Fully mechanized hardwood operations usually rely on manual bucking, which may decrease potential volume and value recovery as compared to automatic bucking. This study aimed at evaluating bucking done in cut-to-length (CTL) operations, hypothesizing a possible increase of value recovery by using a bucking optimizer. Evaluation of bucking was done by a comparison of actual bucking with a mathematical solution – BuckR – a bucking optimizer based on dynamic programming. Results showed that the mathematical approach significantly outperformed manual bucking when quality was not considered, and only main stem products were included. Throughout 315 study trees (Acer saccharum, Acer rubrum, and Betula alleghaniensis) located in the Acadian forests of Eastern Canada, a mean increase of 112% in value and 84% in volume recovery per tree was reached through mathematical optimization. Contributions of mathematical bucking can be summarized by 1) a significantly higher number of sawlogs and a greater mean log length of sawlogs and pulp logs, 2) a two-fold increase in processed height, and 3) a considerable reduction in minimal diameters as compared to actual bucking. Those results illustrate the possibility to increase value and volume recovery in mechanized CTL operations and therefore to utilize the wooden resource more efficiently.

Fully mechanized hardwood operations usually rely on manual bucking, which may decrease potential volume and value recovery as compared to automatic bucking. This study aimed at evaluating bucking done in cut-to-length (CTL) operations, hypothesizing a possible increase of value recovery by using a bucking optimizer. Evaluation of bucking was done by a comparison of actual bucking with a mathematical solution - BuckR - a bucking optimizer based on dynamic programming. Results showed that the mathematical approach significantly outperformed manual bucking when quality was not considered, and only main stem products were included. Throughout 315 study trees (Acer saccharum, Acer rubrum, and Betula alleghaniensis) located in the Acadian forests of Eastern Canada, a mean increase of 112% in value and 84% in volume recovery per tree was reached through mathematical optimization. Contributions of mathematical bucking can be summarized by 1) a significantly higher number of sawlogs and a greater mean log length of sawlogs and pulp logs, 2) a two-fold increase in processed height, and 3) a considerable reduction in minimal diameters as compared to actual bucking. Those results illustrate the possibility to increase value and volume recovery in mechanized CTL operations and therefore to utilize the wooden resource more efficiently.

Contamination of wood chip fuels with soil material, e. g. during logging operations, might result in a deterioration of fuel quality and in elevated gaseous and particulate emissions, corrosion, or slag formation during combustion in biomass boilers. In contrast, fuel upgrading by washing/screening might improve combustion behaviour. To investigate these effects, wood chips from stemwood and forest residues originating from three different forest growing regions (A to C) in Bavaria, Germany, were deliberately contaminated with mineral soils at two mixing ratios (5 %, 10 %) or were mechanically upgraded by screening or washing (only fuels from region A). Contamination was performed using three typical forest soils from Bavaria that corresponded to the selected fuel growing areas (soil A, B and C). Wood chips were tested for fuel quality according to ISO standards for solid biofuels and combusted in a 30 kW biomass boiler. Throughout the trials, fuel upgrading usually improved combustion behaviour. Total particulate matter (TPM) emissions increased for wood chips from stemwood when contaminated with soil A and C from 50 mg/m3 up to 236 mg/m3 (STC), compared to the reference fuel, while emissions for wood chips contaminated with soil B remained constant at approx. 50 mg/m3 (STC). Carbon monoxide (CO) emissions decreased for contaminated wood chips by up to 89 %. For NOX emissions, no clear effect was observed. In addition, contamination mostly increased slagging of fuels when contaminated with soil. Overall, the contamination of woody biomass had a noticeable effect on pollutant emissions and slag formation and should be avoided.

It is common to have large trees in mature hardwood-dominated stands. This is especially true for European beech (Fagussylvatica L.), which can also have a complex architecture. Such trees have predominantly been harvested using motor-manual operations. However, in an effort to increase occupational safety and allow for a more continuous wood flow to processing facilities, fully-mechanized systems are also being employed more frequently. This study was established to determine the effect of season (Fall or Winter) and harvester type (wheeled or tracked) on the performance of semi- and fully- mechanized harvesting systems deployed in beech-dominated stands. Time-and-motion analysis was conducted on a total of 927 trees located in two forest sites in Germany. The study indicated that new silvicultural prescriptions make it impossible to harvest all trees exclusively with mechanized systems, even in the case of the tracked harvester with its 14.5 m boom. Motor-manual intervention was needed with trees that were too large, malformed or out of reach. Motor-manual intervention was significantly more frequent for the wheeled (30%) than for the tracked harvester (18%). Once again, tree size had the strongest effect on time consumption in a linear model, which varied from 0.5 to over 6 min per tree. Season and machine effect were also significant but could only account for a small fraction of the total variability. For the same tree size, time consumption was higher with the wheeled harvester and during the fall. The model also indicated a significant relationship between tree form and time consumption, even though the explanatory contribution of this independent variable was relatively small, too. Good stem form resulted in a lower time consumption. The larger tracked harvester was generally more efficient, but also more expensive to own and operate: its higher costs must be weighed against the higher revenues. New silvicultural trends make it difficult to achieve full mechanization, but the results of this study may guide managers towards technical solutions that minimize motor-manual intervention to the advantage of higher productivity and better occupational safety.

Technical drying of wood chips by active ventilation is often applied, i. a. to reduce dry matter and energy losses due to decomposition during storage. However, wood chips are often dried to very low moisture contents (MC) of below 10-15 % (moisture mass fraction on wet basis), that might lead to a waste of drying energy. But the storage-stable MC has not been investigated intensively yet. Therefore, the purpose of our study was to determine as accurately as possible the threshold of MC where significant decomposition starts. Outdoor-stored spruce wood chips were conditioned to different MC and filled into air-tight containers. The oxygen concentration inside the containers was measured after 48 h and the dry matter loss was stoichiometrically calculated. Two different assortments (high/low quality), two storage temperatures (cold/warm) and varying storage durations before sampling were examined. The results showed that MC and oxygen consumption were significantly positively correlated for both assortments and storage temperatures. Even very low MC caused a slight decrease in oxygen concentration. A multiple linear regression model had a high predictive accuracy. For example, during a storage period of 6 months a dry matter loss of 1.1 % would be caused by a moisture content of 20 % for the low-quality assortment at a storage temperature of 21 degrees C. In conclusion, this study shows that there is no absolute storage-stable MC for wood chips. The point of storage-stability rather depends on the definition of the acceptable dry matter loss. However, the results indicate, that it might not be efficient to dry wood chips below approximately 20 % MC.

This paper discusses Helmedag’s article concerning Faustmann’s formula (Helmedag in Eur J For Res, 2018. https://doi.org/10.1007/s10342-018-1101-8). He computed the present value of a fully regulated forest, including standing timber and theoretical land values. He showed that the optimal rotation in a fully regulated forest would always be the one that maximized the sustainable forest net revenues. Helmedag concluded that the discount rate would have no significance for the optimal rotation in a fully regulated forest, while his solution would still fulfil the optimality condition implied by Faustmann’s formula. Here, we refute this assertion. In fact, the assumption of a fully regulated forest has no impact on the optimal rotation period, or may even reduce it. We illustrate this by appropriately considering the actual costs of achieving a fully regulated forest with altered rotation. One must not ignore the alterations of the financial flows when changing the underlying rotation age in an established fully regulated forest. When the opportunity costs of the transition period are included, the optimal rotation becomes the same as that of a single even-aged forest, or shorter, depending on the transition regime. Under the optimal transition regime, the diminishing marginal rate of return for extending the rotation period in a fully regulated forest matches the discount rate, when we achieve the Faustmann rotation. We conclude that the optimal rotation period is independent of the status as fully regulated forest, provided efficient harvest operations during the transition period.

Due to careless operation during fuel production, considerable shares of mineral soil might be added to wood chips and wood pellets leading to contamination of the biofuels. This can result in high gaseous and particle emissions, corrosion or slag formation during combustion. To investigate this effect, four pellet fuels were produced at TFZ using coniferous wood of Norwegian spruce, i. e. a pure sample as reference and three pellets that were contaminated with different mineral soils (2 w-%). The contaminated pellets had significantly higher ash contents (2.47 to 2.63 w-%, d.b.) compared to the reference fuel (0.83 w-%, d.b.). The reference sample fulfilled the specifications for A2 according to DIN EN ISO 17225-2, while the contaminated pellets only met the I3 quality criteria. A visual differentiation between contaminated and uncontaminated fuels was not possible. The pellets were combusted in a pellet boiler with a nominal heat output of 15 kW. During combustion of the contaminated pellets, the boiler shut down because of to severe slag and ash formation after 118, 120 and 278 minutes of full load operation. Particle emissions and NOX decreased for the soil contaminated pellets compared to the reference fuel while SOX increased. Carbon monoxide was on a very low level for all fuel assortments. A final interpretation of the correlation between soil contamination of wood pellets and their respective emission behaviour is not possible yet as analysis of chemical element concentration is still ongoing. Overall, the contamination of woody biomass had a noticeable effect on emissions and on the continuous, undisturbed operation of the pellet boiler and should be avoided.

Wood chip quality is essential for failure-free and low-emission combustion in automatically stoked small-scale biomass boilers (< 100 kW). Screening and drying of wood chips might be suitable to reduce fuel heterogeneity and to guarantee defined qualities according to ISO 17225-4. Six case studies on screening and drying of forest residue wood chips were performed at German biomass terminals. Screening was done using drum, star, or jigger screens. Drying was done in storage piles and in rolling bed, walking floor, belt, and batch container dryer. Throughput rate was assessed on-site. Fuel properties were analyzed according to international standards. Moisture content of fresh wood chips was too high for small-scale biomass boilers (up to 51 w-%). Raw materials did not comply with specifications of ISO 17225-4 (i.e., ash content, particle size) while fuel indices (Fe/Mn, Al/200) implied contamination with soil material. Technical drying provided moisture contents ≤ 15 w-% while natural drying in piles resulted in values often exceeding 35 w-%. Screening reduced ash content, fines, oversized particles, and the share of N, S, Cl, K, and Si. Screening of pre-dried fuels seems to be beneficial compared to screening of fresh wood chips. After processing, chips could usually be classified as specification A2 to B1 according to ISO 17225-4.

Within six case studies, different drying and sieving process steps were employed for the removal of adhering soil and other extraneous impurities from wood chips. Consequently, it was systematically investigated to which extent this strategy can be used to jointly mitigate the risk of bottom ash slagging and high pollutant emission levels during combustion in an automatically stoked small-scale boiler. Throughout all combustion tests, slag and emission formation were recorded. Formation of agglomerates in the bottom ash was not observed in the fuel bed. However, fuel processing resulted in an increase of the ash shrinking–softening range up to 230 K indicating a lower slagging risk in the bottom ash. An asymptotic trend for the ash melting temperatures was observed as a function of the molar (Si + P + K)/(Ca + Mg) ratio. It was also found that potassium is less efficiently retained in the bottom ash with lower Si content in the fuel. Lower moisture contents in the wood chips typically resulted in lower CO emissions and higher boiler efficiencies for the investigated range of moisture content. The sieving of the unprocessed wood chips reduced NOx emission levels up to 28%. However, fuel processing did not necessarily reduce the level of particulate matter emissions.

The height of tree stumps following mechanized forest operations can be influenced by machine-, tree-, terrain-, and operator-related characteristics. High stumps may pose different economic and technical disadvantages. Aside from a reduction in product recovery (often associated with sawlog potential), leaving high stumps can complicate future entries if smaller equipment with low ground clearance is used, particularly in the case where new machine operating trails are required. The objective of this exploratory study was to examine if correlations existed between the height of tree stumps following mechanized harvesting and the shape of the above-ground root collar, stump diameter, and distance to the machine operating trail. In total, 202 sample stumps of Norway spruce (Picea abies (L.) Karst.) and the surrounding terrain were scanned with a terrestrial laser scanner. The collected data was processed into a 3D-model and then analyzed. Stump height was compared with different characteristics such as stump diameter at the cut surface, distance to the machine operating trail, number of visible root flares per stump, and the root collar. The number of root flares per stump had a positive influence on stump diameter and height, showing a general trend of increasing diameter and height with the increasing number of root flares. Root angles also had an influence on the stump diameter. The diameter of a stump and the shape of the root collar at the cut surface together had a significant effect on stump height and the model reported explained half of the variation of stump heights. Taken together, these findings suggest that other factors than the ones studied can also contribute in influencing stump height during mechanized harvesting operations. Further investigations, including pre- and post-harvest scans of trees selected for removal, are warranted.

The storage of wood chips is important for the biomass supply chain as it compensates for temporal differences in production and consumption. Typical storage-related problems are dry matter and energy losses due to microbial activity. In extensive field trials, we investigated the storage of spruce wood chips from forest residues (FRC) and from energy roundwood (ERC) with and without rain protection under Central European conditions. Additionally, we examined the storage of unchipped piles. The results indicate that the investigated factors, i.e. storage duration, season, assortment and rain protection, have a statistically significant influence on moisture content and dry matter loss of wood chips. During five months of storage, the highest decline in moisture content was 22.6 %-points, the highest dry matter loss 11.1 %. In winter, energy losses reached up to 11.3 %. In summer, energy contents did not change or even increased slightly (max. 4.7 %). Pile temperature and dry matter losses were significantly positively correlated in FRC. Formation of different layers within the piles could be detected. Storage performance was better in unchipped than in chipped energy roundwood. Storage of unchipped forest residues was not beneficial concerning energy content, but fuel quality increased due to reduced ash and fine particle content. Clear best practice recommendations could be drawn regarding wood chip storage under Central European conditions. During winter, FRC should be stored with rain protection or as short as possible while during a dry and warm summer, wood chips can be stored with only few restrictions. (c) 2017 Elsevier Ltd. All rights reserved.

The project KUP-Scout developed a plot detailed digital map to predict the yield of poplar in short rotation coppice for Bavaria. This map was implemented in the Bavarian Forest Information System and planned to be used as consultations material to support “Energiewende”. The objective was to develop or utilise an existing yield predicting model that represents the Bavarian growth conditions for poplar perfectly and gives realistic yield estimation. The realization process was focused on high resolution of geo data and high model quality. Therefore plausibility checks, statistic quality factors, sensitivity analyses were carried out and validity ranges of input data were detected. This way the best model was found out and improved using adjustment factors to compensate regional deficiency.

Critical elements such as chloride (Cl), potassium (K), silicon (Si) or heavy metals might determine high temperature corrosion, emission behavior and slagging of solid biofuels during combustion. Using the fact that plant material contains distinctly less iron (Fe) than mineral soils and vice versa manganese (Mn) in plant tissue is significant higher than in soil material and that the Al-content of plant biomass is limited, indexes could be created to classify wood chips into "contaminated with soil" and uncontaminated "biomass" by building ratios of Fe/Mn and Al/200 Thus, "biomass fuel indexes for the contamination with mineral soil" (BFICS) were developed. Plausibility checks were carried out, using tree biomass of various species und soil datasets from other research projects. They showed that for Pinus sylvestris an adapted Al-index is necessary. 2.7 % of the tree samples, Pinus sylvestris excluded, exceed the threshold value of 1 for the Fe/Mn-index and only 3.3 % exceed 1 for Al/200. The newly developed BFICS are useful to determine the source of critical elements for combustion, for aerosol formation, to detect high shares of non-combustible material and to prove the effectivity of pre-treatments like screening.

Critical chemical elements such as nitrogen (N), sulphur (S), chloride (Cl), potassium (K) or heavy metals might determine high temperature corrosion, emission behavior and slagging of solid biofuels during combustion. Recently developed biomass fuel indexes may help to pre-evaluate fuel behavior during combustion. In total, 92 samples of wood chips (n = 75), of unchipped material (n = 11) and of twigs and needles (n = 6) were collected and analyzed for elemental composition (As, Ca, Cd, Cl, Cr, Cu, Hg, K, Mg, N, Na, Ni, Pb, S, Si, Zn). Biomass fuel indexes were calculated from the results to pre-evaluate NOX and particle emissions, high temperature corrosion, K release and slagging behavior. Wood chips from natural wood showed low concentrations of critical chemical elements. With increasing shares of mineral soil, critical elements also increased. Biomass fuel indexes such as the sum of aerosol forming elements (K, Na, Pb, Zn) or the molar Si/(Ca+Mg) ratio for ash-melting behavior indicated that biomass fuels without contamination with mineral soil, road salt or other anthropogenic waste can be considered uncritical for combustion.

Biological degradation processes during storage of fresh wood chips in large piles may cause high dry matter (DM) losses and a decline in fuel quality. To monitor storage effects, two field trials were conducted i.e. one during winter and one during summer. For each trial, four piles of wood chips were established using two raw materials and two treatments: wood chips from forest residues (FRC) and from energy roundwood (ERC) covered with a fleece and uncovered. Additionally, two piles of unchipped raw material were established during each trial to compare storage effects of chipped and unchipped material. During winter storage, drying of wood chips was low. DM losses amounted up to 1.6 w-% and 0.7 w-% per month for FRC and ERC, respectively. During summer, fuel quality of wood chips in piles improved due to large drying effects. However, DM losses of ERC were twice as high compared to winter. Storage of unchipped material did not improve drying but led to lower DM decomposition compared to storage in wood chip piles. Thereby, the loss of bark material and needles increased total DM losses in unchipped storage piles but positively affected fuel quality parameters.

For a case study the production costs for high quality wood chips from forest residues including secondary treatment steps were analyzed. With 91.03 (sic)/tODT for chipping, screening and drying a higher marginal return could be achieved than with fresh, untreated wood chips. Furthermore, changes in fuel quality were determined. Using these additional production steps moisture and ash content could be lowered and particle size distribution could be enhanced considerably. This offers the possibility to sell these high quality wood chips profitable.

Utilisation of forest biomass for power and heat production plays an important role in Bavarian forestry. On sites with low levels of available nutrients, sustainability might be impaired when whole crowns and forest residues are extracted. Harvesting can be adapted by roughly delimbing coniferous crowns, leaving most branches and needles on site. During field trials, biomass exports were reduced by 17 w-%. The additional work needed amounted to 38.4 min/ODT (motor-manual variant) or 4.2 min/ODT (fully mechanised variant). Forwarding productivity was increased and could in favourable conditions (i.e. when driving longer distances) compensate the extra costs. The method can be used as an economically and ecologically feasible alternative for energy wood utilisation on sites with low levels of available nutrients.

In mechanized timber harvesting, it is common practice to build brush mats from logging residues on skid trails. Protective effects of brush mats against soil compaction are documented by several studies. On the other hand, a large quantity of nutrients is concentrated on the skid trail. Fully mechanized harvesting has been criticized frequently for this reallocation of nutrients. Is there really a risk of nutrient leaching below skid trails or imbalances? Are the nutrients redistributed through nutrient uptake by roots of adjacent trees? Effects of fully mechanized thinning on soil, water and nutrient balance were examined in a seventy years old spruce stand on a nutrient poor site in Bavaria. Sections of the trails were covered with brush mats, while other sections remained uncovered. For five replications, soil physical properties, soil chemistry, matter and water balances and the density offine roots were measured in the middle of the trail, under the tire tracks, at the transition of trail and stand and inside the stand over a period of two years. Logging operation caused soil compaction. The macro pore volume decreased and both hydraulic conductivity and air permeability were severely reduced. The nutrients were largely kept in the forest ecosystem. Results of the soil moisture monitoring indicate that, within the sections covered by a brush mat, tree roots extracted water from the soil between the tracks. Without cover, the trees scarcely extracted water from this area. Hence, building a brush mat can facilitate water availability and thus enable redistribution of nutrients.