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The south face carries a water debt

8 minJames

Revised 11 August 2026

Two slopes, four hundred metres apart, with the same rain passing over both. In a Skåne barrskog the north face is dark under spruce and still cool at noon; the south face is open enough to catch the low October sun. The first fruit bodies appear on the shaded side. A week later the south side has changed. The temptation is to call this a temperature story. It is more nearly a story about water that has already left the ground.

Aspect changes the energy before it changes the soil

A slope is not merely a direction on a map. In a Swedish autumn, a south-facing surface is presented more directly to the low midday sun than a north-facing one. The distinction matters most on clear days and on ground with a view of the sky. SMHI separates direct from diffuse radiation: cloud turns part of the incoming light into diffuse light arriving from many directions, while a clear sky preserves the directional force of the sun. A north-facing slope can therefore spend a bright day in its own shade while the opposite face receives direct energy.

That is the beginning of the mechanism, not its conclusion. Incoming radiation can warm the surface, but it can also supply the energy that turns liquid water into vapour. SMHI describes evaporation as both direct loss from soil and wet surfaces and transpiration through vegetation, together called evapotranspiration. Wind, air humidity and temperature decide whether that water vapour is carried away. Radiation is one part of the account; a dry, moving air mass can make it an expensive part.

A south face in September may thus run a larger atmospheric demand than its north-facing neighbour. It dries needles, moss, litter and the upper mineral soil sooner after rain, provided the canopy and soil permit it. It may also be warmer. Those are not competing descriptions. The same energy that makes a surface warmer can increase the scope for water loss.

But a forest is not a bare slope. A dense spruce canopy intercepts rain and reduces the direct light reaching the forest floor. SMHI notes that a substantial share of rain falling on dense barrskog can be caught on trees and evaporate before reaching the ground. A broadleaf stand, recent thinning, a rock outcrop and a wind-exposed edge all alter the calculation. Aspect is a useful way to notice a difference in energy; it is not a substitute for the stand above the ground.

The important rain is the rain that remains

The rain gauge at a nearby station reports what arrived from the sky. It does not report how much passed the canopy, infiltrated the humus, remained available in the rooting zone, moved sideways through a slope, or went back to the air. Those are the quantities that separate two faces after the same shower.

SMHI’s water balance writes the matter plainly: precipitation is balanced by evaporation, runoff and change in storage. On the south face, stronger drying can deepen the storage deficit before autumn rain arrives. Rain first pays that deficit. On the north face, where energy demand has often been lower, less water may be required to refill the same layers. This is why a generous downpour can leave one side apparently revived and the other still waiting. It does not mean that the rain was different.

Soil decides how long the difference lasts. Coarse, shallow, stony soil stores little; a thick organic layer, fine material and a receiving position in the slope can hold water longer. Water also does not obey aspect. It follows gravity, pores, bedrock fractures, ditches and old machine tracks. A south-facing hollow receiving seepage may remain wetter than a north-facing shoulder. Seibert and Stendahl’s boreal-forest work is useful precisely because it treats topography as a control on hydrological and soil processes, rather than treating a whole hillside as one condition.

The hydrological year supplies the larger seasonal setting. Across Sweden, summer commonly draws down soil water as plant demand and evaporation are high; in autumn that demand falls and rain can replenish moisture. Yet “autumn” is not a reset button. A dry summer, a thin humus layer or a freely draining slope can carry its deficit well into the period when the air feels damp again. The local difference is not a rule about a certain amount of rain. It is the accumulated difference between water in, water out and water stored.

Fruiting is a response to a sequence rather than a single weather event

A visible mushroom is a fruit body, an intermittent reproductive structure, not the whole fungus. The mycelium, its host where relevant, the substrate and the weather history all precede the object seen on the moss. That is why a single warm afternoon, cold night or rain total so often fails as an explanation for the difference between hillsides.

Long records establish that fungal fruiting is responsive to climate and weather at broader scales. Kauserud and colleagues’ historical European data showed a marked change in autumn fruiting phenology over time. Büntgen, Kauserud and Egli assembled a large European record set to examine climate variability alongside mushroom productivity and phenology. Straatsma, Ayer and Egli followed fruit bodies for twenty-one years on one Swiss forest plot. Such studies are valuable because they make the annual variability impossible to ignore.

They do not establish that a particular north face in Småland will fruit before the south face. The records pool species, places, forest histories and observation effort. The Swiss plot is not a Swedish slope; the continental-scale analyses are still further away from the scale of moss, litter and roots. Koskinen and colleagues’ recent work on fruiting fungal communities adds another warning: environmental responses are structured by relatedness. “Mushrooms” do not make one climatic organism.

What the slope mechanism can reasonably say is narrower. If moisture retention is a limiting part of the sequence for the fungi present, a north-facing face may reach a moisture condition favourable to fruiting before a more exposed south face. It is a statement about conditions in ground and a possible biological response, not a claim that anything will be found there. A warm south face may instead be ahead when temperature is the limiting part of the sequence, when its soil stores water well, or when lateral flow supplies it. The direction of the difference is contingent.

Soil temperature does not cancel the moisture story

It is easy to oppose “warm south” to “cool north” and stop there. Soil temperature does matter, but it behaves differently at different depths. SMHI’s account of jordtemperatur notes that surface temperature can vary more than air temperature, while variation weakens and the seasonal peak arrives later with depth. The top centimetres under litter can see a strong aspect effect over one clear day; a deeper, wetter layer responds more slowly.

This creates two clocks. One is the energy clock: sun, air temperature, wind and vapour demand. The other is the water clock: infiltration, storage, drainage and the rate at which vegetation and evaporation draw down the soil. On a south face the energy clock may run ahead while the water clock runs behind. The surface is warm, the litter crisp, and the useful moisture is less available. On the north face, less direct radiation can slow warming while preserving the water that makes the later stages of a fruiting sequence possible.

Neither clock belongs solely to aspect. Canopy closure changes both; the tree canopy reduces ground radiation and controls interception and transpiration. Soil texture alters storage. A late-season cold spell changes the energy side without replenishing water. Persistent cloud suppresses the direct-radiation difference between faces. Once the soil is near field capacity, extra potential evaporation does not describe the same situation as it does on a dry slope. SMHI’s definition is helpful here: potential evaporation is an atmospheric demand from a surface that is not short of water, whereas actual loss falls when soil moisture is low.

The familiar pattern of north first and south later is therefore most plausible after the south face has accumulated a water debt and the autumn has begun to repay it. It can be absent in a wet year, reversed by seepage, or erased under a closed canopy. The hill is doing physics, but not simple physics.

Maps can describe the ground without promising an outcome

Terrain data are good at showing why two nearby pieces of ground need not share the same physical setting. Lantmäteriet’s national Markhöjdmodell is built as a one-metre grid from laser-derived ground and water points. From elevation come slope, aspect, curvature and likely flow paths. SLU’s Markfuktighetskarta uses terrain-related information and field observations to describe long-term relative wetness; its published raster product is much finer than a regional weather map.

That does not turn a map cell into a probe in the forest floor. A markfuktighetskarta concerns the site’s average tendency to be dry or wet, not the moisture available beneath this week’s moss. SMHI makes the equivalent caution about its own national S-HYPE outputs: they are modelled, and local features can make a model inadequate at small scale. SMHI does not measure soil moisture across Sweden; its public soil-water information is modelled against what is normal for the time of year.

The honest use of terrain is therefore habitat-suitability. A north-facing receiving slope with a thick organic layer may be a more moisture-retentive setting than a south-facing convex shoulder. It is not a probability of a find, and an unmapped area is not evidence of dry or empty ground. Even a detailed terrain input cannot settle canopy interception, recent disturbance, subsurface flow or the water status of a particular fungal network.

There is another reason to keep the read modest. A terrain layer can improve spatial understanding where it is well mapped, while weather and soil-water inputs carry their own temporal uncertainty. After a changeable week, a clear-looking slope difference can still be softened by uncertain present moisture. Conditions can be favourable without any promise about the outcome.

The useful conclusion is conditional

Settled: aspect changes the receipt of direct solar energy; radiation, wind, humidity and temperature contribute to evaporative demand; and soil storage, drainage and topographic position determine how much of a rainfall remains. These mechanisms make it entirely credible that opposing slopes on one Swedish hill carry different moisture histories through autumn. They also explain why the warmer-looking south face can lag after dry weather.

Not settled: the soil depth and moisture condition that best precede fruiting for the fungi in a given Swedish stand. I could find no published slope-pair experiment that follows Nordic forest soil moisture, fungal mycelia and fruit-body emergence closely enough to assign a general number of days to north and south faces. The major fruiting studies are broad European record sets or a Swiss long-term plot, not replicated opposing slopes in Söderåsen or Uppland.

That missing experiment matters. It leaves room for a competing explanation: species composition, host trees, soil chemistry, disturbance and observer attention may all create the apparent aspect effect. A good test would repeatedly measure moisture and temperature at several depths on matched faces, record canopy and substrate, and survey the same plots through several autumns. Until that exists, “the south side comes later” is best held as a field observation with a credible water-balance mechanism behind it, not as a calendar rule.

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