Introduction to TIPSY and VDYP

TIPSY (Table Interpolation Program for Stand Yields) and VDYP (Variable Density Yield Prediction system) are Ministry of Forests models for predicting the yield of managed and natural stands, respectively. Both systems contribute to timber supply analyses of Tree Farm Licences and Timber Supply Areas. TIPSY can also be used to evaluate silvicultural treatments, and address other stand-level planning options.

Forest-level Planning

Use VDYP for natural stands. Use TIPSY for natural or planted managed stands of known (or assumed) initial density. Since VDYP is based on randomly located inventory plots, its yields incorporate the full range of stand conditions encountered in naturally regenerated forests. Consequently, VDYP predicts the average yield of naturally regenerated forests that have not been treated.

TIPSY, in contrast, is based on growth trends observed for up to 120 years in fully-stocked research plots and permanent sample plots growing in a relatively pest-free environment. The yields are expected to be very close to the potential of a specific site, species and management regime. These yields are generally not attainable under operational conditions. However, they are the only source of data for managed stands subjected to various silvicultural treatments. We can predict the yield of our best managed stands with reasonable confidence.

The reduction necessary to approximate the yield of the average managed stand is not known. Our best estimate suggests that the productivity of the average managed stand will be about 15% below the unadjusted yields generated by TIPSY. For forest planning purposes, we recommend that yields be reduced by an Operational Adjustment Factor (OAF) of 15%, unless there is strong evidence that local conditions warrant a different value.

Stand-level Planning

Do not use VDYP. Use TIPSY for natural or planted managed stands of known (or assumed) initial density. TIPSY provides yield curves for evaluating treatments such as planting densities and precommercial thinning. Yield curves for untreated, naturally regenerated stands are also included in TIPSY to provide a baseline for comparison.

Yields from TIPSY and VDYP are not comparable at the stand level because the databases and models are quite different. It is tempting to compare the curves for natural stands generated by TIPSY and VDYP. A casual visual comparison shows that VDYP produces approximately 75% of the yield of TIPSY for roughly similar conditions. This implies that the productivity of unmanaged stands of natural origin is about 25% below the potential of the site. More detailed comparisons are not warranted.

TIPSY yield tables include total and merchantable volumes, basal area, diameter, number of trees, mean annual increment, and statistics for the largest 250 "prime" trees. This information is provided to forest managers who need to evaluate the merit of silvicultural treatments in terms of yield and harvest age. For example, what initial plantation density will produce an average diameter of 30 cm at a harvest age which will coincide with the culmination of mean annual increment?

For more information about TIPSY, please refer to TIPSY's online documentation.