Chapter 7 comparison with combined mulitple layers’ projection

As noted the provincial NSYT process generates yield projections using only the Rank 1 (R1) forest layer from each VRI polygon. This approach was adopted to support efficient province-wide yield generation while maintaining consistency with the leading stand layer that typically represents the majority of forest volume and site productivity within a polygon. However, many VRI polygons contain multiple forest layers, such as a primary overstory layer combined with residual, secondary, or understory layers. In a standard VDYP7 workflow, each eligible layer can be projected independently, and the resulting layer-level yields are subsequently aggregated to produce a polygon-level yield projection. As a result, differences may arise between projections generated using only the R1 layer and projections generated by combining all forested layers within a polygon.

This chapter evaluates the magnitude and significance of those differences by comparing the provincial NSYT “R1-only” projections against yield projections derived from the full multi-layer representation of VRI polygons. The analysis examines two distinct polygon types: (1) polygons containing a primary (P) layer and one or more additional live forest layers, and (2) polygons containing only a primary layer with one dead (D) layer. By comparing projected merchantable volume through time for both approaches, the assessment quantifies the effects of simplifying polygon representation to a single layer and evaluates whether the resulting provincial yield estimates remain representative of the yields that would be obtained using the complete multi-layer inventory structure. The results provide an important validation of the NSYT methodology and help identify circumstances where multi-layer stand dynamics may contribute meaningfully to differences in projected yield.

Table 1. numbers multi-layer polygons across the province (from vdyp_input_layer_and_polygon_2024)

category	N	pct
onelayer	5115447	83
multilayer	239892	4
P_and_D	812801	13

*“multilayer” means the polygon has P layer and at least one other layer that is not D layer; “P_and_D” means the polygon only has P layer and D layer

7.1 set1. r1 only vs multilayer set1 (containing P layer and at least one other layer that is not D layer)

Number of multi-layer polygons that were included in this comparison: 239198

Mean live projected volume net dead, waste and breakage (PRJ_VOL_DWB) across projected stand age were calculated from the live layers that were projected in the multiple-layer projection, and the results were compared with R1-only projection. The mean PRJ_VOL_DWB was very similar between the two projections, with the multiple-layer projection generally producing slightly higher mean values at younger ages and converging with the R1-only projection at older ages (Figure 1 and Figure 2). Even though mean PRJ_VOL_DWB was close, the individual differences can vary widely in both positive and negative directions and the variation increases over projected age (Figure 3).

Figure 1. Mean PRJ_VOL_DWB by projected age

Figure 2. Mean differences of PRJ_VOL_DWB by projected age

Figure 3. Variation of PRJ_VOL_DWB by projected age

7.2 set2. r1 only vs multilayer set2 (only containing P layer and D layer)

Number of multi-layer polygons that were included in this comparison: 811118

The projection of P layer from multiple-layer set 2, of which each polygon contains one P layer and one D layer, was compared with R1-only projection. “R1-only” projection produces consistently higher mean PRJ_VOL_DWB than multiple-layer projection after approximately age 25, and the mean differences increased with the increase of projected age (Figure 4 and Figure 5). Individual differences varied widely mainly in the positive direction and the variation increases over projected age (Figure 6).

Figure 4. Mean PRJ_VOL_DWB by projected age

Figure 5. Mean differences of PRJ_VOL_DWB by projected age

Figure 6. Variation of PRJ_VOL_DWB by projected age

7.3 Overall - one-layer polygons + multiple-layer polygons (set1+set2)

Mean live PRJ_VOL_DWB were calculated by averaging the PRJ_VOL_DWB from all polygons provincially, including single layer polygons and multiple layers polygons, and compared with R1-only projection. At the provincial scale, The differences is not significant with the largest differences from this test is around 3m3/ha at age 200 (Figure 8 & Table 2).

Figure 7. Mean PRJ_VOL_DWB after combining provincial polygons

Figure 8. Mean differences(R1 only projection - multilayer projection) after combining all polygons

Table 2. statistic summary table of all polygons

PRJ_TOTAL_AGE	PRJ_VOL_DWB_r1	PRJ_VOL_DWB_multilayer	mean_diff	p_value	conf_low	conf_high
0	0.0000043	0.0000043	0.0000000	NaN	NaN	NaN
10	0.0281453	0.0736559	-0.0455105	0	-0.0476008	-0.0434203
20	1.0852617	1.1959393	-0.1106776	0	-0.1140340	-0.1073213
30	6.3378247	6.4554847	-0.1176600	0	-0.1214046	-0.1139154
40	18.1189195	18.1510706	-0.0321511	0	-0.0359946	-0.0283075
50	35.7041697	35.5997730	0.1043967	0	0.1005060	0.1082875
60	56.8703606	56.9124880	-0.0421274	0	-0.0472716	-0.0369831
70	79.5466662	79.4647204	0.0819458	0	0.0766872	0.0872043
80	102.2248499	102.0196341	0.2052158	0	0.1997373	0.2106943
90	124.0115760	123.6633117	0.3482643	0	0.3424540	0.3540745
100	144.5266269	144.0118668	0.5147601	0	0.5085241	0.5209962
110	163.2818778	162.5794821	0.7023957	0	0.6956641	0.7091272
120	180.1846673	179.2610162	0.9236511	0	0.9163616	0.9309406
130	195.2633307	194.0588249	1.2045058	0	1.1965708	1.2124409
140	208.6505357	207.0979542	1.5525815	0	1.5438993	1.5612637
150	218.8410095	216.9068308	1.9341787	0	1.9247177	1.9436398
160	225.3601844	223.0510313	2.3091530	0	2.2989591	2.3193469
170	229.2210614	226.5909221	2.6301394	0	2.6193135	2.6409652
180	231.2160737	228.3500062	2.8660675	0	2.8547676	2.8773673
190	231.9197136	228.8994187	3.0202949	0	3.0086807	3.0319092
200	231.7569251	228.6581189	3.0988063	0	3.0870294	3.1105831