## Site Index

Site Index is a indirect measure of Forest productivity. In site index we use the height of a tree at a specified base age. Carmean et al. 1989 compiled collection of Eastern United States Site Index curves for 110 species. This function takes parameters for these site index equations and produces a tree specifi site index estimate.

In imperial units:

$$SI = BH + c_1 H^{c_2} \left( 1 - e^{c_3A} \right)^{c_4 H^{c_5}}$$

where: $SI$ is the Site Index in feet, $BH$ is the Breast Height value, $H$ is the total tree height in feet, and $A$ is total tree age in years.

Imperial units
age = 45 yr
height = 70 ft
species = "QUAL"

### Code

#### Visual Basic

 
Function siteidx(spp As String, age As Single, height As Single) As Double
' Function to calulate the site index estimated from a single tree
' given species using USDA codes, age in years, and height in feet
'
' As presented in "Site index curves for forest tree species in
' the eastern United States" by W. Carmean, J. t. Hahn and R. D. Jacobs
' NCFES General Technical Report NC-128
'
' by David Larsen, Copyright July 21, 2000

If (spp = "QUAL") Then
' Carmean, 1972
bht = 0#
c1 = 0.3387
c2 = 1.0135
c3 = -0.0076
c4 = -0.9644
c5 = -0.0176
ElseIf (spp = "QUVE") Then
' Carmean, 1972
bht = 0#
c1 = 0.2598
c2 = 1.1721
c3 = -0.0107
c4 = -2.3272
c5 = -0.2825
ElseIf (spp = "QUCO2") Then
' Carmean, 1972
bht = 0#
c1 = 0.7423
c2 = 0.9677
c3 = -0.0223
c4 = -1.026
c5 = -0.0137
ElseIf (spp = "mcquilkin") Then
' McQuilkin, 1978
bht = 0#
c1 = 0.0064
c2 = 1.6595
c3 = -0.0003
c4 = -1.866
c5 = -0.2916
ElseIf (spp = "upland") Then
' Schnur, 1937
bht = 0#
c1 = 0.189
c2 = 1.2031
c3 = -0.0081
c4 = -2.1975
c5 = -0.2582
Else
si = 0#
MsgBox ("Unknown species ")
End If

siteidx = bht + (c1 * (height ^ c2)) * (1 - Exp(c3 * age)) ^ (c4 * (height ^ c5))

End Function
Excel® Visual Basic Code

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