Crossbill Forestry
Site index & Yield class
Is it time to re-think how we measure forest productivity?
In UK forestry, yield class remains one of the most fundamental measures of site productivity, it is a simple yet powerful way of describing how fast a stand of trees is growing in terms of volume, but are historic top-height/age curves still fit for purpose?
Yield class explained
Yield class is maximum mean annual increment (MMAI), expressed as an integer, and rounded to the nearest even number. In other words, a stand of trees whose mean annual increment (MAI) will peak at 18.5m³ ha⁻¹ yr⁻¹ is growing at yield class 18.
Yield class is used in production forecasting (UK yield tables are mapped to yield class divisions and site treatments). It is perhaps most useful, however, for benchmarking site productivity: for example, a stand of Sitka spruce growing at yield class 24 is somewhat more productive than a stand of Sitka growing at yield class 18.
Yield class cautions
We need to be careful when comparing yield classes between species and regimes, because the age at which MMAI is reached varies greatly. For example, a stand of yield class 22 Sitka spruce will reach its MMAI aged just 39, whereas a stand of yield class 22 Norway spruce will not reach its MMAI until age 61, and it may not be immediately obvious which of the two is the more productive.
According to the yield tables, we can expect a stand of YC22 Norway spruce to accumulate 1,165m³ ha⁻¹ of standing volume in 61 years. YC22 Sitka spruce, on the other hand, if felled at its age of MMAI and restocked the following year, will accumulate 1,011m³ ha⁻¹ in the same 61 year period. It is less productive.
Another common mistake is using yield class alone to estimate volume per ha by multiplying yield class by stand age. This approach works at exactly one point in the rotation – the age of MMAI. To apply this method to a 20-year-old YC20 Sitka stand, will result in an error of 80% (if historic yield tables are to be believed).
Yield class vs Site index
Yield class is usually derived using static top height-age curves, which were developed in the 1980s. For a number of reasons, including the approach that was used to derive them, and the nature of the data that was available at the time, these are no longer considered reliable by some. However, they remain widely, if not universally used. They are well bedded into workflows, and many practitioners are simply unaware of any alternatives.
In 2021, Manso et al (2020) challenged the top height model status quo by publishing a paper in Forestry exhibiting brand new dynamic top height models for UK tree species. These dynamic models are based on more extensive data, covering a wider range of age-classes, and are far more statistically robust than the historic top height age curves.
These models do not predict yield class directly, instead, they predict top height at a reference age, a metric better known site index (SI). This is reported along with the reference age, for example, SI50 would be the height of dominant trees when the stand reaches 50 years of age.
Site index is a metric which is used internationally as a benchmark of site productivity. It is more species agnostic than yield class, and it isn’t constrained by the extent of the current yield class curves (i.e., <= 36 for Sitka spruce). Further, it could be used in combination with existing top-height dependent yield simulators to eliminate some of the bias in existing production forecasting workflows.
A final note of caution is that neither the historic top-height/age curves that we use to derive general yield class, nor the newer dynamic models, account for growth differences associated with genetically improved Sitka spruce, or other species with active breeding programmes.
Summary
Together, yield class and site index provide complementary perspectives on forest productivity, one grounded in historic yield models, the other in more modern, and statistically rigorous approaches. As dynamic modelling becomes the norm, site index may soon take the lead as the preferred benchmark for UK forest site productivity, but for now, why aren’t more of us making use of both?
Access the site index calculator here
Ben Crisford MICFor
5th November 2025