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.