Site Index Curve
Site Index Curve Analysis · Also known as: site productivity, growth intercept
A site index curve is a family of curves relating tree height to stand age, used to quantify the productivity of a forest site. Site index is conventionally defined as the height of dominant trees at a reference age (typically 50 years in temperate forests). These curves enable foresters to classify sites by productivity class and to predict growth rates for planning timber harvests and silvicultural treatments. Site index curves are among the most fundamental tools in forest growth and yield modeling.
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When to use it
Use site index curves when conducting strategic forest inventory or planning long-term management in even-aged stands. They are essential for validating growth models, setting harvest schedules, and comparing site productivity across regions. Site index is most reliable in temperate and boreal forests with clear age structure and predictable height-growth relationships. Less applicable in tropical multi-aged forests, severely disturbed stands, or where climate change has altered growth patterns.
Strengths & limitations
- Simple, practical tool for field classification of site quality without complex growth models
- Directly based on observable measurements (height, age), requiring no specialized equipment
- Enables comparison of forest productivity across regions using standardized site index classes
- Strongly linked to timber yield potential, informing management and investment decisions
- Decades of empirical validation across diverse forest types and regions
- Integrates easily with growth-and-yield simulators and forest management planning systems
- Height growth may be suppressed in young stands recovering from competition or stress; using young-stand data inflates site index estimates
- Site index does not capture multi-factor productivity (climate, disease, insects, water stress). A stand may have high site index but low actual growth due to pest pressure.
- Assumes height-age relationship is stable over time; climate change may shift growth curves, making historical curves unreliable
- Reference age (e.g., 50 years) is arbitrary; curves derived for different reference ages are not directly comparable
- Requires accurate age determination, which is labor-intensive and subject to error in old stands or when rings are densely packed
Frequently asked
What reference age should I use for site index?
The reference age is a convention that allows standardized comparison; 50 years is standard in many temperate regions. However, different species and regions use different standards (e.g., 100 years in some boreal forests). Always check the standard for your region and species. If comparing data from different sources, convert to a common reference age using the fitted growth model.
My stand is only 20 years old. Can I estimate site index?
Site index curves are less reliable for young stands because height growth is still accelerating and competition effects are unstable. If possible, wait until the stand is older (30+ years) for a more robust estimate. Alternatively, use a dynamic site index model that adjusts for stand age, but validate the result with older reference stands if available.
Can I use remotely sensed data (LiDAR, aerial imagery) to derive site index?
Yes. LiDAR can measure canopy height at high spatial resolution and across large areas, enabling landscape-scale site index mapping. However, you still need field data to calibrate height measurements to dominant tree height and to determine stand age. LiDAR-derived site index is faster than traditional plot sampling but requires careful validation.
How do I account for site index changes due to climate change?
Historical site index curves may not reflect future productivity if climate has shifted or will shift. Some researchers fit time-dependent models that allow site index to change over years. Others use climate-based productivity models that explicitly incorporate temperature, precipitation, and other climatic variables. For long-term planning, consider both historical curves and climate projections.
Sources
- Clutter, J. L., Fortson, J. C., Pienaar, L. V., Brister, G. H., & Bailey, R. L. (1983). Timber Management: A Quantitative Approach. John Wiley & Sons. link ↗
- Davis, L. S., Johnson, K. N., Bettinger, P. S., & Howard, T. E. (2001). Forest Management to Sustain Ecological, Economic, and Social Values. McGraw-Hill. link ↗
How to cite this page
ScholarGate. (2026, June 3). Site Index Curve Analysis. ScholarGate. https://scholargate.app/en/forestry/site-index-curve
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- Stand Density IndexForestry↔ compare