage value is $25 ton⫺1 or higher. This analysis indicates that investments in fertilization that produce low-value pulpwood produce poor financial returns, regardless of the growth response that occurs. In contrast, fertilization treatments that increase the amount of higher-value sawtimber produced in a stand usually will generate substantially better financial returns. Summary The growth of many pine plantations in the southern United States is limited by soil nutrient availability, which limits leaf area production. The amount of leaf area in the stand is the main factor determining the current growth rate of the stand. When stand leaf SOUTH. J. APPL. FOR. 31(1) 2007

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Table 1. Effect of stumpage price and 8-yr growth response after fertilization on the IRR from a $90/ac investment in forest fertilization (assumes 1 tn/ac ⴝ 33.3 ft3/ac). Growth response tn/ac/yr

Composite stumpage price ($/tn)

3

ft /ac/yr

$5

$10

$15

$20

17 25 33 42 50 58 66 75

⫺17% ⫺13% ⫺10% ⫺7% ⫺5% ⫺3% ⫺1% 0%

⫺10% ⫺5% ⫺1% 1% 4% 6% 7% 9%

⫺5% 0% 4% 7% 9% 11% 13% 15%

⫺1% 4% 7% 10% 13% 15% 17% 19%

$25

$30

$35

$40

1% 7% 10% 14% 16% 19% 20% 22%

4% 9% 13% 16% 19% 21% 23% 25%

6% 11% 15% 19% 21% 24% 26% 28%

7% 13% 17% 20% 23% 26% 28% 30%

IRR 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25

area index is less than 3.5, light capture by the stand is restricted and growth is negatively affected. Fertilization can increase soil nutrient availability, which will in turn increase leaf area of the stand. Nitrogen and P are the nutrients most commonly limiting growth of pine plantations in the South. On wet clay soils in the lower Coastal Plain and on some well-drained soil in the upper Coastal Plain, severe P deficiencies exist. On these soils, P fertilization with 25–50 lb of P per acre at the time of planting produces a large and sustained growth response, on the order of 50 ft3 ac⫺1 yr⫺1 (1.5 tn ac⫺1 yr⫺1) throughout the rotation. On most other soils in the South, chronic deficiencies of both N and P exist. On these sites, soil nutrient availability often is adequate early in the rotation when tree demand is small. However, around the time of crown closure, N and P frequently become limiting. Fertilization with both N and P in these intermediate ages stands typically increases growth for 8 –10 years. The growth response to a combination of 25 lb of P per acre plus 200 lb of N per acre averages around 55 ft3 ac⫺1 yr⫺1 (1.6 tn ac⫺1 yr⫺1) for an 8-year period. The financial return after fertilization depends on the growth response that occurs, the cost of the treatment, and the stumpage value of the timber produced. Using the mean growth response and the costs and stumpage values from the first quarter of 2006, the internal rate of return from midrotation fertilization of a loblolly pine plantation with N and P would be approximately 16%.

Literature Cited ALBAUGH, T.J., H.L. ALLEN, P.M. DOUGHERTY, L.W. KRESS, AND J.S. KING. 1998. Leaf-area and above- and belowground growth responses of loblolly pine to nutrient and water additions. For. Sci. 44:317–328. ALBAUGH, T.J., H.L. ALLEN, B.R. ZUTTER, AND H.E. QUICKE. 2003. Vegetation control and fertilization in midrotation Pinus taeda stands in the southeastern United States. Ann. For. Sci. 60:619 – 624. ALBAUGH, T.J., H.L. ALLEN, P.M. DOUGHERTY, AND K.H. JOHNSEN. 2004. Long term growth responses of loblolly pine to optimal nutrient and water resource availability. For. Ecol. Manage. 192:3–19. ALLEN, H.L. 1987. Forest fertilizers: Nutrient amendment, stand productivity, and environmental impact. J. For. 85(2):37– 46. ALLEN, H.L. 1990. Manipulating loblolly pine productivity with early cultural treatment. P. 301–317 in Sustained productivity of forest soils. Proceedings of the 7th North American forest soils conference, Gessel, S.P., D.S. Lacate, G.F. Wweetman, and R.F. Powers (eds.). Univ. of British Columbia, Faculty of Forestry, Vancouver, B.C., Canada. ALLEN, H.L. 1994. Enhancing southern pine productivity with fertilization. The Consultant 1994:12–17. ALLEN, H.L. 2001. Silvicultural treatments to enhance productivity, Chap. 6. in The forests handbook, Vol. II, Evans, J. (ed.). Blackwell Science, Ltd., Oxford, UK. 382 p. ALLEN, H.L., P.M. DOUGHERTY, AND R.G. CAMPBELL. 1990. Manipulation of water and nutrients—practice and opportunity in southern U.S. pine forests. For. Ecol. Manage. 30:437– 453.

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