Willow should be harvested at the end of each rotation cycle (2-5 years), normally in fall, after leaf shedding. All willow stems should be cut at a height of 5 — 10 cm above the soil surface in order to leave a stump from which new buds will form sprouts the following spring. Essentially, there are three ways to harvest willows, the choice largely depending on the final destination of biomass and the equipment available. When willows are grown to produce rods to be used in environmental engineering structures such as sound barriers, snow fences and wind breaks along highways and streets [72-73] or to produce new cuttings, plants are harvested with trimmer brush-cutters. Whole willow rods can also be stored in heaps at the edge of the field and chipped after drying.

Another option involves the use of direct-chip harvesting machines (e. g. Class Jaguar and Austoft). This technique uses modified forage harvesters specifically designed to harvest and direct chip willow stems: the stems are cut, chipped and dropped into a trailer either driven parallel to the harvester or connected directly to it. Although this harvest model is very economically efficient and recommended in many countries, it also presents several disadvantages that should be carefully evaluated. Willow biomass has a moisture content of 50-55% (wet basis) at harvest. Consequently, storage and drying of the freshly chipped wood may cause problems. It has been shown that stored, fresh wood chip in piles can heat up to 60°C within 24 hours and start to decompose. Biomass piles require careful management because internal fermentation can cause combustion and the high level of fungi spore production can lead to health problems for operators. Decomposition processes cause a loss of biomass of up to 20% and a significant reduction in calorific value (i. e. energy value) of the biomass [74]. Thus, this type of harvest system requires infrastructures to mechanically dry the biomass (e. g. ventilation, heating, mixing machinery) and these post­harvest operations increase the production cost. Alternatively, the freshly chipped material should be delivered to heating plants as soon as possible.

A third harvest system recently developed in Canada, mainly adapted to willow short — rotation coppice, is a cutter-shredder-baler machine that performs light shredding and bales willow stems [22], producing up to 40 bales hr-1 (20 t hr-1) on willow plantations (Figure 6).

The main advantage is that, since bales can be left to dry before being chipped, the risks linked to handling wet biomass are reduced [75]. In Quebec, willow biomass harvest is usually done in fall after leaf shedding.

As with any other agricultural crop, biomass yield of willow short-rotation coppice depends on many co-occurring factors including cultivar, site, climate and management operations. Soil type, water availability, and pest and weed control also affect yield. Data from existing commercial sites in the UK suggest that average yields of around 8-10 odt ha-1yr’1 are representative of plantations using older cultivars, whereas biomass yields as high as 15-18 odt ha-1yr-1 can be obtained by using selected genetic material [31]. In other northern European countries, an average annual growth of 15-20 odt ha-1yr-1 has been observed in early experiments [76], although more recent figures suggest that an average of 10 odt ha-1yr-1 is more realistic [77]. Experimental yields of short-rotation willow ranging from 24 to 30 oven dry tonnes (odt) ha-1 yr-1 have been measured in the US and Canada [43-44], although typical yields are more often in the range of 10 to 12 odt ha-1 yr-1 [78].

Long-term trials show that under southern Quebec’s pedoclimatic conditions, short-rotation willow coppice can provide high biomass yields over many years, although results vary according to variety. In one clonal test for instance, at the end of the third (3-years) rotation cycle, the most productive willow cultivars were SX64 (19 Odt ha-1 yr-1) and SX61 (17 Odt ha-1 yr-1) (Figure 7). Also, indigenous (i. e. North-American) willow cultivars, especially S. eriocephala (S25 and S546) and S. discolor (S 365) cultivars, show high biomass potential (13 — 15 Odt ha-1 yr-1). A scientific follow up of an old willow plantation established in Huntingdon in southern Quebec (Canada), showed that willows were still able to maintain a high level of productivity after five coppicing cycles. Plants can remain vigorous and produce high yields (14 Odt ha-1 yr-1) even after 18 years of cultivation (Table 4). This represents a very important demonstration of the viability of long-term economic exploitation of willows.