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Forest maintenance with harvesters and forwarders

The progressive worsening of climatic conditions requires that Italy's forest heritage be managed with ever greater care and diligence. From the point of view of mechanisation, the adoption of state-of-the-art machinery can facilitate better forest management, while ensuring the highest respect for the environment and a secure income for forestry companies

by Aldo Calcante
January - February 2023 | Back

The Italy is one of the European countries with the largest area occupied by forest. The latest INFC (National Inventory of Forests and Forest Carbon Sinks) survey of 2015 estimates a forest cover of more than 11 million hectares, occupying about 36.7% of the Italian territory. The total volume of plants present in forests is over 1.5 billion cubic metres, with an average of about 165.4 m3/ha. It should be noted that compared to 2005, the total volume has increased by a good 18.4%.

These data are of great significance, if we consider the role of plants in the carbon cycle: Italian forests are able to store about 539 million tonnes of CO2 (9.4 t/ha on average), to which we must add about 30 million tonnes contained in dead wood (3.3 t/ha of CO2 on average). In addition, from a hydro-geological point of view, forests perform a protective function for the underlying soil, especially on steep slopes, those subject to a greater risk of erosion and instability.

The protection of forests is therefore of paramount importance, especially in view of the increase in extreme weather phenomena that have caused (and are still causing) extensive damage to forest areas. One example of this was storm Vaia in October 2018, which severely affected the mountainous areas of Veneto, Trentino Alto Adige and Friuli Venezia Giulia in particular, with around 41,500 hectares of forest destroyed and 14 million trees felled.

For these reasons, the role of Italy's 6,471 forestry companies (with over 12,000 employees) has changed considerably in recent years. Although they derive most of their profit from cutting and trading woody biomass, these companies are also involved in naturalistic engineering, fire prevention, pest defence, reforestation, hydro-forestry and restoration/maintenance of forest roads.

Levels of forest mechanisation. In recent years, the forestry sector has experienced a slow increase in the level of mechanisation. The main obstacle is related to the considerable slopes that characterise Italian forests: about 35% of timber is harvested from territories with slopes between 60 and 80%, which complicates the organisation of modern work sites with a high production capacity.

Another not insignificant problem is the lack of adequate forest roads, which severely limits access to the forests, especially for specialised machinery.

Three levels of mechanisation can be identified for carrying out forestry work. A first level of traditional mechanisation is based on the availability of multi-purpose machines, not specifically designed for forestry use. The basic equipment is the chainsaw. It evolves into "advanced traditional mechanisation" if machines specifically set up for forestry use are used, such as forestry tractors with a winch or with a forestry loader. A second level of advanced mechanisation regards forestry machines capable of performing certain routines independently, but not an entire processing cycle (e.g. excavator with grapple). The third, advanced, level of mechanisation involves the use of specialised forestry machines capable of carrying out the entire felling and timber processing cycle (e.g. harvester-mounted or excavator-mounted processors). This type of mechanisation includes the "Austrian method", typically adopted in mountain coniferous forests, which consists of felling and cutting with a chainsaw, extraction with a skidder or cable crane, and rigging with a processor on a logging road, and the "Scandinavian method", which on the other hand involves the harvester-forwarder combination, i.e. the most complex and highly efficient worksite currently available in forestry.

Harvesters. The harvester, or felling machine, is a machine consisting of a drive unit and a hydraulically driven arm with a felling head mounted on the end. The harvester head cuts the tree to be felled at the base, directs its direction of fall, and at the same time removes the limbs and, if necessary, stacks the material thus prepared.

The header includes: a gripper that holds the log, allowing it to be felled and moved in a controlled manner; a chainsaw driven by a hydraulic motor, for cutting (in some cases the chainsaw can be replaced by a disk saw or a shear mechanism); a series of devices (usually two or more hydraulically driven rollers) that push the log against special blades to allow the tree to be cut. The same chainsaw is then used to section the log according to predefined measurements. Once the timber has been stacked, it can be harvested using forestry tractors and trailers, skidders and forwarders.

The drive unit can be equipped with wheels or tracks. In the former case, there are 4-, 6- and 8-wheel drive harvesters on the market. Obviously, as the number of driving axles increases, the stability and lift of the machine increases, at the expense, however, of its manoeuvrability and agility in the forest. The tracked version, on the other hand, can have either a single track on each side of the traditional two axles, or a trapezoidal track on each hub, to create a drive unit with four independent, self-levelling tracks. This makes it possible to operate safely even on steep slopes (up to 60%) that cannot be reached with wheeled models (< 30%). From a construction point of view, a harvester features an engine of between 100 and 200 kW, with hydrostatic transmission. It is a machine of considerable size: the mass can reach 25 tonnes, and the footprint up to 8 m long by 3 m wide. The development of the hydraulic arm can reach up to 11 m, and the head can process trees with a cutting diameter between 35 and 70 cm. It can reach a speed of 20 km/h on the road, with a steering angle of up to 40°.

Regardless of the characteristics of the drive unit used, the harvester is a complex machine with a high working capacity: this limits its use almost exclusively for intense and concentrated cuts (e.g. cultivated poplar groves) and on conifers, such as fir, larch and pine. Furthermore, as it is characterised by a certain maximum diameter of the trees that can be processed, we need to consider that some trees must necessarily be felled with the chainsaw. In any case, the felling header leaves taller stumps than the cut made by hand.

Forwarders. These are articulated tractors capable of combining the functions of tractor, trailer and loader in a single vehicle. These machines perform the same functions as agricultural tractors equipped with forestry trailers with a hydraulic crane, but with greater efficiency, so much so that their working capacity is tripled, with the same dimensions. From a construction point of view, a forwarder consists of two parts, a front part that includes the driver's cab and the engine, and a rear part equipped with a loading platform, a hydraulic crane with a hydraulic grapple and a reach of between 7 and 10 m, and sometimes with one or two winches for hauling and/or concentrating timber. The front end, with one or two axles, and the rear end, usually with two axles, are connected by a strong articulated joint. All axles are motorised and self-levelling, so the forwarder is able to provide high tractive force while still maintaining a fair degree of manoeuvrability.

The drive components consist of isodiametric forestry tyres on which forestry chains or special semi-tracks can be applied to increase the machine's grip. The cab - itself self-levelling - can rotate 290° to improve the view during work and facilitate manoeuvring. These machines are able to move mainly on forest tracks and on medium slopes (up to 40 per cent) logging routes and can also work under full load over fairly long distances (up to 4-5 kilometres). Forwarders have an engine power of between 100 and 200 kW and usually have a hydrostatic transmission to speed up manoeuvring. Like harvesters, forwarders are also imposing machines, with lengths of up to 12 m, widths of around 3 m and masses of between 12 and 20 tonnes. The load-bearing capacity of the larger models can be up to 20 tonnes. The forward speed does not exceed 20 km/h and the steering angle is up to 45°, allowing good manoeuvrability despite the large dimensions. As with harvesters, given the considerable size of these machines, the suitability of the forest road must be carefully assessed. To remedy the most critical situations, so-called "mini-forwarders" are available which, while maintaining the same architecture as the classic models, have lower power and dimensions. More specifically, the power varies between 20 and 40 kW, the mass does not exceed 6 tonnes, the track width can be less than 2 m and the overall length less than 8 metres. These characteristics are suitable for official approval for use on public roads, among other things, at a speed of 40 km/h. On the other hand, compared to larger versions, it is obvious that the load capacity, which is - as a rule - within 5 tonnes, decreases drastically with these models.


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