Why the north slope goes first
Revised 10 August 2026
Two hillsides, four hundred metres apart. Same rain, same stand age, same spruce, same week. One is up and one is not. Every forager in Sweden has stood there and quietly concluded the forest is arbitrary. It isn’t. Most of the time this is an energy-balance problem, and the variable doing the work is which way the ground faces.
The arithmetic of a low sun
A slope tilts the ground relative to the beam. The geometry has been solved since Lee (1964) and Swift (1976) and it is unforgiving: a slope of angle β facing due north at latitude φ takes the direct beam roughly as a flat surface at latitude φ + β. A 15° north-facing hillside outside Falun, at 60°N, is on the radiation budget of flat ground at 75°N — north of Nordkapp. The south side of the same hill is at 45°N, near Bordeaux.
Run that clear-sky geometry — my own calculation, not a measurement — for 1 September at 60°N and a 10° north slope takes about 10.6 MJ/m²/day of direct beam against 16.1 on the matching south slope: a surplus of 5.5. Vaporising a kilogram of water takes 2.45 MJ. If even a third of that surplus goes into evaporation rather than warming air and wood, the south slope loses roughly 0.7 mm of water a day more than the north. Over a ten-day dry spell, 7 mm — a respectable rain event, quietly deleted from one side of the hill.
Why it gets worse the further north you go
Same calculation, 10° slopes, south-to-north ratio: mid-July at 60°N, about 1.2 — the sun is high and the hill barely notices. By 1 September, 1.5. By the equinox, 1.9. The curve also steepens with latitude: on 1 September that same pair reads 1.44 in Skåne (56°N), 1.62 near Umeå (64°N), 1.76 in Norrbotten (68°N). Lower sun angles make the cosine term bite harder. The aspect effect sharpens exactly as the mushroom season arrives, and fastest in the north.
One honest correction: those are clear-sky numbers, and a Swedish September is not a clear sky. Diffuse light from an overcast dome does not care which way a slope faces, only how much sky it sees. Redo the calculation with a realistic 60% diffuse fraction and the 60°N ratio collapses from 1.5 to about 1.18. Aspect is not a switch but a slow accumulator: it separates two hillsides over the sunny fortnight, not the grey one.
The canopy is a bigger lever than the hill
Anyone who has walked a clear-cut in August knows this, and the Swedish data agrees. Greiser et al. (2018), modelling understorey microclimate from 203 loggers across some 16,000 km² of central Sweden, found vegetation structure dominates physiography as a microclimatic driver in the warm season. De Frenne et al. (2019) showed the same globally: closed canopies buffer sub-canopy maxima well below open-ground temperatures. A spruce roof takes most of the beam before it reaches litter, so on shaded ground aspect is largely neutralised.
It cuts the other way too. Dense spruce intercepts a large share of rainfall in the crown, where it evaporates without reaching the floor — published fractions are high but vary so widely with stand age and shower depth that no single percentage is worth quoting. So the reading is the interaction, not either factor alone: aspect matters most where the canopy is broken — edges, gläntor, thinned stands, skogsbilväg verges. Under an unbroken roof both slopes are already shaded, and the hill has less to say.
Water also arrives sideways
Aspect governs the loss term. It says nothing about supply, and supply is topographic too. The topographic wetness index of Beven and Kirkby (1979) — upslope contributing area over local slope — remains the workhorse: water collects where a lot of ground drains into a flat or concave patch. Sweden has this unusually well mapped. Ågren et al. (2014) tested terrain wetness indices against field data in the Krycklan catchment; Ågren et al. (2021) extended that to a national soil-moisture map at 2 m resolution, trained on some 20,000 field plots over Lantmäteriet’s 1 m LiDAR elevation model; Larson et al. (2022) tested how well those indices track measured moisture in a boreal catchment.
The compounding case is the one worth walking: a concave, north-to-north-east-facing mid-slope bench with upslope ground feeding it — receiving water, losing little. A different substrate entirely from a convex north-facing nose, which faces the right way and sheds everything anyway.
How much of the fruiting link is actually established
Be clear which parts of this chain are solid. The geometry is settled physics. Radiation-to-evapotranspiration-to-soil-moisture is well established and directly measured — Bilir, Fung and Dawson (2021) documented slope-aspect differences driving land-atmosphere water exchange. Moisture-to-fruiting is well established at stand-and-season scale: Krebs et al. (2008) predicted 15 years of boreal Yukon mushroom crops from June rainfall plus the previous May’s (R² = 0.85, crops 0 to 117 kg/ha); Karavani et al. (2018) found precipitation and soil moisture dominant in Mediterranean pine stands; Boddy et al. (2014) reviewed the field, leaning on the 21-year Swiss plot record of Straatsma, Ayer and Egli (2001).
The weak link is the last: aspect to sporocarp yield, measured directly. It is thin, and it does not always point where you would expect. Mumcu Küçüker (2019) found Boletus edulis yields lower on north aspects in a Turkish planning unit — which is what you would predict once you notice the sign of the effect depends on which side of the moisture optimum a site sits. Geml et al. (2019) showed aspect strongly structures soil fungal community composition in Pannonian forests — community, not yield. I could not find a published Fennoscandian study measuring yield of Cantharellus cibarius or Craterellus tubaeformis stratified by aspect. If one exists, I would like to read it. Until then the aspect-to-fruiting step is an inference from moisture ecology and should be labelled as one. Nordic berry work is further along — Miina et al. (2021) describe how Finland and Sweden built cover-and-yield models for Vaccinium myrtillus from national forest inventory plots, a real response variable, which is exactly what mushrooms lack.
Standing in the forest, deciding which way to walk
In a dry autumn, walk the north and north-east flanks, favouring concave ground and seep lines over noses. Below about 5° of slope the radiation difference is small enough to ignore; from 10–15° it does real work. Expect the north side to lead by days, not weeks, with the gap widest after a sunny stretch and nearly closed after a grey one.
In a wet autumn, invert it. On saturated ground the constraint stops being water and becomes warmth, and the south flank — warmer at litter level by the same surplus that dried it out in August — goes first. Once nights turn cold, that reversal becomes the rule. Greiser et al. (2020) built their microrefugia work on exactly this: the cool topographic pockets sheltering boreal understorey species from warming are, to a forager, the ground that stays damp longest and warms last.
None of this tells you what is standing there. It tells you where the substrate has been inside the moisture band longest, which is a different and more tractable question — and the one a hillside will actually answer.