Site index is a measure of the productivity of a site and is defined as the height of a site tree at breast height age 50. Site index is used in a variety of ways, such as in formulating silviculture prescriptions. It is also one of the most important parameters in estimating forest stand growth and yield. These growth and yield estimates are subsequently used in timber supply analyses, and hence play an important role in determining the allowable annual cut.

Site productivity research involves developing tools to estimate site index and models that predict height growth of site trees.  Trees that have well-defined characteristics and that reflect the productivity of a site are called site trees.  In general, for a tree to be a site tree, its growth cannot be altered by non-site factors such as biotic and abiotic damage, suppression, repression, and so on.  Much of the research in this area depends on the stem analysis technique for sampling.  The models that result from site productivity research are available in the SiteTools software, which presents tools to estimate both height and site index in interactive and batch modes.  These models are also an integral part of the main growth and yield models used in British Columbia, including TASS, TIPSY, and VDYP.

The main tools for estimating site index are height growth models (also called site index models), growth intercept models, SIBEC (Site Index Biogeoclimatic Ecosystem Classification), and site index conversion equations.  Height growth models predict height from site index and breast height age, but can also be used to predict site index from height and age.  These models form the backbone of site productivity work.  Growth intercept models also predict site index from height and age, but are tailored for use in young stands.  SIBEC allows site index to be predicted from the BEC ecosystem classification and species.  This method is particularly useful when other methods cannot be used, for example, on denuded sites, stands where the site trees have not yet reached breast height, and on very old stands.  SIBEC is also intended to be used with OASIS and predictive ecosystem mapping (PEM).  Site index conversion equations predict site index for a species from the site index of another species.  These equations are designed for use in mixed species stands and for stand conversion situations.

Site Productivity Publications

Journal Publications

Nigh, G.D.  2004.  A comparison of fitting techniques for ponderosa pine height-age models in British Columbia.  Ann. For. Sci. 61: 609-615.

Nigh, G.D., B.A. Love, and K. Thomas. 2003. A comparison of the time required for two methods of selecting site trees in juvenile stands. North. J. Appl. For. 20: 101-103.

Nigh, G.D. and M.G.E. Mitchell. 2003. Development of height-age models for estimating juvenile height of coastal Douglas-fir in British Columbia. West. J. Appl. For. 18: 207-212.

Brisco, D., K. Klinka, and G. Nigh. 2002. Height growth models for western larch in British Columbia. West. J. Appl. For. 17: 66-74.

Nigh, G.D., P.V. Krestov, and K. Klinka. 2002. Height growth of black spruce in British Columbia. For. Chron. 78: 306-313.

Nigh, G. 2002. Site index conversion equations for mixed trembling aspen and white spruce stands in northern British Columbia. Silva Fennica 36(4): 789-797.

Nigh, G.D., P.V. Krestov, and K. Klinka. 2002. Trembling aspen height-age models for British Columbia. Northwest Sci. 76(3): 202-212.

Nigh, G.D. and P.J. Martin. 2001. A method to assess the performance of growth intercept models in British Columbia. For. Chron. 77(3): 491-499.

Nigh, G.D. 2001. Species-independent height-age models for British Columbia. For. Sci. 47(2): 150-157.

Nigh, G.D. and B.A. Love. 2000. Juvenile height development in interior spruce stands of British Columbia. West. J. Appl. For. 15:117-121.

Nigh, G.D. and G. Kayahara. 2000. Site index conversion equations for western redcedar and western hemlock. Northwest Science 74:146-150.

Nigh, G.D. and B.A. Love. 1999. How well can we select undamaged site trees for estimating site index? Can. J. For. Res. 29: 1989-1992.

Nigh, G.D. and B.A. Love. 1999. A model for estimating juvenile height of lodgepole pine. For. Ecol. Manage. 123: 157-166.

Nigh, G.D. 1998. Prediction intervals for estimates of site index based on ecosystem type. Environ. Manage. 22(2): 197-202.

Nigh, G.D. 1998. A system for estimating height and site index of western hemlock in the interior of British Columbia. For. Chron. 74: 588-596.

Nigh, G.D. and P.J. Courtin. 1998. Height models for red alder (Alnus rubra Bong). In British Columbia. New For. 16: 59-70.

Nigh, G.D. 1997. Early height growth and site index of lodgepole pine under wet and dry soil moisture regimes in British Columbia. West. J. Appl. For. 12(1): 5-8.

Nigh, G.D. 1997. A Sitka spruce height-age model with improved extrapolation properties. For. Chron. 73(3): 363-369.

Nigh, G.D. and V. Sit. 1996. Validation of forest height-age models. Can. J. For. Res. 26: 810-818.

Nigh, G.D. 1996. Growth intercept models for species without distinct annual branch whorls: western hemlock. Can. J. For. Res. 26: 1407-1415.

Nigh, G.D. 1995. The geometric mean regression line: a method for developing site index conversion equations for species in mixed stands. For. Sci. 41(1): 84-98.

B.C. Ministry of Forests Publications

Nigh, G.D. 2004. Growth intercept and site series-based estimates of site index for white spruce in the Boreal White and Black Spruce biogeoclimatic zone. Res. Br., B.C. Min. For., Victoria, B.C. Tech. Rep. 13.

Mah, S. and G.D. Nigh. 2003. SIBEC site index estimates in support of forest management in British Columbia. Res. Br., B.C. Min. For., Victoria, B.C. Tech. Rep. 004.

Nigh, G.D. and K.R. Polsson. 2002. Splicing Height Curves. B.C. Min. For., Res. Br., Victoria, B.C. Ext. Note. 60.

Nigh, G. 2002. Growth Intercept, Years-to-Breast-Height, and Juvenile Height Growth Models for Ponderosa Pine. B.C. Min. For., Technical Report 002.

Nigh, G. and K. Klinka 2001. Growth Intercept Models for Black Spruce B.C. Min. For., Res. Extension Note 57.

Nigh, G.D. 2000. Western redcedar site index models for the interior of British Columbia. B.C. Ministry of Forests, Forest Science Program, Victoria. Research Report 18. 24 p.

Mitchell, K.J., K.R. Polsson, M. Di Lucca, G.D. Nigh, S.E. Grout, S. Stearns-Smith 2000. SiteTools version 3.2. Online. Ministry of Forests, Research Branch, Victoria B.C. Available: http://www.for.gov.bc.ca/hre/sitetool/. July 2000.

Nigh, G. and P. Krestov 1999. Site Index Conversion Equations for Mixed Black Spruce - Lodgepole Pine Stands B.C. Min. For., Res. Br. Extension Note 39.

Nigh, G., D. Brisco, and D. New 1999. Growth Intercept Models for Western Larch B.C. Min. For., Res. Br. Extension Note 38.

Nigh, G.D. 1999. Smoothing height estimates from two lodgepole pine height models. B.C. Min. For., Res. Br., Victoria, B.C. Ext. Note.30.

Nigh, G.D. 1999. Years to breast height and green-up age models based on a juvenile height model for lodgepole pine. B.C. Min. For., Res. Br., Victoria, B.C. Ext. Note. 31.

Mitchell, K.J., K.R. Polsson, G.D. Nigh, M. Di Lucca, S.E. Grout, S. Stearns-Smith. 1998. SiteTools version 3.1. Online. Ministry of Forests, Research Branch, Victoria B.C. Available: http://www.for.gov.bc.ca/hre/sitetool/. July 1998.

Nigh, G.D. 1998. Site index adjustments for old-growth stands based on veteran trees. B.C. Min. For., Res. Br., Victoria, B.C. Work. Pap. 36. 17 p.

Nigh, G.D. 1998. Site index conversion equations for mixed western hemlock-amabilis fir stands. B.C. Min. For., Res. Br., Victoria, B.C. Ext. Note 26. 4 p.

Nigh, G.D. and B.A. Love. 1997. Site index adjustment for old-growth coastal western hemlock stands in the Kalum Forest District. B.C. Min. For., Res. Br., Victoria, B.C. Work. Pap. 27. 13 p.

Nigh, G.D. 1997. Interior Douglas-fir growth intercept models. B.C. Min. For., Res. Br., Victoria, B.C. Ext. Note 12. 6 p.

Nigh, G.D. 1997. Revised growth intercept models for lodgepole pine: comparing northern and southern models. B.C. Min. For., Res. Br., Victoria, B.C. Ext. Note 11. 6 p.

Nigh, G.D. 1997. Growth intercept models and tables for British Columbia - interior species. 2nd ed. B.C. Min. For., Res. Br., Victoria, B.C. Land Manage. Handb. Field Guide Insert 10. 22 p.

Nigh, G.D. 1997. Growth intercept models and tables for British Columbia - coastal species. 2nd ed. B.C. Min. For., Res. Br., Victoria, B.C. Land Manage. Handb. Field Guide Insert 9. 14 p.

Nigh, G.D. 1997. A growth intercept model for coastal Douglas-fir. B.C. Min. For., Res. Br., Victoria, B.C., Res. Rep. 10. 20 p.

Nigh, G.D. 1996. A variable growth intercept model for spruce in the Sub-Boreal Spruce and Engelmann Spruce - Subalpine Fir biogeoclimatic zones of British Columbia. B.C. Min. For., Res. Br., Victoria, B.C. Res. Rep. 05.20 p.

Nigh, G.D. 1996. Identification and simulation of the spatial pattern of juvenile lodgepole pine in the sub-boreal spruce biogeoclimatic zone, Stuart dry warm and Babine moist cold variants. B.C. Min. For., Res. Br., Victoria B.C. FRDA Rep. 244. 28 p.

Nigh, G.D. 1996. A variable growth intercept model for Sitka spruce. B.C. Min. For., Res. Br., Victoria, B.C. Ext. Note 03. 7 p.

Nigh, G.D. 1996. Interim validation of the western hemlock growth intercept model. B.C. Min. For., Res. Br., Victoria, B.C. Ext. Note 04. 3 p.

Nigh, G.D. 1995. Site index conversion equations for mixed species stands. B.C. Min. For., Res. Br., Victoria, B.C. Res. Rep. 01. 20 p.

Nigh, G.D. 1995. Variable growth intercept models for lodgepole pine in the Sub-Boreal Spruce biogeoclimatic zone, British Columbia. B.C. Min. For., Res. Br., Victoria, B.C. Res. Rep. 02. 22 p.

Nigh, G.D. 1995. Site index conversion equations for mixed Sitka spruce / western hemlock stands. B.C. Min. For., Res. Br., Victoria, B.C. Ext. Note 02. 3p.

Nigh, G.D. 1995. Compatibility improvements and bias reduction in height-age models. B.C. Min. For., Res. Br., Victoria, B.C. Res. Rep. 03. 10 p.

Conference Proceedings

Nigh, G.D. 1996. Growth intercept method of determining site index. In: Proc. NIVMA Annual General Meeting, Jan. 24-25, 1996, Smithers, B.C. P. Tollestrup (editor). pp. 1-8.