United States Department of Agriculture
Forest Service
Pacific Southwest Research Station

General Technical Report
PSW-GTR-155-Web

Appendix C—Site Class

SITE CLASS

Reflects ability of site to grow trees, a measure of quality or productivity.
Is determined by tree age and height .
Requirements of a site tree:

Must be predominant or dominant and grown freely all its life.
Must be at least 50 years old.
Must be ponderosa, Jeffrey, or sugar pine, Douglas–fir, white or red fir.
Must be suitable form class, not a cull or deformed tree.
Must have its original top.

540 – DETERMINING AGE OF LARGE TREES

Use an increment borer at least 16 inches long. When determining the age of a tree that has a radius greater than the length of an increment borer, use the following procedure: Bore into tree as far as possible, extract core, and count the rings. Measure the diameter of the tree and divide by two, then subtract the bark thickness. This gives the radius of the wood part of the tree. Measure the length of the core and subtract from the radius of wood to determine how much longer the core would have to be to reach the pith. Count the number of rings in the last (innermost) inch and extrapolate to the center. Add this to the ring count on the extracted core and then add 10 years to account for the time required to grow 4–1/2 feet tall. Dunning recommends 10 years in all cases.

Calculation for determining age of large trees;

DBH / 2	=	__________	(inches)
Bark Thickness	=	__________
Difference	=	__________
Core length	=	__________
Difference	=	__________
No. of rings innermost inch	=	__________
Sum = Age	=	__________

Example

Determine the age of a Douglas–fir 60.0 inches DBH with bark thickness of 2.0 inches when a core 16.0 inches long has 100 rings and the innermost inch has 5 rings.

60.0 / 2 = 30.0 inches (radius of wood and bark)
30.0 – 2.0 = 28.0 inches (radius of wood)
28.0 – 16.0 = 12.0 inches (short of hitting center)
5 × 12.0 = 60 rings
100 + 60 + 10 = 170 years old

REGION FIVE SITE CLASSES

(Height by Age and Site Class Code)¹

¹Based on ponderosa pine, Jeffrey pine, sugar pine, Douglas-fur, red fir, and white fir. Age is in years. Total height is in feet of average dominant and predominant trees with tree age of at least 50 years. Adapted from Dunning's site index curves for height at 300 years (Dunning 1942). (Predominant and dominant defined in Field 21, Crown Position).

Site Class (Field 9)

Age	0	1	2	3	4

40	95	81	66	49	43
50	106	90	75	56	49
60	115	98	82	63	53
70	122	105	88	68	58
80	129	111	93	73	61
90	135	116	98	77	64
100	140	121	106	81	67
110	145	125	106	84	70
120	149	129	109	87	72
130	153	133	112	90	74
140	157	136	115	93	76
150	160	139	118	95	78
160	163	142	120	98	80
170	166	144	123	100	81
180	169	147	125	102	83
190	172	149	127	104	84
200	175	152	129	106	86
220	179	176	133	109	88
240	184	160	136	112	90
260	188	163	139	115	93
280	191	166	142	117	95
300	195	169	145	120	96
320	198	172	147	122	98
340	201	175	150	124	100
360	204	177	152	126	101
380	206	180	154	128	103
400	209	182	156	130	104