Seth Nordberg, forestry adviser in Northern Finland, established in 1923 a willow (Salix) trial in Rovaniemi forestry school (ref 1). The major aim 100 years ago was to produce raw material for basket making. Testing of suitable willow species or varieties in various ecological conditions, was necessary.
The early Rovaniemi trial of Nordberg was quite small, of a size of one acre (10 m x 10 m). But it was very pedantic and accurately planned trial. The trial was established on cultivated mineral soil, on an agricultural field which had been cultivated for several years. It was a slightly sloping field, towards east. The gradient confirmed the movement of water in the soil. The direction to east confirmed sufficient sunshine to the stand. The field belonged to the forestry school farming area (ref 2). The field was earlier used as a nursery area. During previous years Scots pine seedlings were grown on that field. Precise soil cultivation had preceded seedling growing.
The soil was plowed with shovels to depth of 30 cm. The plowing was apparently done in the previous autumn, before planting the cuttings (the timetable is not clearly described in the report, ref 1). No fertilization was given. Instead, a thin layer of ordinary sand was spread to the top of the soil, to eliminate apparently emerging growth of weeds.
The willow cuttings of different Salix clones were cut to the length of 30 cm. Because of the relatively small trial area the preparation of cuttings was done especially carefully, aiming at even quality of and well sprouting cuttings. The cuttings were planted in spring 1921, immediately after the winterly ground frost had melted enough. The cultivated soil was spongy enough, and the 30 cm cuttings could be manually embedded. The top of the cuttings was adjusted to the soil surface level.
The cuttings were planted into rows with 50 cm distance between the rows. Inside the rows the planting distance was 15 cm. Thus, the planting density was 133 000 cuttings per hectare. This planting density was remarkably higher than the current standard planting density in willow husbandry. For instance, the Swedish recommendation from 2010 is 14 200 cuttings/ha (ref 3); the American recommendation from year 2020 is 14 800 cuttings/ha (ref 4).
After the planting the weeding was done manually several times, in spring and early summer 1921. In the following years the weeding was done only once a year, in the spring before copping was in full power. In the old, well cultivated northern agricultural field the most strong-rooted competing weeds belonged to the genii Trifolium, Ranunculus, Taraxatum and Rumex.
The selected five willow species or clones for the trial were
Salix ”amygdalina” (Ref 5)
Salix ”lanceolata” (Ref 6)
Salix polyphylla (Ref 7)
Salix undulata (Ref 8)
Salix viminalis (Ref 9)
Salix ” amygdalina” and Salix ”lanceolata” are today considered to be different clones of Salix triandra. Salix triandra is also an indigenous willow species in Northern Finland (Ref 10).
The willows were grown on a basket willow husbandry manner, in a one-year rotation cycle. After each growing season three random samples per each Salix variety were taken for measurement. The annual growth was measured for three dimensions: dominant height (cm), coppicing vigor (average number of coppices per stool) and wet biomass growth (grams per stool per annum).
Dominant height. For the dominant height the longest coppice was first identified, for the random three stumps. For Salix ”lanceolata” and Salix polyphylla the measurement moment was: 1 year shoots, 3 years stems. For Salix ”amygdalina”, Salix undulata and Salix viminalis the measurement moment was 1 year shoots, 2 year roots. The dominant height is the average of three shoots for each species or hybrid (Figure 1).
Figure 1. Dominant height of Salix varieties in Seth Nordberg´s Rovaniemi trial 1921. Mean height (in cm) was calculated for three randomly selected longest stems for each variety. Some varieties with 2 year roots, the others with 3 year roots.
By height Salix ”lanceolata” grew as best. The one-year coppices after the third year grew into dominant height of 235 cm. Salix viminalis was number two with dominant height of 230 cm.
Coppicing vigor. Especially the basket willow husbandry of the old times was searching for – and also the more modern biomass willow husbandry is searching for – Salix species and varieties which develop several and strong coppices after the harvest. That is also why the common recommendation for the willow husbandry is to cut the stand down after the first growing season. It is recommended even if there would not be any use or economic value for the first year harvest.
The coppicing vigor in Rovaniemi trial was measured by recording after the autumn the number of all at least 50 cm long coppices per stump and per Salix species. The coppicing vigor is the average for the three stumps. For Salix ”lanceolata” and Salix polyphylla the measurement moment was: 1 year shoots, 3 years stems. For Salix ”amygdalina”, Salix undulata and Salix viminalis the measurement moment was 1 year shoots, 2 year roots.
Figure 2. Coppicing vigor of Salix varieties in Seth Nordberg´s Rovaniemi trial 1921. Coppicing vigor (average number of stems per stool) was calculated for three randomly selected stools for each variety.Some varieties with 2 year roots, the others with 3 year roots.
Salix undulata was strongest in producing the coppices. Already after the second year the number of coppices was on average 14,5 per stool. This order is interesting because Salix undulata was the last by measuring the dominant height (Figure 1). Also Salix ”lanceolata” was strong in coppicing: 14 coppices per stool.
Assuming that the original planting density 133 333 cuttings per hectare remained as the density of living stumps, the coppicing vigor of about 14 coppices per stool leads into magnificent final stem density:
14 * 133 000 = 1,87 million stems per hectare.
This final density data was not, however, recorded or mentioned in the Rovaniemi trial (ref 1).
Wet biomass growth. The third dimension for willow biomass growth that was measured in Rovaniemi trial was the wet biomass yield per stool for each Salix variety. The wet biomass after the growing season was measured as grams per stump, by harvesting and weighing the coppices (Figure 3). It was left unknown (ref 1) whether the wet biomass weight included only the fresh stems, or the fresh stems + the remaining fresh leaves in the autumn. It is logical to assume that the autumn leaves were still included. The harvesting and measurement, often at cold and wet late autumn weather, is quickest and easiest if the remaining leaves need not be removed. In any case, the wet biomass that was recorded gives the best relative order for the ability of the tested willows to produce biomass.
Figure 3. Wet biomass growth of Salix varieties in Seth Nordberg´s Rovaniemi trial 1921. Wet biomass (average grams per stool) was weighed for three randomly selected stools for each variety.Some varieties with 2 year roots, the others with 3 year roots.
The most productive willow species in biomass production was Salix undulata, with average weight 487 grams per stump. It is worth of notifying that that Salix undulata was number one in coppicing ability as well. Salix ”amygdalina” was the last, with 225 grams per stump. It was also the last with its coppicing ability (6,3 coppices per stump, Figure 2).
Dry matter production. Theoretically and further assuming that the wet biomass measurements did include also the leaves, the dry matter production of Salix varieties in Seth Nordberg’s Rovaniemi trial can be calculated using the data and equations from a later Salix trial in Rovaniemi (Ref 11). With Salix gmelinii ”Aquatica Gigantea” (grown only for the first year, 1 year shoots, 1 year shoots) the theoretical maximum values were
195 wet grams per stump, stems and leaves
41 dry grams per stump, stems and leaves
29 dry grams per stump, stems
Using the same relationships, the measured and theoretical values for instance for Salix undulata in Rovaniemi trial are
487 wet grams per stump, with leaves
102 dry grams per stump, with leaves
72 dry grams per stump, only stems
Next, assuming that the final stool density is the same as cutting density, that is 133 333 cuttings per hectare, the measured and theoretic annual yield figures for Salix undulata are
64,9 tn/ha/a wet biomass, with leaves
13,7 tn/ha/a dry biomass, with leaves
9,66 tn/ha/a dry biomass, only stems
Finally, the calculated, indicative dry matter yields for the stems for each willow species in Seth Nordberg´s trial are shown in Figure 4.
Figure 4. Theoretical dry biomass yield of stems, tn/ha/a, of Salix varieties in Seth Nordberg´s Rovaniemi trial 1921. Dry biomass is theoretically calculated from wet biomass measurements (average grams per stool) weighed for three randomly selected stools for each variety.Some varieties with 2 year roots, the others with 3 year roots.
The order of Salix varieties by dry stem biomass is the same as for wet biomass per stool (Figure 3). It is more essential to compare the average growth capacity of the old basket willow husbandry to the later biomass willow husbandry. The theoretical Salix dry biomass production in Seth Nordberg’s early Rovaniemi trial compares well with the later Salix study (ref 11) in Rovaniemi.
(Ref 1) Nordberg, S. 1928. Vertaileva katsaus pajun viljelykseen ja sen edellytyksiin ulkomailla ja Suomessa. Deutsches Referat: Die Weidenkultur und ihre Voraussetzungen im Ausland und Suomi (Finnland). Silva Fenn. 9:1-63. https://silvafennica.fi/pdf/article4450.pdf
(Ref 2) Old location of Rovaniemi forest school https://email@example.com,25.412806,609m/data=!3m2!1e3!4b1!4m2!7m1!2e1
(Ref 3) Salix planting density recommendation in Sweden
(Ref 4) Salix planting density recommendation in USA
(Ref 5) Salix amygdalina = Salix triandra http://www.plantsoftheworldonline.org/?q=Salix+amygdalina
(Ref 6) Salix lanceolata = Salix triandra http://www.plantsoftheworldonline.org/?q=Salix+lanceolata
(Ref 7) Salix x polyphylla = Salix x rubra http://www.plantsoftheworldonline.org/?q=Salix+polyphylla
(Ref 8) Salix undulata = Salix x mollissima http://www.plantsoftheworldonline.org/taxon/urn:lsid:ipni.org:names:778210-1
(Ref 9) Salix viminalis http://www.plantsoftheworldonline.org/taxon/urn:lsid:ipni.org:names:779152-1
(Ref 10) Siira, J., Heino, E. & Pohjonen, V. 1981. Harvinainen jokipaju – tulvarantojen vaeltaja. Summary: Salix triandra L., a rare inhabitant of alluvial river shores. Dendrol. Seur. Tied. 12(1):11-20.
(Ref 11) Pohjonen, V. 1974. Istutustiheyden vaikutus eräiden lyhytkiertoviljelyn puulajien ensimmäisen vuoden satoon ja pituuskasvuun. Summary: Effect of spacing on the first year yield and height increment in some species undergoing short rotation culture. Silva Fenn. 8(2):115-127. (PDF)