3.2. Available Biomass for Soil Protection
With the use of BiOS, on-the-ground biomass obtained from stand 1 (115 year old softwood, naturally regenerated) was determined to range from 16 to 54 GT ha−1
for 30 and 100% tree removals, respectively (Table A1
). Increasing total trail area, through a reduction of trail spacing and increasing width, lowered the average amount of brush applicable for soil protection on machine operating trails. The lowest applicable amount of brush was associated with the full (100%) trail coverage and increased as overall trail coverage was reduced.
In a clear-cut operation, the amount of brush applicable on the 5-m trail segment varied from 24 kg m−2
(16 m spacing, 4.5 m width) to 68 kg m−2
(22 m spacing, 3.0 m width). When considering partial harvests where only 30% of the merchantable volume was harvested, brush amounts applicable to protect a 5-m long trail segment were significantly reduced and varied from 7 kg m−2
(16 m spacing, 4.5 m width) to 21 kg m−2
(22 m spacing, 3.0 m width). In the event where harvesting was restricted to the area covered by machine operating trails, a brush mat of 5 kg m−2
would be produced over the entire trail network, regardless of tested widths and spacings. Although rarely applied, this technique was considered in a situation where a harvester entered a harvesting block only to harvest material from the machine operating trails, while subsequent tree removal located in between trails would be performed at a later entry. For clarity and to fit within the scope of this article, Table A1
, Table A2
, Table A3
, Table A4
and Table A5
focus on biomass and brush related results. However, all removal rates presented can also be calculated on an m3
basis when referring to the merchantable volume to obtain an operational context.
On-the-ground biomass availability, presented in Table A1
, Table A2
, Table A3
, Table A4
and Table A5
, was highest for stand 2 (139.0 GT ha−1
; black spruce plantation) and lowest for stand 3 (17 GT ha−1
; immature white spruce plantation). Due to its young age and relatively low merchantable volume, the effect of a clear-cut operation was not studied in stand 3 whereas partial harvests (50 and 30% tree removals) were considered. During a commercial thinning operation with a 30% removal, stand 3 only produced between 4 and 8 kg m−2
of brush when considering full trail coverage for the 4.5-m-wide trail spaced at 16 m and the 3.0-m-wide trail spaced at 22 m, respectively (Table A3
). In the case where soil bearing capacity was very low, maximum brush that could be placed on a 5 m segment of machine operating trail was 13 kg m−2
and corresponded once more to the 3.0-m-wide trail spaced by 22 m.
The amount of on-the-ground biomass varied considerably with stand properties, in particular species composition. Despite having similar merchantable volumes (95 and 100 m3
), stands 1 and 5 yielded very different brush amounts for trail protection (Table A1
and Table A5
). At 100% removal, stand 1 provided, at varying trail widths and spacings, between 19 and 40 kg m−2
of brush for use over the entire trail network whereas stand 5 (deciduous) produced 34 to 71 kg m−2
of brush, corresponding to a 79% increase in brush availability for the deciduous stand in comparison to its softwood counterpart. However, Labelle and Jaeger [6
] showed that softwood brush mats had slightly better load distributing capacities than hardwood brush mats when compared at same mass per square meter.
3.3. Available Biomass on a Per Tree Basis
Using the properties from stand 2 (black spruce plantation), we determined the number of trees required for on-trail delimbing for providing the needed brush amounts and the number of trees whose brush is remaining for alternative use. Knowing the width of a machine operating trail and assuming equal tree spacing and tree size, the number of trees to be delimbed over a fixed segment of trail is easily calculable. This approach would likely provide numbers more applicable for machine operators considering the ease at which the trail segment length can be estimated using the reach of the boom as reference.
The effect of varying brush mat requirements for trail protection on the number of trees to be delimbed over the machine operating trail can be explained using Figure 1
. Based on a 10-m-long boom, and limiting its movement to 90 degrees on each side of the harvester, the area covered equaled 157.1 m2
. Using the properties from stand 2 (dbh = 23 cm, height = 20 m, on-the-ground biomass = 139 GT ha−1
, and 1003 stems ha−1
) and applying a clear-cut CTL operation with 3.5-m-wide trails spaced by 20 m (twice the length of the 10 m boom), the effect of increasing brush mat requirements for trail protection on the use of tree biomass can be observed. According to the area covered by the harvester boom, the number of stems per ha, and assuming equal distribution of trees within the stand, a total of 16 trees would be located within the path of the boom. As the amount of brush required for soil protection increased from 5 to 30 kg m−2
, more trees needed to be delimbed on the trail, and biomass available for alternative use decreased from 128 to 72 GT ha−1
, respectively (Figure 1
). In Figure 1
F, limbs and tree tops of 8 out of the 16 total trees were required to reach the 30 kg m−2
target brush mat on the 10-m-long machine operating trail segment.
The complete dataset for stand 2 presented on a per tree basis is available in Table A6
. Brush mat amounts requiring more biomass than what was available within boom reach are identified with a negative value in the corresponding “A” (additionally available) column. A positive number in this column indicates that the target brush amount can be achieved. The number of trees required to build a respective brush mat is in relation to a trail length of equal distance to the reach of the boom. For specific trail width, increasing trail spacing through use of a longer machine boom provided access to a larger area and the corresponding number of accessible trees increased (Table A6
). Conversely, for a specific trail spacing, an increase in trail width required more brush for trail protection compared to a narrower trail. As trail brush mat requirements increased from 5 to 30 kg m−2
, the number of trail spacing and width options available was reduced as indicated by the frequency of shaded cells. Focusing on clear-cut operations, brush mats from 5 to 30 kg m−2
could be maintained over the entire machine operating trail network, regardless of the tested trail widths and spacings. When the target brush amount was set to the maximum tested (30 kg m−2
), the number of trees with brush remaining for other uses varied from 2.3 to 11.9 for the 4.5-m-wide trail spaced at 16 m and the 3.0-m-wide trail spaced at 22 m, respectively. During partial harvests, the frequency of shaded cells increased significantly compared to clear-cut harvesting. When applying the lowest degree of tree removal tested (30%), a brush mat beyond 15 kg m−2
could only be sustained on certain trail dimensions whereas brush mats of 25 and 30 kg m−2
were not possible, regardless of trail dimensions, during a 30% partial cut of the merchantable volume.
From Table A6
, the effect of varying brush mat requirements can also be observed on the number of trees that could potentially be delimbed on the side of the machine operating trail, thus enabling a bioenergy operation. The highest number of remaining trees (18) available within boom reach was associated with a clear-cut operation performed over a 3.0-m-wide trail spaced by 22 m, whereas the fewest number of trees (9) available was associated with the 4.5-m-wide trail spaced at 16 m.
Focusing on clear-cut operations, these trends can be easily observed in Figure 2
. When combining all trail spacings, an increase in trail width leads to a reduction in the average number of available trees, whose residues could be used for a biomass operation. This trend, apparent for all brush amounts tested, was more pronounced as brush amounts increased. If a 5 kg m−2
brush mat was allocated for protecting the entire trail network, increasing trail width from 3.0 to 4.5 m reduced the number of trees with brush available for bioenergy use by 3.7%. For the 30 kg m−2
brush amount, the number of trees with available brush was reduced by 38% when increasing trail width from 3.0 to 4.5 m.
Knowing the area covered by the sweep of a boom and the average biomass per tree, the number of trees with brush available for bioenergy use can easily be used to estimate the available brush amount in GT ha−1
to assess the feasibility of a bioenergy operation (Table A7
). Similar to Table A6
, negative values indicate the scenarios where brush mat requirements exceeded available biomass, thus resulting in a deficit. In these instances, no biomass remained for other use aside soil protection. The amount of biomass available for other use, aside from trail protection, increased considerably when brush required for soil protection decreased from 30 to 5 kg m−2
). When achieving a 5 kg m−2
brush mat during a clear-cut operation, the biomass remaining on site varied between 121 and 130 GT ha−1
, for the 4.5-m-wide trail spaced at 16 m and the 3.0-m-wide trail spaced at 22 m, respectively. In comparison, only a maximum surplus of 104 GT ha−1
was possible when applying a 20 kg m−2
brush mat, indicating a 20% reduction compared to the same trail dimensions as used in the 5 kg m−2
analysis. When targeting the heaviest brush mat of 30 kg m−2
, biomass remaining varied between 32 and 87 GT ha−1
for the 4.5-m-wide trail spaced at 16 m and the 3.0-m-wide trail spaced at 22 m, respectively. At this brush mat amount, most 16-m and 18-m trail spacing options produce a biomass deficit when applying partial harvests. In fact, during a 30% removal, only 3.0-m-wide trails spaced by 18, 20, and 22 m were viable options.
3.4. Relationship Between Trail Area and Brush Mat Amounts
Based on different on-the-ground biomass availability, we also determined the relationship between the area covered by machine operating trails (m2
) and the amount of brush (kg m−2
) applicable for trail protection. The semi-log graph (abscissa) indicates reverse exponential functions whereas for a respective on-the-ground biomass availability, decreasing trail area increases the amount of brush available for trail protection (Figure 3
). Once on-the-ground biomass in GT ha−1
is determined through BiOS or another biomass supply model, an appropriate trail area or brush amount for soil protection can be established. For a respective trail area, brush for soil protection increased with higher on-the-ground biomass availability. For example, the maximum amount of brush that could be placed uniformly over the entire trail network is 50 kg m−2
for a stand with 40 GT ha−1
of on-the-ground biomass. Relating the trail area throughout the harvest block on a per hectare basis, and assuming equal coverage over the entire trail network, it is possible to determine the maximum amount of brush available for soil protection.
To determine which of the three factors (brush amount, trail width, and trail spacing) has the most significant impact on the amount of brush required for trail protection, we used standard trail dimensions of 20 m spacing and 3.5 m width covered with a 10 kg m−2 brush mat. The intentions were to: (1) assess the effect of reducing trail spacing from 20 m to 19 m, (2) determine the impact of increasing trail width from 3.5 m to 4.0 m, and (3) evaluate the amount of brush required for trail protection when increasing brush mat amount from 10 to 11 kg m−2 on the overall brush requirements on a per hectare basis.
First, increasing brush amount required for trail protection from 10 to 11 kg m−2 required an additional 1750 kg ha−1. Second, an increase in machine operating trail width from 3.5 m to 4.0 m required an additional 2500 kg ha−1 of brush. Third, reducing trail spacing from 20 m to 19 m required an extra 1940 kg ha−1 of brush. This basic analysis demonstrates that in order of importance, factors affecting needed brush for trail protection would be trail width, trail spacing, and brush amount (kg per m2). This would mean that in thinning operations or harvest blocks in proximity to where bioenergy demand is high (prime locations for thinner brush mats), a reduction of trail width would be the most cost-effective factor to increase the amount of brush available for bioenergy use.
3.5. Economic Impact of Leaving Brush Mats for Trail Protection
As explained earlier, spatial distribution and magnitude of brush mats in mechanized forest operations depend on soil conditions, machine configurations, degree of tree removal, and type of silvicultural treatment. Labelle and Jaeger [6
] suggested brush mats of 20 kg m−2
(green mass) for trail protection to minimize machine peak surface contact pressures on sensitive sites and for trail segments with highly susceptible soils brush mats of up to maximum available brush amounts. During a clear-cut operation of stand 2 (Figure 1
D), allocating a 20 kg m−2
brush mat for soil protection over an entire trail network of 1750 m2
(3.5-m-wide trail spaced by 20 m) would leave 94 GT ha−1
of biomass for other use such as bioenergy supply (Table A7
). Leaning on the side of caution, only 70% of the remaining biomass, corresponding to 66 GT ha−1
, will be used for the biomass calculation, thereby leaving 52.5% of the total on-the-ground biomass on site.
Several key parameters of biomass harvesting must first be addressed before we can evaluate the financial impact of harvesting the surplus biomass. This example will again focus on stand 2 (high-yield black spruce plantation), where a clear-cut CTL operation is performed on the entire 20 ha harvest block. The origin of the biomass, harvesting residues (limbs, tree tops, and foliage) pre-piled on the side of machine operating trails, will be transported with a forwarder to a roadside landing where a horizontal grinder will be utilized for comminution.
In BiOS, site conditions were classified as defined in Section 2.1
and transport distances equal to 75 km from the roadside yard to the mill were programmed per road class (5 km road class 3–4; 20 km road class 1–2; and 50 km on paved roads).
In terms of operating costs, harvesting costs were non-existent since these were allocated in relation to the processed industrial round wood. All costs presented are based on Canadian dollars. Costs associated with cutover handling (use of forwarder to transport harvesting residue from the trail to a roadside yard) were estimated at $11.32 per GT, while roadside and landing costs associated with the horizontal grinder were $6.86 per GT. Transport costs for the distances mentioned above were calculated at $10.05 per GT. Total operating costs of removing the biomass from stand 2, comminuting this biomass at roadside and transporting it to a mill located 75 kilometres away were calculated at $28.23 per GT.
Comminuted biomass delivered at the mill generally sells at between $25 and $35 per green ton. Considering the operation costs of $28.23 per GT and an average selling price of $30 per GT, the biomass operation of the 20-hectare stand described above (1320 GT of biomass) would produce a revenue of $2336 (66 GT ha−1 × 20 ha × $1.77 of profit per GT). The revenue for an operation can fluctuate based on site conditions and machinery used. According to BiOS, using a roadside chipper (600 kW engine) instead of the horizontal grinder would increase the profit for the total operation to $5400.
To quantify the effect of leaving a 20 kg m−2
brush mat on site for machine operating trail protection, we subtracted the amount of brush remaining from the total on-the-ground biomass available to obtain the amount of brush required for soil protection (45 GT ha−1
; Table A7
). This amount of brush was required to cover the entire trail network uniformly. Assuming the same conditions as explained in the scenario described above, using this extra 45 GT ha−1
(892 GT for the entire 20 ha harvest block) of brush for bioenergy operation would yield an additional $
1579 of revenue.
Knowing the loss of revenue associated with allocating a 20 kg m−2
brush mat, we estimated the financial impact of leaving machine operating trails uncovered by brush on the potential loss of forest productivity over the next rotation. Leaving machine operating trails uncovered would result in machine loads being fully and directly exerted to the soil. Consequently, machine traffic would cause a higher increase in soil density and soil displacement, which can significantly reduce plant growth as it limits root growth, particularly in the case when long-term machine operating trails (use of same trail system over multi-stand entries) are not common [20
]. In a review of 142 studies where soil compaction had been reported, Greacen and Sands [21
] found that 82% of the cases reported reduced tree growth. Following subsoil compaction below a depth of 10 cm, Murphy et al. [22
] determined that reduced growth in a radiata pine (Pinus radiata
D. Don) plantation resulted in a decrease of stand volume up to 42% over a 28-year projection period. Froehlich and McNabb [23
] estimated that tree growth would decrease by 6% for every 10% increase in soil density, while Helms et al. [24
] showed that ponderosa pines growing on compacted machine operating trails had 13–50% height growth reductions compared to trees growing on non-compacted soil. The studies that assessed the effect of machine-induced soil compaction on tree growth are all site, machine, and species dependant, which make their application to generalized scenarios difficult. However, to present a balanced and somewhat conservative example, a 30% tree growth reduction will be hypothesized on the area directly affected by machine traffic and will correspond to two 70-cm-wide tracks per machine operating trail. Using the same trail dimensions as the previous example (3.5 m wide and 20 m spacing; 1750 m2
), the two 70-cm-wide machine tracks would translate to an area of 700 m2
. Some area of the stand might not require high traffic frequency or may be located on soil with higher bearing capacity where machine traffic occurring directly on the forest soil would not cause a significant soil density increase, thus not affecting stand productivity. For this reason, this scenario will only consider 75% of the area directly impacted by forest machine tires (525 m2
) to be severely compacted and thus causing a 30% tree growth reduction over the next rotation.
Based on the current merchantable volume of stand 2 (329 m3
, 75 years old), and assuming that machine operating trails are not re-used for multiple entries (often the case in eastern Canada) and for the affected area of 525 m2
(5.25% of entire stand surface area), a 30% reduction in tree growth projected over the next rotation of 75 years would correspond to a loss of 5.2 m3
in merchantable volume. This reduction in volume would translate to 104 m3
for the 20 ha stand. Taking into account the time value of money and assuming a very modest rate of $
25 per m3
(average estimated price for stud wood, pulpwood, and biomass of harvested wood), this would represent a $
49,258 loss in revenue (2600 × (1 + 0.040)75
) at the end of the projection period of 75 years. To obtain an offset, the revenue associated with extracting the 20 kg m−2
brush amount for bioenergy over the entire stand of 20 ha ($
2336) was projected over the same 75-year time horizon equalling a revenue of $
44,256 (2336 × (1 + 0.040)75
). Subtracting the cost associated with a 30% growth reduction ($
49,258 for the loss of 104 m3
) from the revenue of using the 20 kg m−2
brush amount for bioenergy operations ($
44,256), yields a deficit of $
5000, corresponding to an 11% difference. In fact, a reduction in tree growth of approximately 27.5% would be required to offset the costs associated with protecting the entire trail network with a 20 kg m−2
brush mat. However, if biomass prices were to significantly increase, which is quite conceivable due to expanded markets and carbon credits, the cost associated with protecting forest soils against machine traffic with the use of brush mats would increase accordingly. It is important to note that this hypothesized scenario only considered reduced growth in 75% of the area directly impacted by the machine running gear. This area is where most severe soil property alterations would occur, however, tree growth could also be affected beyond the area directly impacted by the machine running gear, which would increase the loss of revenue [25