When comparing edge trees with “normal” trees (found inside a stand) it becomes obvious they can be significantly different. A generalisation is often that edge trees are shorter and fatter (greater taper) and also tend to have large branching. An edge tree therefore represents approx. 30-50% less value than a normal tree. It is therefore important to include these in the inventory planning component. If ignored a systematic error/bias is introduced (Gregoire, 1982). Overall grade / log product mix, stocking (SPH), basal area (BA) and volume (standing and recoverable) will be biased, eventually effecting forest/woodlot value.
The impact of the value loss along the edge is obviously related to severity of the edge effect and the amount of exposure to its surrounding environment. Therefore changing it’s effect on the tree due to stem defects (large branching, forking, lean and sweep).
Value loss also occurs due to over estimating stocking density and basal area (Ducey et al, 2004) since when assuming net stocked area (NSA) the edge of the canopy is used as a mapping feature. Usually 2-3m meter is used to represent the drip line from edge of canopy to the ground. This assumption seems reasonable for edges within forests but for forest edges next to open land the drip line may be larger i.e. 5-12m due to full light and wind exposure (Figure 1). So when providing guidance in your forest inventory proceedures it is often wise to consider 3 types of edges:
- Road boundary definition for a block hard against external roads.
- Boundary definition for between stands or similar forest area.
- Boundary definition for an external edge not bordered by a road or another stand (landuse change e.g. pasture etc.).
The combined effect of forest edges will have a negative effect any volume reconciliation especially as it relates to forest value. This can be large for small woodlots / stands in particular.
So the key is significant errors can occur when sampling a stand which has a lot of forest edge. Plot establishment within small wood lots (<3 ha) may therefore not be the best option (Maclaren, 2000). In reality forest edges should be sampled as equivalent proportion in relation to the forest NSA. If done correct projected estimates of volume and value will most likely be very close.
Recently we have had queries from some of our clients looking to utilise an exisiting Emission Trading Scheme Forest Measurement Approch (FMA) for forest valuation and product mix estimation. It is important to consider that the current FMA inventory procedures do not to include forest edges in their inventory design. This is due to a different set of objectives, this being biomass or carbon estimation while attempting to reduce complexity of the inventory design. FMA plots are moved from the forest edge into the stand thus avoid capturing any of these effects mentioned within. So if thinking of combining a forest product yield inventory (like MARVL, Method of Assessing Recoverable Volume by Logtype) inventory with FMA without careful consideration can very quickly have a large bias on the end answer.
Our team will be happy to assist you in every component of above. If you need further assistance, please contact us.
– Maclaren, J. P. (J. Piers) & Forest Research Institute (N.Z.) 2002, How much wood has your woodlot got? : a practical guide to estimating the volume and value of planted trees, Forest Research, Rotorua, N.Z
– MJ Ducey, JH Gove, HT Valentine 2004, A Walkthrough Solution to the Boundary Overlap Problem. Forest Science 50(4):427-435
– Gregoire, T.G. 1982, The unbiasedness of the mirage correction procedure for boundary overlap. Forest Science 28:504-508