Site Index for Longleaf Pine

A l A b A m A A & m A n d A u b u r n u n i v e r s i t i e s

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ANR-1424

Site Index for Longleaf Pine

Have you ever looked at trees in your woodlot

and thought, “i wish i could see into the future

to know when my trees will be ready to harvest”?

most of us have probably wished for the ability to

predict the future to help us plan a timber sale or

other land-management activity. What you may not re-

alize is that there are tools used by land-management

professionals that can estimate future tree and stand

growth. One of these tools is site index, which is an

important forest measurement that is used to predict

trends in future height growth of forest trees.

What influences the growth of forest trees?the amount of wood that can be grown on an area of land depends on many factors, such as species, age, number of trees, length of rotation, and land quality. the production of wood can be affected by adjusting some of these factors. For example, periodic thinning to reduce the number of trees in an area can increase tree diameter growth rates.

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 palustris) 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 on a site. the following are some of the ways land

managers use site quality estimates in forest growth and yield models:

• to determine the present and future growth and productivity of forest stands

• to determine forest product sizes and values at a given age

• to justify land investments (and divestments)

• to provide a frame of reference for land-manage-ment prescriptions such as timing of thinning or pruning, type of site preparation, and tree species selection

How do we assess forest site quality?there are different ways land managers can assess site quality, but the most commonly used method is site index (si), which is estimated by measuring heights and ages of standing trees in a forest. this measurement is used because tree volume production potential is positively related to height growth. Finally, it is easy to measure the age and height of trees when conducting a forest inventory.

Figure 1. An increment borer can be used to determine tree age at 4.5 feet above the ground by taking a small sample from the tree. (Photo credit: John Gilbert)

2 Alabama Cooperative extension system Site Index for Longleaf Pine 3

darker-colored ring. it is common practice to add 3 to 4 years for loblolly pine and 7 years for longleaf pine.

total heights of dominant and codominant trees are used to estimate site index. dominant trees are described as trees that have crowns that are taller than the surrounding trees and are receiving sunlight from above and from the side (figure 3). Codominant

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)

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

total age is estimated either from stand establishment records or by using an increment borer to obtain a wood sample from the tree at 4.5 feet above the ground (figure 1). the number of annual rings are counted, and then years are added to the count based on species for the time it took the tree to reach 4.5 feet tall (figure 2). An annual ring is made up of two parts, earlywood, a light-colored ring, and latewood, a


trees are similar to dominant trees, but the tree crown receives sunlight from above only, not from the sides. these trees usually make up the majority of the forest canopy. this measurement is most associated with even-aged forests, or forests in which the trees in a stand are all the same age. site index is usually consid-ered most reliable when based on measurements of trees that are 20 years old or older.

Once heights and ages have been measured, those values are used in an equation, table, or graph to determine site index. the following is an example of how site index can be estimated for longleaf pine.

Estimating site index for longleaf pinelongleaf pine is managed as both natural-seeded and planted stands in the southeastern u.s. regardless of the regeneration method, determining early height development is complicated by the grass stage period, during which no height growth occurs (figure 4). the length of time between germination and the start of seedling height growth is usually 4 to 5 years but may range from 1 to 20 years and is followed by a period of rapid height growth (figure 4).

emergence from the grass stage can be unpredictable and may depend on seedling quality for planted

stands, competition from other seedlings or vegeta-tion, insect and disease incidence, and climatological factors. silvicultural practices that reduce competition and improve soil conditions can reduce duration of the grass stage. the incidence of brown-spot needle blight caused by the fungus Mycosphaerellaceae dearnessii m.e. barr, which varies by geographic location, can also increase the number of years a seedling remains in the grass stage.

the accurate description of early height growth in longleaf pine is becoming more important as increasing numbers of landowners implement longleaf pine management and restoration activities on their lands. site index estimation is key to the comparison of management options because active management can increase growth, and, therefore, this species’ value to landowners. However, because of the unpredictable nature of longleaf pine’s early growth, the age of a given tree can be difficult to determine. An error of 1 year in time of emergence could result in a 2- to 4-foot error in height estimates, and in turn cause errors in site index estimation that could over- or underestimate future stand growth. One way around this problem is to reference growth using tree ring count alone at 4.5 feet above the ground (breast height age) in addition to stand age.

Mature

Stemelongation

Grass stage seedlings

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


An updated site index modelA new site index model (lauer and Kush 2010) has been developed for even-aged longleaf pine stands that accounts for variation in tree age and length of the grass stage period using 45 years of height measure-ments from the regional longleaf Pine Growth study (rlGs). the rlGs monitors growth of natural even-aged longleaf stands and is being used to develop new growth and yield models for longleaf pine. An estimate of site index is required to use these new models.

For this current site index model, site index of even-aged longleaf pine stands can be estimated using actual stand age if it is known. Or, if the actual age of the stand or tree is not known, ring count at 4.5 feet above the ground plus 7 years added to that ring count can be used. For example, if the measured ring count was 23 years plus 7 years for the tree to reach 4.5 feet, then site index would be estimated using a tree age of 30 years.

let’s look at an example of how to use the model to estimate site index on your property for an even-aged natural longleaf pine stand with an unknown age.

Assume that about 30 years ago your grandfather naturally regenerated a 20-acre block of longleaf pine on your family farm. You have recently inherited that stand and are starting to plan management activities,

Tree number

Ring count at 4.5 feet

Years added to ring count

Total tree age (years)

Total height (feet)

(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 7 30 58

6 23 7 30 59

7 27 7 34 64

8 23 7 30 62

9 23 7 30 61

10 24 7 31 65

Average 30.4 60.6

Table 1. Estimates of longleaf pine age and height based on a ten-tree sample from a natural even-aged stand.

but you want to know what future height growth for the existing trees might be like. last month, you hired a consulting forester to inventory your 20-acre stand. As part of that inventory, ten trees were sampled for total height and age.

tree age was estimated by boring the trees at 4.5 feet above the ground and counting the rings. the results of that sample are illustrated in column A of table 1.

to estimate site index on your stand, you must first determine the total age of your trees. seven years should be added to the ring count at 4.5 feet to get the total tree age in years (column b, table 1). For example, tree number 1 had a ring count of 23 years plus 7 years, for a total tree age of 30 years (column C, table 1). this process is repeated for all trees sampled.

based on your calculations, the average age of this stand is approximately 30 years, with an average height of about 61 feet (table 1). to estimate site index, find the 30-year mark on the Age axis of the site index graph in figure 4. then, using the corresponding tree height, find 61 feet on the Height axis of the site index graph. estimated site index can be determined by drawing a vertical line from the age axis and a horizontal line from the height axis and finding where those two lines intersect in relation to the curves on the graph. Follow the curve to the right to read its corresponding site index value label, which represents a site index of about 81 feet at a base age of 50 (figure

Figure 5. Fitted curves for natural, even-aged longleaf pine site indices 40, 50, 60, 70, 80, and 90 feet base age 50.


5, table 2). this simply means that if growing condi-tions remain the same, we can expect, on average, the dominant and codominant trees on this stand to grow so that they are about 81 feet tall when they are 50 years old. base age, often thought of as rotation age for most southern pine species, is usually 50 years for natural stands and 25 years for plantations.

site index estimates could also be calculated for each tree sampled and then averaged (table 2). based on this same sample of ten trees in table 1, the estimated average site index based on all trees sampled is 81 feet at base age 50. Calculated either way, the answers should be about the same

the information in table 3 is taken from the curves in figure 5 and can be used to estimate land quality. First, determine the age and average total height of the dominant and codominant trees in the stand. then apply these to the table to determine site index. table 3 estimates site index for longleaf pine stands that took 7 years to reach 4.5 feet in height.

the method above works well for stands more than 20 years old, but for younger stands, you can be more accurate by using the actual equations. (see Forester’s note at the end of this publication.)

Tree number Ring count at 4.5 feet

Years added to ring count

Total tree age (years)

Total height (feet)

Estimated site 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

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.

Predicting the futureWhile we may not truly be able to predict the future in forestry, models are commonly used by land-management professionals to estimate future growth and plan land-management activities. However, there are some limitations to these models that must be remembered when estimating site index.

• determining age in some species can be diffi-cult, and if not done correctly, small errors can cause large changes in site index estimation.

• estimates of site index will improve with stand age. stands must be inventoried periodically to obtain current site index estimates.

• site index does not translate across species. that is, you would not use site index curves for loblolly pine (Pinus taeda) to estimate longleaf pine site index or vice versa.

• Although this model is based on natural stands, it is possible to use it for plantation longleaf if no plantation-based equations are available. early work in this area shows that the model estimates plantations relatively well. the number 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.


Age (at 4.5 feet, plus 7 years)

Site Index (Base Age 50)

40 50 60 40 80 90

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

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.

site index curves are an important tool for estimating future stand height growth trends and for estimating site productivity. remember that models are not substi-tutes for regular forest inventories but can help you make decisions between those inventories. it is impor-tant that you visit your forest often and have regular forest inventories conducted to update these estimates so that you can better plan future forest-management activities on your land.

Forester’s Note:the following equations can be used to predict natural, even-aged longleaf pine height given site index or to predict site index from height. these equations are complicated, but they reference the system of curves as shown in figures 5 and 6 for site indices of 40 to 90 feet (lauer and Kush 2010). in this case, the traditional 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

height and age to estimate site index (for example, an average height of 60 feet at age 30, for instance, is very close to the site index 80 curve), it is more accurate for forestry professionals to use the actual equations.

Future plans include the development of an online site index calculator based on these equations. the comple-tion of this current longleaf pine site index model is the first step in updating a longleaf pine growth model by getting accurate estimates of site productivity (lauer and Kush 2011). the other important aspect of this approach is to be able to estimate growth based on breast height age so that models can be used for stands that vary in origin (planted bareroot, planted container-ized, planted with weed control, naturally seeded, etc.), which is related to how stand age is defined and the number of years it takes seedlings to come out of the grass stage.


ANR-1424

Becky 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

© 2012 by the Alabama Cooperative extension system. All rights reserved.

www.aces.edu

the equation for height prediction is as follows.

where

where

the equation for site index prediction is as follows.

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-rally regenerated, even-aged longleaf pine site indices 40, 50, 60, 70, 80, and 90 feet.

Referenceslauer, d.K., and J.s. Kush. 2011. A variable density stand level growth and yield model for even-aged natural longleaf pine. special report no. 10. Alabama Agricultural experiment station. 16 p. Available online at http://www.aaes.auburn.edu/comm/pubs/specialre-ports/sr10-longleaf-pine.pdf.

lauer, d.K., and J.s. Kush. 2010. dynamic site index equation for thinned stands of even-aged natural long-leaf pine. south. J. Appl. For. 34:28-37.

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 site index base age, H is total height, and the param-eters are b1=77.080, b2=1723.39, b3=1.235. subscripts for s and b indicate how base age is referenced, with s

43+7 indicating base age 50 years with a 43-year dbh

ring count (rc) and 7 years (G) to reach 4.5 feet (G). in practice, G can be measured or assumed.

the traditional base age is 50 years for natural longleaf pine stands, but the number of years added to ring counts from increment cores varies across the range of longleaf pine. this may be due to the wide range of sites and geographical range for this species and the local experience of foresters. this new site index model includes a reference to number of years added, so height growth is calculated based on growth once 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 s

43+7 because

the measured ring count is 43 years and 7 years is added to estimate stand age at 50 years.

Documents

Site Index for Longleaf Pine