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This research was conducted to determine the cause, intensity and location of damage (stem, butt end, root collar, root) and the extent of damage to standing trees during felling and processing by an harvester and timber extraction by a forwarder (cut-to-length system). The research was conducted in the central part of the Republic of Croatia in the Management Unit (MU) “Bjelovarska Bilogora” during the thinning of Subcompartment 14b, area of 18.28 ha, in the stand of hornbeam (Carpino betuli—Quercetum roboris fagetosum Rauš 1975), age 70, and of Subcompartment 14c, area of 9.07 ha, in a stand of common beech (Carici pilosae—Fagetum Oberdorfer 1957) aged 79 years. The thinning intensity was 12.13% in Subcompartment 14b and 13.72% in Subcompartment 14c. Field measurements were carried out on sample plots—the first time in 2017 to determine the intensity and characteristics of the damage to standing trees with regard to the cause of the damage (harvester or forwarder), and the second time in 2018 to determine the overall intensity and features of the damage to standing trees after finishing harvesting operations. For all trees remaining in the stand after the harvesting operations, the following were determined: tree species, diameter at breast height (DBH), the position of the tree in the stand depending on the forest traffic infrastructure, and—if damaged—cause of damage, type of damage, the position of damage on the tree, and dimensions of damage. The intensity of the damage was expressed by the ratio of damaged and undamaged trees, with a detailed analysis of bark damage (squeezed-bark damage and peeled-bark injuries). The results of the research indicate the highest prevalence of peeled-bark injuries. In relation to the total number of standing trees, trees with peeled-bark injuries were more represented in Subcompartment 14c (39%) than in Subcompartment 14b (33%). In Subcompartment 14b, the harvester and the forwarder damaged an equal number of trees, while in Subcompartment 14c, the harvester damaged 59% of the damaged trees. In both subcompartments, an average of 83% of (peeled bark) injuries were up to 1.3 m above the ground. In both subcompartments, the most common (67%) were injuries up to 100 cm2 in size, for which many authors claim the tree can heal by itself. Given the increasing use of harvester-forwarder systems in deciduous stands and research results that indicate possible damage to standing trees, it is necessary to pay attention to all phases of planning and execution of timber harvesting operations, thus minimising negative effects.

The goal of the research was to determine the biomass yield and fuel properties of ten different poplar clones. The research was conducted in an experimental plot established in Forest Administration Osijek, Forest Office Darda, in the spring of 2014. The layout of the plot consisted of three repetitions per clone with 40 plants per repetition in spacing 3x1 m. Based on the DBH distribution, in the early spring of 2018, one sample tree of an average DBH per repetition was selected, thus forming a sample of 30 trees. Average survival rate of the investigated trees after four vegetation periods was 74.54 ±13.85% ranging from 52.08% (Koreana) to 91.67% (SV885 and SV490). Average DBH of the sample trees was 8.2 ±1.9 cm, height 9.3 ±1.8 m and root collar diameter 10.7 ±1.9 cm. Moisture content in fresh state (just after the felling) ranged from 51.6% (Hybride 275) to 55.9% (SV885). Bark content averaged 18.4%, from 15.4% (Baldo) to 21.1% (V 609). Average nominal density of the sampled trees amounted to 383.5 ±35.9 kg/m3. Bark ash content was on average ten times higher (6.44 ±0.65%) than wood ash content (0.64 ±0.07%) resulting in average ash content of 1.7 ±0.1% (taking the bark content into account). The clone SV490 showed the highest biomass yield with 15.8 t/ha/year, while the lowest biomass yield was recorded for the clone Hybride 275 with 2.8 t/ha/year. High inter-clonal productivity variation stresses the importance of selection work to find the most appropriate clones with the highest productivity potential for the given area where the poplar SRC plantations are to be established. Due to high initial moisture content, if direct chipping harvesting systems are preferred, wood chips could be efficiently used in CHP (Combined Heat and Power) plants that operate on the principle of biomass gasification (where a gasifier is coupled to a gas engine to produce electric power and heat). In several CHP gasification plants operating in Croatia, wood chips with high initial moisture content (from traditional poplar plantations) are used as a feedstock that has to be pre-dried using the surplus heat. In this respect SRC poplar wood chips could make an ideal feedstock supplement.

The use of forestry vehicles in mechanised harvesting systems is still the most effective way of timber procurement, and forestry vehicles need to have high mobility to face various terrain conditions. This research gives boundaries of planning timber extraction on sloped terrain with a cable skidder, considering terrain parameters (slope, direction of skidding, cone index), vehicle technical characteristics and load size (5 different loads) relying on sustainability and eco-efficiency. Skidder mobility model was based on connecting two systems: vehicle-terrain (load distribution) and wheel-soil (skidder traction performance) with two mobility parameters: (1) maximal slope during uphill timber extraction by a cable skidder based on its traction performance (gradeability), and (2) maximal slope during downhill timber extraction by a cable skidder when thrust force is equal to zero. Results showed mobility ranges of an empty skidder for slopes between −50% and +80%, skidder with 1 tonne load between −26% and +63%, skidder with 2 tonne load between −30% and +51%, skidder with 3 tonne load between −34% and +39%, skidder with 4 tonne load between −35% and +30% and skidder with 5 tonne load between −41% and +11%.These results serve to improve our understanding of safer, more efficient timber extraction methods on sloped terrain.