Alabama A&M and Auburn Universities Site Index for Longleaf Pine ANR-1424

ave you ever looked at trees in your woodlot managers use site quality estimates in forest growth Hand thought, “I wish I could see into the future and yield models: to know when my trees will be ready to harvest”? • To determine the present and future growth and Most of us have probably wished for the ability to productivity of forest stands predict the future to help us plan a timber sale or • To determine forest product sizes and values at a other land-management activity. What you may not re- given age alize is that there are tools used by land-management • To justify land investments (and divestments) professionals that can estimate future tree and stand • To provide a frame of reference for land-manage- growth. One of these tools is site index, which is an ment prescriptions such as timing of thinning or important forest measurement that is used to predict pruning, type of site preparation, and tree species trends in future height growth of forest trees. selection What influences the growth of forest trees? How do we assess forest site quality? The amount of wood that can be grown on an area of There are different ways land managers can assess land depends on many factors, such as species, age, site quality, but the most commonly used method number of trees, length of rotation, and land quality. is site index (SI), which is estimated by measuring The production of wood can be affected by adjusting heights and ages of standing trees in a forest. This some of these factors. For example, periodic thinning measurement is used because tree volume production to reduce the number of trees in an area can increase potential is positively related to height growth. Finally, tree diameter growth rates. it is easy to measure the age and height of trees when conducting a forest inventory. Some factors, such as land quality, cannot be changed. For timber management, land quality, often referred to by land managers as site quality, can be defined as the timber production potential of a site for a particular tree species or forest type. Although site quality is consistent at a given location, it has meaning for only one species at a time. The words good and poor are often used when discussing site quality, which simply implies high or low timber-production potential; therefore, a tract of land may be of good site quality for longleaf pine (Pinus palustrisARCHIVE) but of poor quality for green ash (Fraxinus pennsylvanica). Why do we assess forest site quality? The proper assessment of site quality is very important in forest land management because it is the key to estimating or predicting what forest growth will be Figure 1. An increment borer can be used to determine tree on a site. The following are some of the ways land age at 4.5 feet above the ground by taking a small sample from the tree. (Photo credit: John Gilbert)

www.aces.edu Figure 2. Rings are then counted from the center of the tree core (noted by the arrow on the left) to the outer edge (noted by the arrow on the right) to determine tree age. (Photo credit: John Gilbert) Total age is estimated either from stand establishment darker-colored ring. It is common practice to add 3 to records or by using an increment borer to obtain 4 years for loblolly pine and 7 years for longleaf pine. a wood sample from the tree at 4.5 feet above the Total heights of dominant and codominant trees ground (figure 1). The number of annual rings are are used to estimate site index. Dominant trees are counted, and then years are added to the count based described as trees that have crowns that are taller on species for the time it took the tree to reach 4.5 than the surrounding trees and are receiving sunlight feet tall (figure 2). An annual ring is made up of two from above and from the side (figure 3). Codominant parts, earlywood, a light-colored ring, and latewood, a ARCHIVE

Figure 3. Forest tree crown classes. D = Dominant trees, C = Codominant trees, I = Intermediate trees, O = Overtopped or suppressed trees. (Graphics credit: Kelly Knowles)

2 Alabama Cooperative Extension System Site Index for Longleaf Pine 3 Mature

Stem elongation

Grass stage seedlings

Figure 4. Longleaf pine stand with trees at various life stages: grass stage, stem elongation, and mature. (Photo credit: John Kush)

trees are similar to dominant trees, but the tree crown stands, competition from other seedlings or vegeta- receives sunlight from above only, not from the sides. tion, insect and disease incidence, and climatological These trees usually make up the majority of the forest factors. Silvicultural practices that reduce competition canopy. This measurement is most associated with and improve soil conditions can reduce duration of the even-aged forests, or forests in which the trees in a grass stage. The incidence of brown-spot needle blight stand are all the same age. Site index is usually consid- caused by the fungus Mycosphaerellaceae dearnessii ered most reliable when based on measurements of M.E. Barr, which varies by geographic location, can trees that are 20 years old or older. also increase the number of years a seedling remains in the grass stage. Once heights and ages have been measured, those values are used in an equation, table, or graph to The accurate description of early height growth in determine site index. The following is an example of longleaf pine is becoming more important as increasing how site index can be estimated for longleaf pine. numbers of landowners implement longleaf pine management and restoration activities on their lands. Estimating site index for longleaf pine Site index estimation is key to the comparison of Longleaf pine is managed as both natural-seeded and management options because active management can planted stands in the southeastern U.S. Regardless of increase growth, and, therefore, this species’ value to the regeneration method, determining early height landowners. However, because of the unpredictable ARCHIVEnature of longleaf pine’s early growth, the age of a development is complicated by the grass stage period, during which no height growth occurs (figure 4). The given tree can be difficult to determine. An error of 1 length of time between germination and the start of year in time of emergence could result in a 2- to 4-foot seedling height growth is usually 4 to 5 years but may error in height estimates, and in turn cause errors in range from 1 to 20 years and is followed by a period of site index estimation that could over- or underestimate rapid height growth (figure 4). future stand growth. One way around this problem is to reference growth using tree ring count alone at 4.5 Emergence from the grass stage can be unpredictable feet above the ground (breast height age) in addition to and may depend on seedling quality for planted stand age. 2 Alabama Cooperative Extension System Site Index for Longleaf Pine 3 An updated site index model but you want to know what future height growth for the existing trees might be like. Last month, you hired a A new site index model (Lauer and Kush 2010) has consulting forester to inventory your 20-acre stand. As been developed for even-aged longleaf pine stands part of that inventory, ten trees were sampled for total that accounts for variation in tree age and length of the height and age. grass stage period using 45 years of height measure- ments from the Regional Longleaf Pine Growth Study Tree age was estimated by boring the trees at 4.5 feet (RLGS). The RLGS monitors growth of natural even- above the ground and counting the rings. The results of aged longleaf stands and is being used to develop new that sample are illustrated in column A of table 1. growth and yield models for longleaf pine. An estimate To estimate site index on your stand, you must first of site index is required to use these new models. determine the total age of your trees. Seven years For this current site index model, site index of even- should be added to the ring count at 4.5 feet to get the aged longleaf pine stands can be estimated using actual total tree age in years (column B, table 1). For example, stand age if it is known. Or, if the actual age of the tree number 1 had a ring count of 23 years plus 7 stand or tree is not known, ring count at 4.5 feet above years, for a total tree age of 30 years (column C, table the ground plus 7 years added to that ring count can 1). This process is repeated for all trees sampled. be used. For example, if the measured ring count was Based on your calculations, the average age of this 23 years plus 7 years for the tree to reach 4.5 feet, then stand is approximately 30 years, with an average height site index would be estimated using a tree age of 30 of about 61 feet (table 1). To estimate site index, find years. the 30-year mark on the Age axis of the site index Let’s look at an example of how to use the model to graph in figure 4. Then, using the corresponding estimate site index on your property for an even-aged tree height, find 61 feet on the Height axis of the site natural longleaf pine stand with an unknown age. index graph. Estimated site index can be determined by drawing a vertical line from the age axis and a Assume that about 30 years ago your grandfather horizontal line from the height axis and finding where naturally regenerated a 20-acre block of longleaf pine those two lines intersect in relation to the curves on on your family farm. You have recently inherited that the graph. Follow the curve to the right to read its stand and are starting to plan management activities, corresponding site index value label, which represents a site index of about 81 feet at a base age of 50 (figure Table 1. Estimates of longleaf pine age and height based on a ten-tree sample from a natural even-aged stand. Tree Ring Years Total Total number count at added tree height 4.5 feet to ring age (feet) count (years) (A) (B) (C) (D) 1 23 7 30 61 2 23 7 30 58 3 25 7 32 60 4 20 7 27 58 5 23 ARCHIVE7 30 58 6 23 7 30 59

7 27 7 34 64 Figure 5. Fitted curves for natural, even-aged longleaf pine site 8 23 7 30 62 indices 40, 50, 60, 70, 80, and 90 feet base age 50. 9 23 7 30 61 10 24 7 31 65 Average 30.4 60.6

4 Alabama Cooperative Extension System Site Index for Longleaf Pine 5 Table 2. Estimates of site index from a ten-tree sample from a natural, even-aged stand using a base age of 50 years with 7 years added to breast height age. Tree number Ring count at Years added to Total tree age Total height Estimated site 4.5 feet ring count (years) (feet) index (A) (B) (C) (D) (E) 1 23 7 30 61 82 2 23 7 30 58 79 3 25 7 32 60 78 4 20 7 27 58 84 5 23 7 30 58 79 6 23 7 30 59 80 7 27 7 34 64 79 8 23 7 30 62 83 9 23 7 30 61 82 10 24 7 31 65 84 Average 30.4 60.6 81

5, table 2). This simply means that if growing condi- Predicting the future tions remain the same, we can expect, on average, the While we may not truly be able to predict the future dominant and codominant trees on this stand to grow in forestry, models are commonly used by land- so that they are about 81 feet tall when they are 50 management professionals to estimate future growth years old. Base age, often thought of as rotation age and plan land-management activities. However, there for most southern pine species, is usually 50 years for are some limitations to these models that must be natural stands and 25 years for plantations. remembered when estimating site index. Site index estimates could also be calculated for each • Determining age in some species can be diffi- tree sampled and then averaged (table 2). Based on cult, and if not done correctly, small errors can this same sample of ten trees in table 1, the estimated cause large changes in site index estimation. average site index based on all trees sampled is 81 feet at base age 50. Calculated either way, the answers • Estimates of site index will improve with stand should be about the same age. Stands must be inventoried periodically to obtain current site index estimates. The information in table 3 is taken from the curves in figure 5 and can be used to estimate land quality. • Site index does not translate across species. First, determine the age and average total height of That is, you would not use site index curves for the dominant and codominant trees in the stand. Then loblolly pine (Pinus taeda) to estimate longleaf apply these to the table to determine site index. Table pine site index or vice versa. 3 estimates site index for longleaf pine stands that took 7 years to reach 4.5 feet in height. • Although this model is based on natural stands, ARCHIVEit is possible to use it for plantation longleaf if The method above works well for stands more than no plantation-based equations are available. 20 years old, but for younger stands, you can be more Early work in this area shows that the model accurate by using the actual equations. (See Forester’s estimates plantations relatively well. The number Note at the end of this publication.) of years it takes trees to reach 4.5 feet is usually less for plantations, so age should be adjusted down for use with this model.

4 Alabama Cooperative Extension System Site Index for Longleaf Pine 5 Table 3. Site index estimates for naturally regenerated, even-aged longleaf pine stands based on average total height in feet of dominant and codominant trees and age at breast height (4.5 feet), plus 7 years. Age (at 4.5 Site Index (Base Age 50) feet, plus 7 40 50 60 40 80 90 years) Average total height of dominant and codominant longleaf pine trees in feet 15 11.4 14.5 18.2 22.6 27.7 33.3 20 16.3 21.2 26.9 33.4 40.5 48.2 25 21.0 27.4 34.6 42.5 50.9 59.8 30 25.4 33.0 41.3 50.1 59.3 68.8 35 29.5 38.0 47.0 56.4 66.1 75.9 40 33.3 42.5 52.0 61.7 71.6 81.6 45 36.8 46.5 56.3 66.2 76.2 86.2 50 40.0 50.0 60.0 70.0 80.0 90.0 55 42.9 53.2 63.3 73.3 83.2 93.2 60 45.6 56.0 66.1 76.1 86.0 95.9 65 48.0 58.5 68.6 78.6 88.4 98.2 70 50.3 60.8 70.9 80.7 90.5 100.2 75 52.4 62.9 72.9 82.7 92.3 101.9 80 54.3 64.8 74.7 84.4 93.9 103.4 85 56.0 66.5 76.3 85.9 95.3 104.7 90 57.7 68.0 77.8 87.2 96.6 105.9 95 59.2 69.5 79.1 88.5 97.7 107.0 100 60.6 70.8 80.3 89.6 98.7 107.9 105 61.9 72.0 81.4 90.6 99.7 108.8 110 63.1 73.1 82.4 91.5 100.5 109.6 115 64.3 74.2 83.4 92.4 101.3 110.3

Site index curves are an important tool for estimating height and age to estimate site index (for example, an future stand height growth trends and for estimating average height of 60 feet at age 30, for instance, is very site productivity. Remember that models are not substi- close to the site index 80 curve), it is more accurate for tutes for regular forest inventories but can help you forestry professionals to use the actual equations. make decisions between those inventories. It is impor- Future plans include the development of an online site tant that you visit your forest often and have regular index calculator based on these equations. The comple- forest inventories conducted to update these estimates tion of this current longleaf pine site index model is so that you can better plan future forest-management the first step in updating a longleaf pine growth model activities on your land. by getting accurate estimates of site productivity (Lauer Forester’s Note: and Kush 2011). The other important aspect of this approach is to be able to estimate growth based on The following equationsARCHIVE can be used to predict natural, breast height age so that models can be used for stands even-aged longleaf pine height given site index or to that vary in origin (planted bareroot, planted container- predict site index from height. These equations are ized, planted with weed control, naturally seeded, etc.), complicated, but they reference the system of curves which is related to how stand age is defined and the as shown in figures 5 and 6 for site indices of 40 to 90 number of years it takes seedlings to come out of the feet (Lauer and Kush 2010). In this case, the traditional grass stage. 50-year base age with 7 years to reach 4.5 feet in height (S43+7) was used. Although it is possible to plot a point on the curves in figures 5 and 6 using average

6 Alabama Cooperative Extension System Site Index for Longleaf Pine 7 The equation for height prediction is as follows.

where

The equation for site index prediction is as follows.

where

For these equations, age is stand age in years, G is the age at which trees reach 4.5 feet, S is site index, B is Figure 6. Fitted curves using the equation above with S43+7 (base age 50 with 7 years for trees to reach 4.5 feet) for natu- site index base age, H is total height, and the param- rally regenerated, even-aged longleaf pine site indices 40, 50, eters are b1=77.080, b2=1723.39, b3=1.235. Subscripts 60, 70, 80, and 90 feet. for S and B indicate how base age is referenced, with S indicating base age 50 years with a 43-year dbh 43+7 References ring count (rc) and 7 years (G) to reach 4.5 feet (G). In practice, G can be measured or assumed. Lauer, D.K., and J.S. Kush. 2011. A variable density stand level growth and yield model for even-aged The traditional base age is 50 years for natural longleaf natural longleaf pine. Special Report No. 10. Alabama pine stands, but the number of years added to ring Agricultural Experiment Station. 16 p. Available online counts from increment cores varies across the range at http://www.aaes.auburn.edu/comm/pubs/specialre- of longleaf pine. This may be due to the wide range ports/sr10-longleaf-pine.pdf. of sites and geographical range for this species and the local experience of foresters. This new site index Lauer, D.K., and J.S. Kush. 2010. Dynamic site index model includes a reference to number of years added, equation for thinned stands of even-aged natural long- so height growth is calculated based on growth once leaf pine. South. J. Appl. For. 34:28-37. trees have reached 4.5 feet in height. Subscripts are used to indicate the years added. Site index using a base age of 50 years with 7 years for the tree to reach

4.5 feet in height would be referenced as S43+7 because the measured ring count is 43 years and 7 years is added to estimate stand age at 50 years.

ARCHIVEBecky Barlow, Extension Specialist, School of Forestry and Wildlife Sciences; Dwight K. Lauer, Biometrician, Silvics Analytic; and John S. Kush, Research Fellow, School of Forestry and Wildlife Sciences, all with Auburn University. For more information, call your county Extension office. Look in your telephone directory under your county’s name to find the number. Published by the Alabama Cooperative Extension System (Alabama A&M University and Auburn University), an equal opportunity educator and employer. Web Only, New March 2012, ANR-1424 ANR-1424 © 2012 by the Alabama Cooperative Extension System. All rights reserved. www.aces.edu 6 Alabama Cooperative Extension System Site Index for Longleaf Pine 7