From 6 October 2003 to 18 March 2004, every Monday and every Thursday, at ten in the morning, a team fells three trees drawn at random. Forty-eight dates, twenty-four weeks, without ever missing an appointment. Four hundred and thirty-two spruces spread over three Swiss sites, Château-d’Œx in the canton of Vaud on a former pasture, Bergün in the Grisons under natural regeneration, Marchissy and Begnins in planted forest. And one hundred and forty-four chestnut trees in a coppice at Quartino, in Ticino. From each felled tree, two discs seven and a half centimetres thick are taken fifty centimetres above the ground, two more at four and a half metres. Cubes twenty millimetres on a side and prisms are cut from them, in the sapwood and in the heartwood. They are weighed, measured, dried for a week at twenty degrees and sixty-five per cent relative humidity, then put in a drying oven at one hundred and three degrees until constant weight. For relative density alone, four thousand and thirty-two values.
All that to check a woodcutter’s saying.
This is the central experiment of Ernst Zürcher’s book, and it says a good deal about who he is. A forest engineer trained at ETH Zurich, with a doctorate in biology and tree anatomy, he taught at EPFL and is professor emeritus of wood sciences at the Bern University of Applied Sciences, on the Biel campus. He is not a dissident, he is a man in a salaried post who has chosen to work on subjects his profession looks askance at.
What is inside
Two hundred and eighty pages, seven long chapters, each cut into about ten sections, plus a set of notes, a fifteen-page bibliography and appendices. Francis Hallé writes the preface, Bruno Sirven the afterword. The dedication goes to the First Nations and to indigenous peoples who carry “another knowledge”, and to the ethnologist Bruno Manser, who disappeared while on his way to join the Penan nomads of Sarawak.
The journey starts with sacred trees and the peoples of the tree, with the yew as its thread, the megalithic sites, Celtic toponymy, the knowledge of the druids. Then comes the matter itself: structures and formation of wood, conifers and broadleaves, anatomy, the hidden side of photosynthesis. Next, a short chapter on polarity and spirality, where the golden ratio meets human blood flow. The fourth chapter is the beating heart of the book, chronobiology: lunar rhythms, four elements and constellations, three forgotten women pioneers of lunar chronobiology, green tides, the pulse of buds, bioelectricity. The fifth deals with wood as a material, felling dates, construction, fire. The sixth is called “Subtle messages” and speaks of odours, weather prediction, tree-antennas and geomagnetism. The seventh, the longest, is a handbook of agroforestry: hedges, sustainable management, biodiversity, terroir understood as an ecological organism. It is the one that justifies the third verb in the subtitle, “act”.
The measurements
Zürcher starts from Theophrastus of Eresus, fourth century BCE, who writes in his Enquiry into Plants that one must cut at the start of the waning moon to obtain wood that is harder and less prone to rot. The German rule says the same thing differently: “soft wood on the waxing, hard wood on the waning”. Three teams set out to test this at almost the same time, at the end of the nineties. Triebel in Dresden on one hundred and twenty trees, Seeling and Herz in Freiburg im Breisgau on sixty, Rösch then Bariska and Rösch in Zurich on thirty. All concluded that there was no effect. All had only six felling dates. It is this thinness of sampling that pushes Zürcher up to forty-eight.
Result, published in the journal Trees in 2010. In spruce, water loss according to the date of cutting varies from a relative minimum of 91.44 to a maximum of 115.66, that is a difference of twenty-six and a half per cent. In chestnut, eighteen per cent. Four time models are tested, and two stand out: the synodic rhythm cut into eight levels, and the sidereal rhythm related to the four qualities of Empedocles. On spruce sapwood, the p-values fall to 0.0000. The most sensitive criterion is not water loss but shrinkage. A delicious detail: the dates situated just before the full moon systematically give higher averages than those situated just after. The authors take good care not to claim victory, they speak of “slight but significant” variations.
Two other pieces of work feed the book. In 1998, in Nature, Zürcher and three colleagues publish a two-page note on the reversible fluctuation in the trunk diameter of young spruces, under controlled conditions, in synchrony with the tides. In 2014, with Schlaepfer, he takes up old spruce germination data, judged negative at the time, and runs it again through modern statistical tools: a lunar rhythmicity appears, weak, in sowings made shortly before the full moon compared with those made shortly before the new moon.
The introduction rests on two theses that have nothing lunar about them and that bowled me over. First, cloud seeds: the volatile organic compounds emitted by trees, fungal spores and pollen grains serve as condensation nuclei, so that the forest makes its own rain. Then the biotic pump of Victor Gorshkov and Anastassia Makarieva, of the Nuclear Physics Institute in Saint Petersburg, taken up by Peter Bunyard in The Ecologist. The idea reverses the usual direction: it is not the moving air masses that set off the water cycle, it is the phase changes of water above the forests that move the air masses. Six hundred calories per gram absorbed on evaporation, given back in the upper atmosphere on condensation. Immediate consequence, and a political one: razing coastal forests dries out the interior of continents.
The rest of the book teems with facts of this kind, noted down as I read and which I have not cross-checked one by one, the elemental composition of wood, which makes it an object more than ninety-nine per cent drawn from the air, the production of new water by photosynthesis, spruce resin used as a collective pharmacy by the ant Formica paralugubris, bacterial self-cleaning of wood, a room in Swiss stone pine and heart rate.
The ideas, and where it snags
The underlying thesis is not mystical, it is methodological. There exists a measurable invisible, and what tradition asserts without proof can be put to the test, where it sometimes holds. The boldest thesis concerns the clock of the living: Zürcher disputes Erwin Bünning’s conclusion, in the thirties, on the strictly internal clock, and proposes that the time base be external, gravimetric, lunar. He gives a possible mechanism for it, which he presents as a hypothesis to be verified: under the attraction of the Moon, water would move inside the cell itself, leaving the contents to lodge in the wall, which would change the ratio between free water and bound water according to the date of cutting.
This is where the contradiction is instructive, and it has to be given. Meriem Fournier, who heads the AgroParisTech centre in Nancy, acknowledges the rigour of the work on lunar cycles and states the exact limit: “these variations are slight, masked by other factors such as the site, the age of the trees”. The effect is not denied, it is declared too weak to be operative, and Zürcher admits it himself since the difference between mountain wood and lowland wood exceeds that produced by the felling date. The objection of the astronomer Robert Lamontagne lies elsewhere, in the physics. I know it only second hand, through the press: the Moon being electrically neutral, it does not produce the electromagnetic effects invoked. These two reservations do not cancel out the four thousand measured values. They say where what the measurements allow stops, and the book sometimes spills over that boundary, on the side of tree-antennas and the prediction of earthquakes by an electrode in the sapwood.
The language
Album format, thick paper, photographs, diagrams, dendrochronograms, tables, boxes that isolate a technical point. The text alternates between dry exposition and open lyricism, as in these opening lines: “Modern man is sick from no longer having roots, neither in the earth nor in the cosmos.” A little further on comes a distinction I have not let go of since, between seeing and looking. Zürcher proposes that the look directed at a tree be accompanied by “regard”, that is to say by an invitation made to that being to take its full place in the field of our consciousness. And he drives home three questions that make up the whole programme of the book: when did we last touch, smell, listen to a tree?
It is the most learned and the most difficult of the tree books I have opened this year. I had to take notes chapter by chapter so as not to lose the thread. What I carry away from it fits in one sentence, and it is the title itself: entre visible et invisible, between visible and invisible. Here is a man who refuses to choose between measurement and meaning, and who pays the reputational price of that refusal without ever letting go of his protocols.
Read in November 2017. Note written in 2026.
