A Place for the Heathlands

During the fifty-year period following 1870, the woodland cover of Jutland increased, on average, by an annual 2 per cent. This tremendous afforestation effort, however, would not have been possible without the collaboration of a wide range of agents of all kinds. This paper attempts to identify some of these agents through scrupulously following their varied interrelations, as represented in the historical documentation of the scientific and practical explorations that enabled the afforestation of the Jutland heathland.

Since prehistoric times, scrub dominated by ling heather (Calluna vulgaris (L.) Hull) has covered large areas of northwest Europe. from northern Portugal to the Atlantic coast of Norway.^{2 ( 2 )}
Odgaard 1994, “The Holocene vegetation”; Kaland 2014, “Heathlands.” In the eighteenth and nineteenth centuries, regional and national authorities together with local landowners made a determined effort to “reclaim” these areas for agriculture. In order to establish a benign climate for this new arable land, they established numerous hedgerows and coniferous plantations. In Les Landes, the French government in 1857 initiated large-scale plantations of pine trees.^{3 ( 3 )}
Temple 2009, “The Natures”; Temple 2011, “Forestation.” Simultaneously, demand for pit props by local coalmines induced extensive afforestation of the Campine (de Kempen) region in central Belgium.^{4 ( 4 )}
Beerten et al. 2014, “The Campine,” 194. Concurrently, extensive parts of Lüneburger Heath south of Hamburg in northern Germany were converted to plantations of conifers, as were large tracts in the Scottish Highland moors.^{5 ( 5 )}
Grewe & Hölzl 2018, “Forestry in Germany”; Oosthoek 2013, Conquering the highlands. In Denmark, afforestation of the heathlands, which by the late eighteenth century covered approximately one-third of the Jutland peninsula, had started as early as 1788.^{6 ( 6 )}
Dam 2008, “Historisk-geografiske”; Nielsen 1988, Hedens grannelund.

Making trees grow in former heathland soil, however, was not always easy. Silvicultural progress frequently built upon clashing interests and ideas, and the afforestation process encountered considerable resistance in the meetings between planters, the available tree species, and actual soil conditions.

By concentrating on the interconnected transformations between different kinds of actors with dissimilar capacities, this paper will show how the gradual, parallel resolution of combined natural and social conflicts made the establishment of sustainable coniferous plantations possible. This will happen through thick descriptions of a number of exemplary decisions about trees, earthworms, scientific procedures, academic disputes, and their interrelations.^{7 ( 7 )}
Geertz 1973, The interpretation of cultures, 3ff.

The investigation will concentrate on the interpretations of – and the reactions to – the widespread experience that uniform plantations consisting exclusively of Norway spruce (Picea abies, (L.) H. Karst.) normally failed, whereas the intermixture with mountain pine (Pinus mugo, Turra) had a far more favourable outcome.^{8 ( 8 )}
E. M. Dalgas 1880, “Hedeselskabets forsøgsstationer,” 56.

In 1866, the distinguished engineer Enrico Mylius Dalgas (1828–94), together with a number of like-minded landowners and townsmen advocating for the economic and social development of rural central and western Jutland, founded Hedeselskabet (The Danish Heath Society) at a meeting in Aarhus.^{9 ( 9 )}
For the following: Skrubbeltrang 1966, Det indvundne. They considered the afforestation of central and western Jutland in order to improve the regional climate a precondition for the project’s success. As they saw it, the presence of wooden macrofossils in the region’s bogs, together with existing oak thickets and relict place names related to woods or tree growth, proved that what was now current heathland had in a distant past been forest. Their obvious inference was that trees could ultimately once again replace heather. Dalgas pondered how:

Nonetheless, after a few years, the plantation cause developed into a matter of patriotism, and eventually it gained widespread national support.

Through his work as a highway engineer in the central parts of the Jutland peninsula, Dalgas had observed minor morphological variations in the landscape originating from its formation during the Weichselian glaciation (117,000–11,600 BP). The ground was not equally flat: extensive plains alternated with almost imperceptible shallow hills (hill islands). Dalgas first published this observation in 1866, and the following year he called attention to some notable soil differences between the heathland plains (dominated by a hardpan layer) and the hill islands (with a higher content of vegetable mould).^{11 ( 11 )}
E. M. Dalgas 1888, En oversigt; Garboe 1961, Geologiens historie, 332ff.

For a long time, geological authorities stubbornly resisted this interpretation; but the issue mattered greatly to Dalgas, since these dissimilar topsoil qualities served his programme for heathland afforestation well. Trees were more likely to grow on the hill-island mould than on the sandy plains:

Despite the overall pattern, the podsolization or extreme leaching of soil prevalent on the plains at this time was frequently also found in the hill-island soils. This resulted in a soil profile with three kinds of challenges for any cultivator. First, the top layer of compacted, slightly decomposed humus with a high content of fungus hyphae called mor (lyngskjold) tended to impede the penetration of roots and the ventilation of underneath soil. Below this followed, second, bleached sand (blysand), with most of its nutritious matter washed out, so that plants would find little nutrition. Third, the layer of bleached sand was replaced downwards by a hardpan (al) resulting from the precipitation of various metal salts washed out from the topsoil, creating a stratum that was virtually impenetrable for plant roots.

In cooperation with forester Carl Emeis (1831–1911), co-founder in 1871 of the Heidekulturverein für Schleswig-Holstein (the Heathland Afforestation Association of Schleswig-Holstein, a sister organization to Hedeselskabet), Dalgas soon arrived at the conclusion that the creation of hardpan in heathland soils was the combined effect of physical and chemical processes.^{13 ( 13 )}
Jensen 1997, “Emeis-kulturerne.” He explained how:

Further, he observed that intake of atmospheric air could rapidly dissolve the hardpan. Therefore, he considered careful soil preparation an effective means to make heathland soils arable. Accordingly, in 1872 Hedeselskabet started to use the Hanoverian subsoil ploughs in most afforestation projects.^{15 ( 15 )}
E. M. Dalgas 1880, “Hedeselskabets forsøgsstationer,” 56. Yet no matter the degree of soil preparation, pure stands of Norway spruce tended to fail. Therefore, based partly on the German experience, the Hedeselskabet society board made the decision as early as 1870 always to mix the young spruces with mountain pines.^{16 ( 16 )}
E. M. Dalgas 1877, “Om plantning.”

Dalgas explained the adversities against the cultivation of spruce by the acidity of the moor – which mountain pines, to the contrary, possessed “a remarkable ability to be unaffected by.”^{17 ( 17 )}
E. M. Dalgas 1880, “Hedeselskabets forsøgsstationer,” 56. The antagonist to cultivation was thus allegedly acids in the soil. The pines, however, also worked exceptionally well as a “protective secondary stand between the spruces,” as they “brought the spruces upwards by driving the ling away from them.”^{18 ( 18 )}
E. M. Dalgas 1877, “Om plantning, 272. The secondary effect of the pines was, accordingly, that above ground they mechanically drove the spruces and closed the soil, thus preventing both regrowth of heather and desiccation of the soil. Still, Hedeselskabet also claimed,

Thus, the pines were also ascribed a certain intrinsic value that was transferable to monetary income.

Further, Dalgas’s penchant for mixed plantations built on overarching imaginaries about naturalness, as he contemplated how “[v]irtually any vegetation obtained naturally without human help contains, to a greater or lesser extent, a mixed population.”^{20 ( 20 )}
E. M. Dalgas 1882, “Exempler,” 124. Dalgas maintained that it was the ability of the mountain pine to “defeat acidity of the ling around the spruces” that did the work.^{21 ( 21 )}
E. M. Dalgas 1882, “Exempler,” 130. Indeed, Dalgas was so enthusiastic about the effects of mountain pine then specified by the name Pinus montana (Miller) that he named his new Aarhus home “Villa Montana.”^{22 ( 22 )}
Skodshøj 1966, E.M. Dalgas, 133f.

At year-end 1872, Hedeselskabet was about to establish a large concentration of plantations in an area approximately ten kilometres south of the town of Herning. Since 1868, this had already consisted of Høgildgaard Plantation (869 hectares) belonging to Hendrik Pontoppidan, owner of the Constantinsborg estate near Aarhus; four years later, this was supplemented with Birkebæk and Provsthus (741 hectares), areas owned by Hedeselskabet itself. In 1873, Tage Reedtz-Thott from the large barony Gaunø on Zealand acquisitioned Harreskov (693 hectares), and a partnership bought the much smaller Holt Plantation (99 hectares).^{23 ( 23 )}
Hedeselskabets Tidsskrift 1894, p. 158; Kristensen 2008, Harreskov Plantage. Finally, in 1874, the even smaller Rind Plantation (44 ha), which was organized as a company based on a large number of local stockholders, was added.

A partnership of three acquired Holt Plantation: the thirty-year-old heir to the entailed estate Broholm on Funen, Hannibal Sehested (1842–1924); 33-year-old Peter Erasmus Müller (1840–1926), who had just taken up the post as only forestry lecturer at the Royal Veterinary and Agricultural University; and Müller’s old school friend, zoologist Jonas Collin (1840–1905). Both Müller and Collin had grown up in wealthy, bourgeois families in Copenhagen.^{24 ( 24 )}
For the following: RA, Stamhuset Broholm, Sager vedr. Holt Plantage 1873–1921.

After buying the land, the partners concluded an agreement with Hedeselskabet about annual payments for the supply of plant material and plantation work to be undertaken by the planter Bertel Nielsen from the neighbouring Høgildgaard; the work was to be superintended by Hedeselskabet’s regional forest supervisor, Peter Borch. However, as a trained forester, Müller was to be solely responsible for planning and revising the afforestation effort in Holt.

Casual wheel tracks passed for local roads, and the plot had no buildings; but just three years after the acquisition, a newly constructed highway from Herning to Arnborg passed through Holt. At the same time, the owners installed Peter Sinding as a new planter, and had a tenement house built for him and his family. During the summer of 1873, the original planter had trenched 160 square metres of the area selected to become the plantation’s nursery; upon this, he had spread twenty loads of marl and the ashes from twenty loads of heath peat over the soil. Finally, he had pricked out 70,000 mountain pines, Austrian pines (Pinus nigra nigra (Arnold)), Norway spruces, and white spruces (Picea glauca (Moench) Voss)). Two years later, his successor transplanted the first of these to the plantation’s Section I. During the following years, afforestation progressed throughout the entire designated plantation area. When visiting Holt in 1877, Müller concluded that this first part of the plantation was in a “most excellent” shape.

In his planning process, Müller followed scientifically supported ideas; in the cultivation of Section IV, he wanted to experiment with a pure stand of Norway spruce. This he believed to be possible through the application of intensive soil preparation alone, since he considered soil structure and the lack of organic metabolism as the primary barriers to successful cultivation. In 1879, a minor part of Section IV was therefore trenched, and during the following years, first all the heather in the rest of the section was burned and the soil was subsequently lea ploughed (high ploughed) and fallowed for two years. Then the planter harrowed, subsoiled, and finally planted it with Norway spruce.^{25 ( 25 )}
RA, Stamhuset Broholm, Sager vedr. Holt Plantage 1873–1921: Excursion guide 1897.

To Müller, by contrast with Dalgas, there was nothing particularly “natural” about mixed stands.

Later, he emphasized that

The until then promising Section I in Holt, however, started to become stunted, and the needles of the oldest trees were turning yellow.^{28 ( 28 )}
For the following: RA, Stamhuset Broholm, Sager vedr. Holt Plantage 1873–1921. In order to prevent something similar befalling the pure stand of Norway spruce in Section IV, Dalgas and Borch recommended the planter to marl each tree individually. This, however, had no lasting effect, and just two years later Müller noted that the entire plantation had achieved

In 1880, Hedeselskabet, based upon experiences similar to those in Holt, concluded that its ten-year-old decision always to mix Norway spruce with mountain pine had been correct. It therefore established as a more specific planting policy that one pine should always mix in for every two spruces; simultaneously, it initiated a large-scale afforestation test programme.^{29 ( 29 )}
E. M. Dalgas 1880, “Hedeselskabets forsøgsstationer,” 56ff. In consequence of the continued stunted growth of the spruces in Section IV, Müller in 1888 finally decided that not even repeated soil preparation would save the pure stands of Norway spruce. He therefore instructed the planter to start planting one pine for every second spruce, exactly as recommended by Hedeselskabet. “By completing the planting of mountain pine, I still hope to save this whole spruce section as pure spruce forest, when in time the later and sparsely planted mountain pines are cut away.”

During the first years of his position as forest lecturer, Müller prepared to bring to fruition his “dear old thought,” articulated when he was only 28, “to look at, study and describe the forest as an organism.”^{30 ( 30 )}
Fritzbøger 2022, Videnskab og venskab, 37. His first step in this direction was to study forest soils through the methodology of a zoologist (he had taken a doctor’s degree in marine zoology in 1871). In 1878, he was therefore able to publish the first results of an investigation of the impact of biological activity on forest soil properties, especially in beech woods.^{31 ( 31 )}
Müller 1906, “Studier I.”

Most importantly, he called attention to the crucial importance of earthworms for the decomposition of organic compounds into humus. Coincident with their consumption of organic matter and subsequent defecation of humus, Müller realized that the earthworms were excellent soil preparators. Based on his observations in the field, he estimated that, in fertile forest mould, worms would produce between eighty and ninety vertical tunnels per square metre, which would strongly conduce to the ventilation and metabolism of the top soil.^{32 ( 32 )}
Müller 1906, “Studier I,” 20.

The realization that worms improved topsoil fertility was not new. In a speech to the Geological Society in 1837, Charles Darwin had concluded that formation of humus ”is due to the digestive process, by which the common earthworm is supported.”^{33 ( 33 )}
Darwin 1838, “On the formation,” 574; Darwin did not, however, produce a major publication on the subject until 1881, two years after Müller. Müller knew this speech from its mention in the popular Danish Magazine for Popular Expositions of Natural Science; at the same time, simultaneously with Müller’s own investigation, the German zoologist Victor Hensen had also made a provisional study of “The contribution of the earthworm to the fertility of soil,” also known to Müller.^{34 ( 34 )}
Reference in Müller 1906, “Studier I,” 74, note 3, to Tidsskrift for populære fremstillinger af naturvidenskaben 4:2, 1870, 234f; Hensen 1877, “Die Thätigkeit.” Inspired by his old zoology teacher Jørgen Christian Schiødte, however, Müller also acknowledged the possible impact of other living organisms, such as “a host of commonly found moulds, pyrenomycetes and slime fungi, in addition to the countless amounts of even lower-standing organisms of the realm of monera.”^{35 ( 35 )}
Müller 1906, “Studier I,” 16; Schiødte, “Insekterne.”

A few weeks before the publication of his first soil thesis, on 8 August 1878, Müller met Dalgas in Holt, and together they drove to the small thicket Skarrild Krat, some five kilometres to the south.^{36 ( 36 )}
RA, KVL, Hdskr. P.E. Müller, the notebook “Studier i Skovjord. P.E. Müller 1878,” the meeting took place on 26 July 1878. Skarrild was one of the many stunted oak groves scattered in the heathland that for Dalgas served to substantiate the claim that woods in former times had covered the landscape of central and western Jutland. It was now of the greatest interest to Müller in relation to his soil studies, so he left the carriage and started comparing soil conditions inside and outside the thicket.

Soon he found that, exactly as in the forest soils in eastern Denmark, the presence of zoological degraders constituted the key distinguishing mark between fertile and infertile soils.^{37 ( 37 )}
Müller 1908, “Studier II.” After some hours of intense exploration, while twilight was falling, Müller walked the short distance across the Skjern River footbridges to Hesselvig Enggaard, where he reunited with Dalgas. During the evening meal, he vividly described how he had found plenty of life forms beneath the brush, but none in the heathland mor.

From its very start, Hedeselskabet had regarded meadow irrigation as its most prominent activity, and Dalgas was personally deeply involved in the subject. Listening to Müller’s account, he suddenly realized that similar variations in the occurrence of life could explain fertility differences in irrigated meadows. He was, he later remembered, “struck by lightning” by this insight, and he immediately called for zoological analyses of meadow soils.^{38 ( 38 )}
E. M. Dalgas, cited in Lütken’s adjudication of 16 August 1893, KB, NKS 4484, 4^o.

Concurrent with Müller’s soil studies, C. F. A. Tuxen, a research assistant at the Chemical Laboratory of the Veterinary and Agricultural University (1850–1903), was conducting a number of physico-chemical tests in order to identify further variations in different types of heathland soil. Hedeselskabet’s forest supervisor, Borch, provided him with samples in Birkebæk.^{39 ( 39 )}
Tuxen 1876, “Nogle analyser.” Tuxen concluded, among other things, that soil profiles from hill islands and heath plain were quite similar: uppermost 5 cm of mor, then 15 cm of bleached sand followed by 5 cm of red soil, and finally a subsoil of red sand. As stated by Dalgas, the hill-island soils proved by contrast much more fertile than those of the plain. Further, not only was the content of nutrients in the mor higher than that of the sandy subsoil, but it was also acid, and contained substances such as ferrous oxide (FeO), which “should be considered venomous for our cultivated plants.” Therefore, it was Tuxen’s conclusion that “it is not unlikely that these two harmful properties put some critical obstacles in the way of the development of plants of which their roots come into contact.”^{40 ( 40 )}
Tuxen 1876, “Nogle analyser,” 290.

As a forestry lecturer, Müller soon established a network of well-off forest owners who were willing financially to support silvicultural experimentation. One of their grants enabled Tuxen to continue working with heathland soils, and in 1878, he published the results of an investigation specifically aiming to identify the possible effects of ventilating the mor through ploughing.

he concluded.^{41 ( 41 )}
Tuxen 1878, “Om lyngskjoldens,” 198. Airing the soil over a period of some years did not appear to have any effect on the composition of the mor, if the humic acid was not simultaneously neutralized. So mechanically breaking and removing the dense tough layer of ling mor seemed to Tuxen the most promising approach for silviculture.^{42 ( 42 )}
Tuxen 1882, “Regnormenes.”

Dalgas, on the other hand, insisted that “[n]ot only do the spruces demand so deep a preparation of the soil that the red soil or hardpan is being penetrated but also so thorough a ventilation that all ling poison is removed.”^{43 ( 43 )}
E. M. Dalgas 1877, “Om plantning,” 274. He therefore suspected an unknown venomous property of heather to be the main barrier to tree growth. Despite his emphasis on other explanations, Müller was at this point in no way dismissive of this idea. In 1888 he suggested that the pine “possibly by its flattened root system contributes to ventilating the acidic mor and to promote the early closing of the young stand.”^{44 ( 44 )}
Müller 1889, “Om bjergfyrren IV,” 34. That is, he suggested a mechanical rather than a biological explanation.

By the end of the nineteenth century, the stands of mountain pine in Holt were beginning to close so densely around the spruces that it was time to thin them out. In 1896, by use of short military sabre, the planter culled the first pines, and out of the resulting branches and twigs some were burned to charcoal.^{45 ( 45 )}
Hedeselskabets Tidsskrift 1896, p. 227.

The following year, a group of forestry students from the Veterinary and Agricultural University visited Holt as part of a major field trip to Jutland. Müller used the occasion to reflect on his own experiences with cultivation of Norway spruce. He readily confessed that Hedeselskabet and Dalgas had been right in recommending mountain pine as an efficient nurse tree.^{46 ( 46 )}
For the following: RA, Stamhuset Broholm, Sager vedr. Holt Plantage 1873–1921: Excursion guide 1897. This was an obvious conclusion based upon the fact that after the admixture with pines, the stunted spruces were actually improving.

Addressing the students, he summed up as follows:

In his attempt to close in on the “X”, he observed a group of spruces in Section IV that had never stopped growing, despite the fact that their soil conditions appeared to be completely identical with that of the rest of the plantation section. This directed Müller’s attention towards some local and unobservable factors of bacteriological or pedological nature and – probably inspired by a recent publication by microbiologist Hjalmar Jensen – he proposed experimenting with inoculating the soil with nitrifying bacteria.^{48 ( 48 )}
Jensen 1897, “Nogle plantefysiologiske.” Bacteria, however, were not the only potential representatives of “X”.

While digging through beechwood topsoil, Müller had frequently run into “some microscopically fine, brown and black-brown mycelia that in particular are growing on the beech roots from where they spread freely in the soil.”^{49 ( 49 )}
Müller 1906, “Studier I,” 16. In 1885, German botanist Albert Bernhard Frank named this kind of fungus–tree relations mycorrhiza; in the following year, Müller published a study in which he demonstrated how the fungi not only entered into symbiotic relationships with the beech roots (as demonstrated by Frank), but that, sometimes, they also served as saprophytes, decomposing vegetable and animal debris.^{50 ( 50 )}
Frank 1885, “Ueber die auf Wurzelsymbiose”; Müller 1886, “Om bjergfyrren I–II,” 26.

In 1884, Müller joined the Royal Danish Academy of Sciences and Letters, and five years later, he ensured that it offered a prize competition for work on the function of, precisely, mycorrhiza. One of its quite specific sub-questions addressed whether there was any

Müller clearly wanted the study to further develop his own inquiries. The deadline was October 1891, and the prize was 600 kroner.

By the closing date, only the anonymous “Nemo” had handed in a paper.^{52 ( 52 )}
Oversigt over Videnskabernes Selskabs Forhandlinger 1892, p. 46. A panel consisting of botanist and mycologist Emil Rostrup (1831–1907), mycologist and microbiologist Emil Chr. Hansen (1842–1909), and Müller himself agreed that the author was entitled to the prize. On opening the sealed envelope accompanying the paper, they disclosed the author’s identity to be forestry graduate Georg Sarauw (1862–1928). Based upon a combination of a literary survey and independent studies in the laboratory, Sarauw had concluded that the mycorrhiza were in general neither harmful nor beneficial for the host.^{53 ( 53 )}
Sarauw 1892–1893, “Rodsymbiose,” 246.

When, in 1888, Dalgas read Müller’s fourth and final article in a series on varieties of the mountain pine, he took strong offence at finding what he believed to be a misrepresentation of his own and Hedeselskabet’s efforts.^{54 ( 54 )}
Sarauw 1892–1893, “Rodsymbiose,” 246. Müller, in order to sort things out peacefully, emphasized in reply that his only wish was to support the cause of heathland reclamation. Further, he used the opportunity to tell Dalgas that in Holt he had surrendered and had instructed the planter to mix pines into the stand of stunted spruces in Section IV.^{55 ( 55 )}
KB, NKS 4484, 4^o: Letter from Dalgas to Müller, 13 February 1888. The subsequent truce, however, was only temporary.

Three years later, Hedeselskabet celebrated its twenty-five-year anniversary. To mark the occasion, Enrico Dalgas wrote a series of four articles in the association’s own periodical about the current state of its plantations.^{56 ( 56 )}
E. M. Dalgas 1891, Skov-kulturer. Reading this historical account, it is clear that Müller disagreed strongly with some parts that described the gradual comprehension of soil conditions. In order not to spoil the jubilee celebration for the then 63-year-old Dalgas, however, he kept silent. But when the following year, the principal of the folk high school in Askov, Ludvig Schrøder (1836–1908), wrote a popular leaflet about the society’s history in which he quoted Dalgas’s account of how he and Müller together had initiated experiments with the preparation of former heathland soils for afforestation, it was too much for Müller.^{57 ( 57 )}
Schrøder 1892, Det danske hedeselskab, 20f.

He first contacted Schrøder and the chair of Hedeselskabet, Ferdinand Mourier-Petersen (1825–98). Only when he found them both evasive did he, in the summer of 1892, confront Dalgas directly. According to Müller, Dalgas had given the impression that the scientific investigations of the 1870s took place in cooperation between the two; for Müller, in reality, “these analyses were carried out on my initiative and paid for solely by me.”^{58 ( 58 )}
KB, NKS 4484, 4^o: Draft of letter from Müller to Dalgas c. 28 March1891; according to the superscription, it was never sent. Further, he maintained that in his view, Dalgas had initially put forward the idea that planters were obliged to turn acid soil into basic or neutral mould before planting, and that he had identified “ling venom” as tannic ferrous oxide, identical with traditional iron gall ink. This was a position from which Dalgas later distanced himself. Lastly, Dalgas’s descriptions of the different varieties of mountain pine appeared to be based on descriptions by the society’s forest supervisors, rather than by Müller.

Academic and practical standards clashed in a conflict that obviously also touched upon the pride and self-esteem of two self-aware individuals. In his reply, Dalgas claimed that their first talk about the need for soil analyses had taken place as early as 1874 and that, therefore, it did actually spark off Müller’s scientific work.^{59 ( 59 )}
KB, NKS 4484, 4^o: Letter from Dalgas to Müller, 29 August 1892. This was, at the end of the day, the most fundamental of their disagreements: the matter of chronology and (ultimately) inspiration.

By the end of the summer and after a lively correspondence, Müller concluded that each believed the other’s memory to be at fault. He therefore suggested that they should involve an arbitrator respected by the both to settle the dispute. Because the two adversaries were so busy, however, the process drew out and a whole year went by before the arbitrator, zoology professor Christian Lütken (1827–1901), could produce an adjudication.

At their landmark meeting in Hesselvig Enggård, Christensen, the local engineer, had dined with Dalgas and Müller. Despite the fact that he supported Müller’s contention that the meeting took place in 1878, Lütken however concluded that it was impossible to date beyond question. Tuxen, the chemist performing the physico-chemical test, declared that his investigations in 1876 only concerned the soil content of plant food, and that they did not aim to identify any particular substance in the mor layer.^{60 ( 60 )}
Annex B to the arbitrator case, KB, NKS 4484, 4^o.

As for Dalgas, he claimed not to have suggested that it was their meeting in Hesselvig that originally prompted Müller’s interest in earthworms, but only that he himself had realized the significance of animal life for heathland plantations “by other means” unrelated to the meeting. Nevertheless, it was Müller’s remarks about Skarrild Krat that made him appreciate the mutual exclusivity of earthworms and heather. Consequently, after some deliberation Lütken saw no reason to find that Müller’s scientific reputation had suffered injury from publications by Dalgas and Hedeselskabet, and the conflict was thus settled.

The two parties acknowledged this settlement, and what had clearly been a matter of principle to both was over – at least for the time being. Enrico Mylius Dalgas died on 16 April 1894. But when Müller, seven years later, gave a lecture on heathland afforestation, Dalgas’s son Christian (1862–1939), a graduate in forestry and then deputy director of Hedeselskabet, strongly objected.^{61 ( 61 )}
Report on the content of Müller’s lecture in Hedeselskabets Tidsskrift 1902, pp. 72–106; Dalgas’ rebuttal in pp. 107–114 and 1904, pp. 23–41, 51–69.

Müller concluded that:

Further, Müller saw no reason why the alleged tannic ferrous oxide should be more harmful to spruces than to pines; and, anyway, the substance would easily decompose in the soil.

Dalgas junior, on the other hand, argued that his “father’s theory that the mor layer contains substances that are toxic for the spruce, and that these must be changed through ventilation to a good plant nutrition, is correct and well documented.”^{63 ( 63 )}
Hedeselskabets Tidsskrift 1902, p. 110. And he stressed that

Müller did not reply. In the first place, he probably did not consider the young Dalgas an equally worthy opponent to his father; second, it was time for new research rather than continued bickering about old disagreements.

As a young scientist, Müller at first had been critical of the revolutionary ideas of Charles Darwin (1809–82), but following some more meticulous reading, they became a great inspiration to him.^{65 ( 65 )}
Fritzbøger 2022, Videnskab og venskab, 97f. Therefore it came as no surprise that among the contributions Müller made to the improvement of afforestation in the plains of Jutland, he attempted to identify the mountain pine varieties best suited for this particular environment. In 1885, he went as state chief forester on a study tour to southern Europe, together with the government inspector of dunes, J. P. F. Bang (1844–1929).^{66 ( 66 )}
Müller 1886, “Om Bjergfyrren I–II”; Müller 1887, “Om Bjergfyrren III”; Müller 1889, “Om Bjergfyrren IV.” And in 1892, during his skirmish with Dalgas, he had gone to Spain to study “the glorious mountain forests of the Pyrenees, which hopefully entrust me with some secret that can be of use.”^{67 ( 67 )}
KB, NKS 4484, 4^o: Draft of letter from Müller to Dalgas, 5 September 1892. Together, these surveys in botanical taxonomy and plant geography resulted, for one thing, in the conclusion that a French (or Pyrenean) variety of mountain pine produced the most straight-boled trees (although it later proved to be ill adjusted to the Danish climate).^{68 ( 68 )}
Müller 1887, “Om Bjergfyrren III,” 270; Møller 1913, “Skovdyrkningens udvikling,” 116f.

More importantly, however, Müller continued his search for the mysterious “X” that the mountain pine might somehow be able to overcome. His work as chief forester was mostly bureaucratic, but in 1901, the ministry appointed him at the same time chair of a newly established Danish Experimental Forestry Service (DEFS), and this opened up new opportunities. During the following years, together with other scientists, he carried out a number of cultivation experiments, all aimed at the improvement of growing conditions in heathland soils.^{69 ( 69 )}
Müller and Weis 1906, “Studier over Skov- og Hedejord I”; Müller & Weis 1908, “Studier over Skov- og Hedejord II”; Müller et al. 1910, “Bidrag til kundskab”; Müller and Helms 1913, “Forsøg med Anvendelse”; Müller & Weis 1913, “Studier over skov- og hedejord.”

At the outset of this intensified enterprise, Müller took the opportunity in a talk at the eighth Ordinary Forestry Meeting in 1901 to ventilate a number of potential “X”s. As heathland soils contained no earthworms or moles, he considered it most likely that “the lower organisms that defecate the decomposition of organic substances under insufficient air access, fungi and especially bacteria, [...] therefore, [must] play a prominent role in the formation of topsoil.”^{70 ( 70 )}
Hedeselskabets Tidsskrift 1902, 80.

In 1903, Müller gave two lectures on “[t]he relation of the mountain pine with the Norway spruce in Jutland heath cultures” in the Forestry Debate Society, subsequently published.^{71 ( 71 )}
Müller 1904, “Nogle nye undersøgelser”; Ferdinandsen & Jørgensen 1938–1939, Skovtræernes sygdomme, 479. In the resulting thesis, he combined laboratory and literary studies of the mycorrhiza affiliated respectively with pine and spruce. Identification of the fungus species engaged in these affiliations was still out of the question, but Müller observed how the mycorrhiza morphology differed.^{72 ( 72 )}
Identification of mountain pine mycorrhiza species first appeared in Melin, “Zur Kenntniss.” On the mountain pine, their form was fork-like and accompanied by numerous tubers, whereas on the spruce, their shape was like lateral branches (as was the case in most fungi affiliated with other tree species). He concluded that:

Seven years later, Müller’s frequent collaborator, plant physiologist Frederik Weis (1871–1933), investigated the nitrification of heathland soils.^{74 ( 74 )}
Weis 1910, “Über Vorkommen.” He concluded that continued spreading of chalk or marl would prevent the trees suffering from nitrogen deficiency. Later that year, Müller together with forest lecturer Johannes Helms (1865–1934), geologist Kristian Rørdam (1860–1939), and forest supervisor Eigil Helge Wøldike (1861–1923) published results from their extensive research in “the growth conditions of Norway spruce in heathland soils of Central Jutland.”

One of their experimental localities was the group of spruces in Holt that had continued growing with no signs of a stunting period. Here, they inter alia analysed the soil content of bacteria and fungi, and they found that “the soil outside the spruce group in Holt Plantation was exceedingly poorer in fungal vegetation than the soil in the spruce islet itself.” Further,

The group concluded that since Weis had been able to show good results from marling, and since such results could not be reproduced elsewhere, the explanation had to be “‘nitrogen hunger’ due to a progressive change in the microcosm of the soil over time, as a result of which the nitrogen becomes insufficiently accessible to the higher plants.”^{76 ( 76 )}
Müller et al. 1910, “Bidrag til kundskab,” 213. Thus the “X” now appeared to be the combined effect of soil chemistry and biology.

The more precise character of the active microcosm, however, remained unclarified, and Müller in particular followed the fungus trail. Having himself been among the first to publish on mycorrhiza, he started in 1911 a long and intensifying correspondence and exchange of fungi specimens with the Czech mycologist Jaroslav Peklo (1881–1955) in Prague.^{77 ( 77 )}
KB, NKS 4484, 4^o. Peklo strongly supported Müller in his assumptions about the role of mycorrhiza in the decomposition of litter; together, however, they made no new discoveries about the symbiotic fungi’s possible assistance by the tree’s assimilation of nutrients.

Therefore, in 1920 Müller together with Christian Dalgas announced a prize essay competition offered by Hedeselskabet for someone “[t]o investigate and experimentally demonstrate whether the mycorrhiza of the mountain pine and the mycorrhiza and its closely related Scots pine are capable of absorbing the free nitrogen of the air.”^{78 ( 78 )}
Hedeselskabets Tidsskrift 1920, p. 30f. One year later, the society received one preliminary dissertation written by Carl Marenus Møller (1891–1978), editor of The Danish Forest Association’s journal and an assistant lecturer in forestry. A panel consisting of Dalgas, Müller, and Weis soon agreed that he should be awarded part of the prize and be encouraged to continue the exploration.

In 1919, a few years before this, the world-renowned botany professor Eugenius Warming (1841–1924) had summed up the then-current knowledge about the mysterious “X” in a landmark monograph on forest ecology:

This was, overall, in accordance with Müller’s view.

As for Carl Marenus Møller, he continued working with mycorrhiza studies for some years, but appear to have reduced this line of inquiry when, from 1927, he held one of two professorships in forestry. Consequently, he did not publish his findings until twenty years later, when he first, and most importantly, pointed out that there was no certain indication that the special capacity of the mountain pine (or the Scots pine (Pinus sylvestris (Linn.))) to grow under difficult conditions depended on its mycorrhizae. Second, he wrote,

Mycorrhiza accordingly appeared to be of no importance.

By the turn of the century, critics were increasingly voicing their discontent with the radical landscape transformations pushed forward by Hedeselskabet. Among these was the poet Jeppe Aakjær, who characterized the association as “a gigantic enterprise, an enterprise under the sign of the crassest materialism, and its cold rectangular principles move like an iron roller over this beautiful and unique landscape.”^{81 ( 81 )}
Olwig 1984, Nature’s Ideological landscape, 86; Aakjær 2017 (1909), Hedevandringer, 36. He lamented about its plantations that

It was, however, not the un-Danishness of the trees that made a number of prominent foresters question the very rationale of transforming ling heathland into a “mountain pine heath.” In 1915, an anonymous silviculturalist made a general attack on Danish forestry in a magazine article in which he specifically deplored the heath plantations that he judged to be:

The forestry professor C. V. Prytz (1857–1928) rejected the overall criticism, but subsequently went along with these critical remarks about Hedeselskabet when he pointed out that:

The “said fungus” was identical with the “vermin” earlier mentioned by Aakjær, namely, heart-rot fungus (Heterobasidion annosum (Fr.) Bref.), which was increasingly becoming a menace to those Norway spruces that the mountain pines were expected to protect.^{85 ( 85 )}
Rostrup 1902, Plantepatologi, 355. The most obvious reason was that when woodmen cut down the pines in order to thin the stands, the stumps and roots that remained became welcoming hosts and vectors for the fungi. And “[w]hen the colourless hyphae have penetrated through a wound or a fine crack [...] in the root bark [and] migrate further partly in the bark layer and partly in the wood of the root” they could soon infect the spruces as well.^{86 ( 86 )}
C. Dalgas 1910, “Om Frembringelse,” 416.

This biotic setback, however, was not Prytz’s only criticism, since he based his reservations mainly upon an economic rationale. “The extensive afforestation on the heath, where the state and patriotic companies and wealthy people have taken the lead, is flawed, and the large amount of money spent here has been thrown out the window!”^{87 ( 87 )}
Østsjællands Folkeblad 16 April 1915 (https://www2.statsbiblioteket.dk/mediestream/). Thus the reasonability of the critique obviously very much depended on the economic expectation concerning heathland afforestation. Only two days later, Müller gave his former student and successor an official reply (alongside a great many counterarguments from Hedeselskabet).

Despite the obvious adversities experienced in many plantations, Müller pointed out that “[i]t is especially in the second round of spruce cultivation on the same area as the fungus rages most dangerously; I do not know of any plantation in heathland, where the destruction in the first round can be said to be fatal to the vegetation.” And he continued by praising the endeavour initiated by Dalgas senior:

During the following years, the world war caused prices of all dimensions of wood to rise; even small timber and fuel wood from the mountain pine obtained a market value. Suddenly producers of wood alcohol, tar, and charcoal were buying plenty of branches from the thinning of pines.^{89 ( 89 )}
Hedeselskabets Tidsskrift, several volumes. Despite recurrent problems with fungus infection, soil conditions were improving, and pure stands of spruce were gradually becoming more common. In 1932, the board of Hedeselskabet therefore finally decided no longer to establish pure mountain pine plantations unless special circumstances justified it.^{90 ( 90 )}
Pedersen 1971, Hedesagen, 317.

Nobody found a singular, crucial “X” to explain the stunting of Norway spruces. Practical experience, however, demonstrated that intermixture of mountain pine worked well, and as soil conditions and the means to counteract heart-rot fungus infections gradually improved, plantations dominated by spruce slowly gained a foothold in a landscape of rapidly receding heathland.^{91 ( 91 )}
Frederiksen 1935, “Hugst og Tilvækst.” As I hope to have shown, many agents took an active part in this long process.

The entities that were actively involved here were of exceedingly different kind, as they ranged from acidity (actually the concentration of hydrogen ions) to bacteria, from planters to microclimate, from laboratory procedures to nationalist temperaments, from ill-concealed academic pride to subterranean hypha. Every step in the process of afforestation resulted not from individual agency, but from often highly complex and contradictory cooperation between dissimilar agents. It was human efforts, soil structures, microorganisms, and tree species properties that together afforded the transformation of heathland to plantation forest in central and western Jutland.

And, by the way: after the negative conclusions about the transfer of nutrients from soil to tree roots by mycorrhiza published by Møller in 1947, international research gradually established that some tree species interact with as many as 2,000 different species of fungi. Further, some of these so-called ectotrophic mycorrhizae (the presence of hypha in the tree roots is only intercellular and not intracellular) do actually, in many cases, supply the host with nutritious matter.^{92 ( 92 )}
Daniel et al. 1979, Principles, 215f.