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Biogeosciences, 12, 4261–4273, 2015
www.biogeosciences.net/12/4261/2015/
doi:10.5194/bg-12-4261-2015
© Author(s) 2015. CC Attribution 3.0 License.
Evidence for biological shaping of hair ice
D. Hofmann1, G. Preuss2, and C. Mätzler3
1IBG-3: Agrosphere, Institute of Bio- and Geosciences, Forschungszentrum Jülich, Jülich, Germany
2Birkenweg 7, 57555 Brachbach, Germany
3Institute of Applied Physics, University of Bern, Bern, Switzerland
Correspondence to: D. Hofmann (d.hofmann@fz-juelich.de)
Received: 07 January 2015 – Published in Biogeosciences Discuss.: 02 April 2015
Revised: 11 June 2015 – Accepted: 15 June 2015 – Published: 22 July 2015
Abstract. An unusual ice type, called hair ice, grows on
the surface of dead wood of broad-leaf trees at tempera-
tures slightly below 0◦C. We describe this phenomenon and
present physical, chemical, and biological investigations to
gain insight in the properties and processes related to hair ice.
Tests revealed that the biological activity of a winter-active
fungus is required in the wood for enabling the growth of
hair ice. We confirmed the fungus hypothesis originally sug-
gested by Wegener (1918) by reproducing hair ice on wood
samples. Treatment by heat and fungicide suppresses the for-
mation of hair ice. Fruiting bodies of Asco- and Basidiomy-
cota are identified on hair-ice-carrying wood. One species,
Exidiopsis effusa (Ee), was present on all investigated sam-
ples.
Both hair-ice-producing wood samples and those with
killed fungus show essentially the same temperature varia-
tion, indicating that the heat produced by fungal metabolism
is very small, that the freezing rate is not influenced by the
fungus activity, and that ice segregation is the common mech-
anism of ice growth on the wood surface. The fungus plays
the role of shaping the ice hairs and preventing them from
recrystallisation. Melted hair ice indicates the presence of
organic matter. Chemical analyses show a complex mixture
of several thousand CHO(N,S) compounds similar to fulvic
acids in dissolved organic matter (DOM). The evaluation re-
veals decomposed lignin as being the main constituent. Fur-
ther work is needed to clarify its role in hair-ice growth and
to identify the recrystallisation inhibitor.
1 Introduction
1.1 Characteristics of hair ice
One of the most exciting types of ice takes the shape of fine
hairs (diameter near 0.02mm, length up to 20 cm). It can be
observed in forests, on dead wood, usually on the ground
and sometimes on trees that are still standing (Fig. 1). This
so-called hair ice or ice wool grows on the surface of the
unfrozen wood body of certain moist and rotten branches
of broad-leaf trees. The hairs are smooth, often with a silky
shine. They are found in bunches of beautiful structures such
as curls and waves, sometimes with clear parting or zon-
ing, but without ramification. Although individual hairs are
mostly separate, they follow a macroscopic order, often with
surprising regularity. The hair base is rooted at bark-free po-
sitions or where the bark is loose, but never on the bark.
The outer end is either free or in contact with an ice crust
or with surrounding material, such as bark or leaves. Some-
times hairs loop back to the branch. Bands of parallel hairs
can sinter along contacting lines without losing the original
shape until melt onset. In cold, dry air, however, hair ice sub-
limates within a short time. Therefore it can be observed un-
der calm, humid conditions only, at air temperatures slightly
below 0◦C. We observed branches that produced hair ice re-
peatedly over more than 2 years. During this time, the dry
density decreased to about 0.3g cm−3while the wood was
brightening as a consequence of white rot.
Time-lapse videos of hair-ice growth support visual ob-
servations, in that the growth is controlled and synchronised
over macroscopic parts of the wood. The hair curvature re-
sults from lateral gradients in the growth velocity. The pri-
mary direction is radial, the hairs being in the prolongation
Published by Copernicus Publications on behalf of the European Geosciences Union.
4262 D. Hofmann et al.: Evidence for biological shaping of hair ice
Figure 1. Hair ice on a stem of dead beechwood (26 Decem-
ber 2009, Moosseedorf), hair length up to 10 cm.
of wood rays (Fig. 2). Indeed, our observations indicate that
the hairs are rooted at the mouths of these rays (Wagner and
Mätzler, 2009). Furthermore, the hair-ice thickness (Fig. 3)
corresponds to the diameter of the cells (Figs. 8 and 9 in
Sect. 2) forming the channels in the wood rays (Schwein-
gruber, 1990).
The fine hairs can keep their shape over a long time – many
hours or even days. This is surprising because recrystalli-
sation of small ice crystals to larger ones is a fast process,
especially at temperatures close to melting point. The ob-
served stability is an indication that hair ice is doped with
a recrystallisation inhibitor. Examples are antifreeze proteins
(AFP), also called ice-binding proteins (IBP); see Griffith
and Yaish (2004).
When the air temperature exceeds 0◦C, hair ice starts to
melt. Hairs fixed at both ends are transformed to a type of
micro-string, covered by small water droplets (Fig. 4) be-
fore they decay. Hairs with free ends have been observed
to collapse in length. The melted liquid is clear water with
a slightly brownish colour, indicating the content of organic
material.
We received many reports on the observation of hair ice,
mainly at latitudes between 45 and 55◦N from Canada,
Figure 2. Cross section of a hair-ice-producing beech branch
(∅22 mm) with radial wood rays.
Figure 3. Enlargement of hair ice on beechwood, image width
3.2mm.
France, Germany, India, Ireland, Netherlands, Russia, Scot-
land, Slovenia, Sweden, Switzerland, USA, and Wales.
1.2 References to earlier work and the fungus
hypothesis
Although the literature on hair ice is not abundant, it reaches
back about one century. Wegener (1918), the founder of the
theory on continental drifts, published the most relevant pa-
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D. Hofmann et al.: Evidence for biological shaping of hair ice 4263
Figure 4. Melting hair ice, showing tiny droplets on hardly visible
strings.
Figure 5. Reduction and suppression of hair-ice growth after fungi-
cide treatment; durations from 0 to 120 min are indicated.
per of earlier work. He observed hair ice in winter 1916/1917
in the Vosges Mountains, and in February 1918 in north-
ern Germany at Rheinsberg in der Mark. He was able to
grow hair ice again on the branches that he found. Wegener
assumed a relationship between the formation of hair ice
and the mycelium visible on the branch surface. His con-
sultant, Arthur Meyer, confirmed that these branches con-
tained fungus mycelia. He assumed a type of Ascomycota;
however, he was unable to determine the species. Mühleisen
and Lämmle (1975) described the reproduction of hair ice
on rotten wood of broad-leaf trees in a climate chamber.
They assumed that some type of osmotic pressure is acting.
Lenggenhager (1986) observed hair ice in areas near Bern,
Switzerland from 1979 to 1985. Being aware of these obser-
vations, Wagner (2005) supposed the involvement of a fun-
gus activity without the knowledge of Wegener’s work. Wag-
ner found that the presence of fungus mycelia in the wood
body of hair-ice-producing branches is manifested in the mi-
croscopic and macroscopic view. Fruiting bodies of fungi
were observed as shown by Wagner and Mätzler (2009).
They developed a method to repeatedly grow hair ice on
a covered balcony during nights with freezing conditions.
Their more specific hypothesis, that the metabolism of these
fungi is a prerequisite for the formation of hair ice, was sup-
ported by their tests. A further hypothesis was the assumption
that gas pressure, caused by fungal metabolism, ejects water
through wood rays to the wood surface. Furthermore, they
concluded that hair ice is formed from water stored in the
wood. Dash et al. (2006) considered hair ice as a frost-heave
phenomenon associated with ice segregation. In the present
work we recognised signatures of this effect during hair-ice
growth.
1.3 Hair ice and ice segregation
Hair ice grows on porous substrates containing liquid wa-
ter. We call this ice type a “basicryogen”, in order to distin-
guish it from ice that grows from atmospheric water. Differ-
ent basicryogens grow on different substrates and with differ-
ent ingredients. The co-existence of liquid water, ice, and the
porous substrate at the ice front are common to all basicryo-
gens. This general phenomenon is called ice segregation. The
thermodynamic phenomenon of frost heave (Vignes and Di-
jkema, 1974; Dash, 1989; Ozawa, 1997; Dash et al., 2006) is
a common driver for these ice types.
Hair ice grows on the wood surface while a connected net-
work of water inside the wood remains in the liquid state.
Due to molecular interactions at the large specific interface
area between wood cells and the water, the melting point in-
side the wood is reduced to below 0◦C (the melting point
of bulk water), which is known as premelting (Rempel et
al., 2004; Dash et al., 2006). The reduction increases with
increasing specific surface, i.e. with decreasing size of cap-
illaries and pores of the wood. The depression can keep the
water unfrozen inside the pores while ice is growing outside.
The freezing front remains at the wood surface during ice
growth. In this way, the ice is segregated from the reservoir
of liquid water.
How can we understand this process? The large dipole mo-
ment of the water molecule causes charges on the surface of
liquid water and ice. Energy is needed to set up the associ-
ated electric field. Surface energy can be reduced if a liquid-
water film remains between the wood and the ice surface due
to the flexibility of the liquid to neutralise the local charges.
Under freezing conditions, suction forces are set up, caused
by the interface energy of the wood–water–ice sandwich, to
attract liquid water from the pores of the wood toward the
freezing front. In this way the liquid-water film is main-
tained. Frost-heave processes in fine-grained soil also act in
this way (Ozawa, 1997; Dash et al., 1995, 2006). Although
the theory of ice segregation is still incomplete (Dash et al.,
2006), empirical insight in the behaviour was found by ex-
perimental studies. Ozawa and Kinosita (1989) investigated
ice growing on the surface of a microporous filter. The thin
liquid film, separating the ice from the filter material, was
confirmed. The authors observed that the growth rate of ice
increases proportionally with the depression 1Tf=T0−Tf
of the temperature Tfat the freezing front, where T0=0◦C,
is the melting temperature of bulk ice (1Tfvalues ranging
from 0.0 to 0.3◦C). The observations are in agreement with
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4264 D. Hofmann et al.: Evidence for biological shaping of hair ice
an experiment by Vignes and Dijkema (1974), who grew ice
at the mouth of a capillary channel that was in contact with
a reservoir of supercooled water (1Tfvalues ranging from
0.3 to 0.77◦C). With respect to growth rate and temperature
range, these experiments agree with our observations of hair-
ice growth as will be shown.
1.4 Outline
Advancing the understanding of the hair-ice phenomenon
was the objective of this work. In Sect. 2, we will confirm
that a winter-active fungus is needed for hair-ice formation.
We will identify the acting fungus. In the physical analysis of
Sect. 3 we will study the temperature variation of wood dur-
ing ice formation. Wood samples with and without the acting
fungus will be studied. The results point us to the search for
a recrystallisation inhibitor produced by the fungus and thus
also to the chemical analysis, detailed in Sect. 4. Final dis-
cussions and conclusions will be given in Sect. 5.
2 Observations and experiments related to the fungus
hypothesis
2.1 Suppression of hair-ice growth by killing the fungus
2.1.1 Experiments with fungicide
In winter 2010/2011 Gerhart Wagner refined the hair-ice ex-
periments of Wagner and Mätzler (2009). A hair-ice-active
branch was cut into five pieces (length 35cm, diameter
2cm) and exposed to a commercial fungicide (a Miocolor
“Schimmelentferner”, sodium hypochlorite, conc. 4.4%):
piece no. 0 remained untreated, no. 1 was exposed for 15 min,
no. 2 for 30min, no. 3 for 60 min, and no. 4 for 120 min.
On the first evening (18 December 2010), the air tempera-
ture was close to −10◦C. Consequently the growth of hair
ice was fast, but ended after 1h at a hair length of 1cm. The
result at 21:30 LT is shown in Fig. 5. Hair ice was reduced on
pieces treated with the fungicide, but complete suppression
was observed for the longest treatment only.
The following morning all pieces melted at room tempera-
ture before they were exposed to the cold again. The air tem-
perature rose from −5 to 0 ◦C. Two hours later, a very similar
result was obtained. In the afternoon, the experiment was re-
peated again after wetting the branches in distilled water for
a short time. The results showed the expected gradation even
more clearly. All results demonstrate that the 15min treat-
ment causes a significant reduction in hair-ice production,
and a treatment for 120min stopped the production com-
pletely. During the following weeks, the same branches were
repeatedly exposed to the cold. Slow recovery occurred for
the fungicide pieces, in that they gradually started to grow
hair ice again. The pieces with different treatment duration
became more and more similar.
2.1.2 Experiments with heat
A hair-ice branch of about 20cm in length and 2cm in di-
ameter was cut into four similar pieces on 03 January 2011.
Three of them were exposed to hot (90 to 95◦C) water for
1, 2 and 4min, respectively. After the frost night following,
the reference was covered with dense hair ice; however, the
heated pieces did not show any. The repetition with a thin
branch (1 cm) produced the same result. Some days later, the
heated pieces started to produce traces of hair ice again.
2.1.3 Other treatments
Treating the branches with alcohol (70%) or with the weak
fungicide, Imaverol – an imidazole, also called Enilconazole
(C14H14Cl2N2O), often used in veterinary medicine – did not
affect hair-ice growth.
2.1.4 Results
Heat treatment showed the most radical hindrance to hair-
ice growth. 1min in hot water was sufficient for a com-
plete suppression. For fungicides, the type and duration of
the treatment played a role. Whereas our weak fungicide did
not have any effect, complete suppression was achieved with
the stronger fungicide after treatment lasting 2h. The results
are plausible in view of the fungus hypothesis. At a tempera-
ture of at least 90◦C the active cells of the fungus are killed
rapidly. Fungicides act much more slowly. Several days or
weeks after the treatments, hair-ice growth starts again, either
due to the recovery of the mycelium or due to the survival of
spores.
2.2 Identification of the fungus and microscopic studies
on hair-ice-producing wood
2.2.1 Methods
From January to March 2012, from December 2012 to
April 2013, and from November 2013 to March 2014, a
total of 78 hair-ice-bearing twigs and branches were col-
lected in the forests near Brachbach/Sieg in Germany (north-
ern slope of “Windhahn”). Some of them were kept on a
moss-covered shadowy area or temporarily on a wet towel
in a closed plastic box in the garden of one of the au-
thors (G. Preuss) to simulate forest-floor conditions. Addi-
tionally, 41 hair-ice-bearing trees, boles, and branches were
observed in their natural location in the forest. Most of
the species, or at least the genus of the dead branches,
were identified by species-specific characteristics visible in
the microscopic wood anatomy (Schoch et al., 2004, and
Richter and Dallwitz, 2000). Fungi growing on the exam-
ined branches and logs were identified if necessary by mi-
croscopic characteristics with the help of Gminder (2008),
Haller and Probst (1983), Jülich (1984), Krieglsteiner (2000),
Laux (2010), Moser (1963), and Rothmaler (1994). Samples
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D. Hofmann et al.: Evidence for biological shaping of hair ice 4265
of the fruiting bodies were prepared as squash mounts and
sometimes coloured by Phloxine B to stain the basidia. Hand-
cut sections of hair-ice-bearing wood samples were prepared
as wet mounts and afterwards coloured by the dye “Cotton
Blue”, following the instructions of Bavendamm (1936) ac-
cording to the description of Riggenbach (1959). This is an
approved method to make fungal hyphae visible within wood
cells.
2.2.2 Observations and results
The type of examined hair-ice wood varied from fallen twigs,
attached and fallen branches, to dead standing trees and
fallen or felled boles, except for one case: a hazel tree, which
was still living, bore hair ice several times on a dead part of
the wood.
In the majority of cases the consistency of the dead wood
was “rather hard” – indicating an initial stage of decay – but
four samples had a “distinctly softened” texture, adopting the
terms of Heilmann-Clausen and Christensen (2003). The in-
tegrity of the bark differed from almost intact to completely
lost. If it still existed at all, the bark peeled off from the hair-
ice-producing wood surface and often got lost during the ob-
servation period.
Hair ice was observed on ten different broad-leaf tree
species belonging to five different plant families (Table 1).
Eleven different fungal fruiting bodies were identified on
the wood samples – in some cases up to three species on the
same piece of wood. The list is certainly not complete. One
of these fungi, Exidiopsis effusa (Ee), deserves closer atten-
tion, as it colonised all our hair-ice-producing wood samples.
In more than half of the samples, it was the only detectable
fungus, but in most cases it was not yet visible macroscopi-
cally when hair ice occurred for the first time in late autumn.
Its unspectacular fruiting body appeared some weeks later as
a thin whitish coating (Fig. 6) exactly on those areas of the
wood surface where hair ice had grown before (Fig. 6b, d,
f) or next to the hair-ice area (Fig. 6d). In the first observa-
tion period (winter 2011/2012) the coating was present on
every observed sample from February onwards; in the sec-
ond and third winter, the first fruiting bodies were visible in
January. The colour was sometimes slightly bluish, greyish
or light-pink tinted, depending on the humidity and colour of
the wood.
Under humid conditions (in nature or in a closed plastic
box containing a wet towel) and at temperatures above freez-
ing, small water droplets often appeared on the surface of the
Ee fruiting body overnight (Fig. 6f). This might be a case
of guttation, which is the active exudation of an aqueous so-
lution. The phenomenon is well-known related to different
vascular plants, but also related to some fungi (Knoll, 1912;
Thielke, 1983). The observation of Sprecher (1959), who de-
scribes a reinforcing effect at temperatures near +4◦C on the
guttation activity of a fungus, is particularly interesting with
Figure 6. Three wood samples, first with hair ice, later with the
fruiting body of Exidiopsis effusa (Ee): Corylus avellana (∅3.8cm)
on 29 January 2012 (a) and 15 March 2012 (b),Carpinus betulus
(∅3.2cm) on 25 January 2012 (c) and 7 March 2012 (d),Fagus
sylvatica (∅4.9cm) on 3 December 2013 (e) and 11 January 2014
(f, branch rotated by about 180◦).
regard to the formation of hair ice. Further studies on Ee are
necessary to clarify this aspect.
Cross sections of the fruiting body were 30 to 100 µm thick
with great variability (Fig. 7a, b, c). The body mainly con-
sists of ovoid to pyriform basidia in a layer of thin, clamped
hyphae. The basidia, too, are clamped at their base, are 4-
septate (Fig. 8a, c, d), longitudinally divided (Fig. 8a), about
10 µm in diameter and ca. 15µm long without the sterigmata.
The spores (Fig. 8b) are weakly allantoidal, 13–15µm long
and 4–5µm in diameter and often contain one big oil drop
in the middle. Based on these characteristics the fungus was
identified as Exidiopsis effusa (Ee) with the help of the man-
ual of Jülich (1984). The descriptions at www.mycobank.org
(Robert et al., 2005) and www.mycokey.com (Petersen and
Læssøe, 2003) confirm the result, while Krieglsteiner (2000)
classifies the fungus as Exidiopsis grisea var. effusa.
Beneath the fruiting body, the superficial wood cells are
almost filled by hyphae (Fig. 7a, b). The fungus seems to
grow along the wood vessels (Fig. 9a) and intrude into the
deeper wood tissue along the wood rays (Figs. 7c and 9b).
Some hair-ice wood samples found in November were ex-
amined by microscope, too. At that time of the year, there
was no macroscopic sign of a fungal fruiting body on the
wood surface. But when microscopic mounts were coloured
by the dye “Cotton Blue”, they showed, that the wood tissue
was pervaded by fungal hyphae running along the wood ves-
sels and rays (Fig. 9b). Except for the density of the hyphae,
the findings looked exactly like those from the wood samples
with an Ee fruiting body.
As our main result, we unravelled the mystery about the
whitish covering described by Wegener (1918) and Wag-
ner (2005) by the identification of Ee. The opinion of We-
gener’s consultant, A. Meyer, who did not expect the appear-
ance of a fruiting body and thought that the determination
of the species would be impossible, has been disproved. Fur-
thermore, the result specifies the fungus hypothesis of Wag-
ner and Mätzler (2009).
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4266 D. Hofmann et al.: Evidence for biological shaping of hair ice
Table 1. Hair-ice-bearing wood species, range of diameters and identified fungal species D is Dacrymyces spec., Dd is Diatrype disciforme,
Dst is Diatrype stigma, E is Exidia spec., Ee is Exidiopsis effusa, Ff is Fomes fomentaria, Hf is Hypoxylon fragiforme, Hm is Hypoxyon
multiforme, Sch is Schizopora paradoxa, Pc is Pycnoporus cinnabaria, and Tm is Tremella mesenterica.
Fam. Species (in brackets, number of finds) Diameter (cm) Fungi
Aceraceae Acer pseudoplatanus (4) 1.3–4.8 Ee
Adoxaceae Sambucus nigra (3) 1.5–2.5 Ee
Betulaceae Betula pendula (3) 0.5–13.0 D, Dd, Dst, E, Ee, Hm, Sch
Carpinus betulus (15)
Corylus avellana (16)
Alnus glutinosa (14)
Fagaceae Fagus sylvatica (32) 0.5–18.0 Dd, Ee, Ff, Hf, Sch
Quercus spec. (12, robur/petraea/bastards)
Rosaceae Prunus avium (2) 0.6–6.0 Dd, E, Ee, Pc, Tm
Sorbus aucuparia (18)
Figure 7. Microscopic views of a hair-ice wood sample (Alnus glutinosa twig, ∅9mm) with the Ee fruiting body. In images (b) and (c),
the fungal hyphae and basidia were coloured by “Cotton Blue” (scale bar 10µm, B are basidia, UR are uniseriate rays). Panels (a) and (b)
show cross sections of an Alnus glutinosa twig with a thin Ee fruiting body. The wood vessels are shown in the cross section. (c) Radial
section of the Alnus glutinosa twig with a thick Ee fruiting body, showing two uniseriate rays in longitudinal section. The wood vessels are
cut longitudinally, too.
3 Physics of hair ice
3.1 Thermal effects during hair-ice formation
Insight in the physics of hair ice is found from thermal
signatures of hair-ice wood, i.e. the temperature enhance-
ments caused by heat sources associated with the ongo-
ing processes. These include (1) heat generated by fungal
metabolism, (2) latent heat of fusion when water freezes in
contact with the wood, and (3) heat used or generated by re-
crystallisation.
Heat generated by these processes diffuses into its envi-
ronment by radiation, heat conduction, air turbulence, evap-
oration, and sublimation and therefore exact measurements
are difficult to achieve. However, by careful design of the ex-
periments, the main heat loss can be reduced to black-body
radiation. For heat sources that change rapidly with time, the
temperature changes are damped according to the heat capac-
ity of the wood. On the other hand, under stationary condi-
tions the heat power Pgenerated in the wood (at temperature
T1)is equal to the net power that the wood loses to its envi-
ronment (at temperature T2). The temperature difference can
then be expressed by
1T =T1−T2=0.25P σ −1A−1T−3,(1)
where σ=5.673×10−8Wm−2K−4is the Stefan–
Boltzmann constant, Athe branch surface area, and T
the mean absolute temperature of T1and T2. The pro-
portionality allows us to express the generated power by
temperature differences. Two examples are given below.
3.1.1 Fungal metabolism
For the combustion of nutrients by fungal metabolism, we as-
sume a heat of combustion Qb=15.6kJ g−1, representative
for glucose. A typical branch used in our experiments has
a volume of V=82cm3and a surface area of A=150cm2
(length l=26cm, diameter d=1.8 cm). Such a branch is
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D. Hofmann et al.: Evidence for biological shaping of hair ice 4267
Figure 8. Microscopic characteristics of the Ee fruiting body (scale
bar: 10µm). (a) Ee fruiting body showing longitudinally divided
basidia with four sterigmata (squash mount); (b) Ee spores; one is
germinating; (c) 4-septate basidia in a cross-sectional view (squash
mount); (d) basidium with four long sterigmata (squash mount,
coloured by Phloxine B).
Figure 9. Microscopic view of hair-ice wood showing fungal hy-
phae coloured by “Cotton Blue” within the wood tissue. The vary-
ing thickness of the fungus hyphae was caused by the colouring pro-
cedure (scale bar: 10µm, UR is an uniseriate ray, MR is a multise-
riate ray). Panel (a) shows the tangential section (similar to the look
on the surface) of an Alnus glutinosa twig (∅8 mm), that was partly
covered by the Ee fruiting body. Several uniseriate rays are shown in
the cross section; the wood vessels are cut longitudinally. Panel (b)
shows the cross section of a Fagus sylvatica twig (∅6mm) bearing
hair ice, that had not yet developed the fungal Ee fruiting body. The
wood vessels are shown in cross section; one multiseriate ray is cut
longitudinally.
able to produce hair ice over about 1 year. For the active
phases of the fungus we assume a duration of 3000h. Fur-
thermore, assuming 5g of nutrients, the rate of combus-
tion is on the order of 1.5mg h−1. Multiplying this value
with Qb gives P=7mW, and for T=273K in Eq. (1) we
get 1T =0.1K, a value that should be measurable under
favourable conditions.
3.1.2 Heat of fusion
Now, assuming the same branch geometry, latent heat is
generated by freezing 1g of water per hour. The generated
heating power follows from the latent heat of fusion of ice,
Lf=333J g−1at T=273 K, as P=92 mW; inserting this
value in Eq. (1) gives 1T =1.3K. This temperature differ-
ence should be easy to measure. Note that it is proportional
to the freezing rate.
3.1.3 Measurement setup
Branches of beechwood with hair ice were collected in a
forest in Moosseedorf, Switzerland. The samples were thor-
oughly wetted in rainwater. Then they were arranged on a
plastic socket put on wet snow to stabilise temperature and
to keep the humidity at saturation to avoid evaporation and
sublimation. The entire stack was put in a plastic tub that
was covered and further insulated with a towel to reduce
vapour fluxes and heat conduction inside the tub. The tub
was put in a garden hut from mid January to early March
2011, and again in January 2012. Hair ice formed on wood
samples during cool nights. From time to time wetting was
repeated. With this setup we realised conditions for pre-
cise temperature measurements and we therefore approached
the assumption that the net heat flux can be represented
by Eq. (1). For temperature measurements we used Pt-100
sensors (length 10mm, diameter 2.6 mm). After calibration
with an ice–water mixture at 0.00◦C, the sensors were in-
serted into the wood with the cylinder axis along the radial
wood rays (Fig. 10). Temperatures were registered once per
second to monitor changes with sufficient time resolution.
The effective temperature of the environment was a mean
of T0=273.15K of the underlying wet snow and of the tub
walls cooled from outside. To represent this value of T2we
either used the air temperature inside the tub or the tempera-
ture of a dry, passive tracer (hazel wood in 2011) close to the
wetted wood samples.
3.1.4 Experiment of 18–20 January 2011
First measurements were taken in January 2011. During the
first night, 18–19 January, the temperature stayed above 0◦C
(Mätzler et al., 2013). The time-averaged temperature of the
hair-ice branches was 1.213◦C, whereas the value of the pas-
sive tracer was 1.203◦C. The difference of 0.010◦C is very
small, similar to the measurement uncertainty.
First hair-ice growth was monitored from 19 to 20 January
2011 with temperature variations shown in Fig. 11. Before
midnight all temperatures decreased linearly with time, in-
cluding the passive tracer. At midnight, sudden increases oc-
curred for all hair-ice branches, indicating ice nucleation and
the start of latent-heat release by hair-ice growth. The start
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4268 D. Hofmann et al.: Evidence for biological shaping of hair ice
Figure 10. Situation in the morning of 15 January 2012: heat-
treated piece (1, bottom right), piece (2, top) with abundant hair
ice, air temperature sensor (3, bottom left) with red insulation.
Figure 11. Temperature variation during the first experiment:
beechwood samples (1, 3, 4, 5, 6, and 7) with growing hair ice,
and dry hazel wood (8) as a passive tracer of ambient temperature.
is almost simultaneous in all branches, with slightly differ-
ent nucleation temperature (−0.4 to −0.7◦C). Ice grew until
the end of the measurement period the following morning.
During this period, the temperature of the hair-ice branches
remained nearly constant near −0.2◦C. On the other hand,
the passive tracer (8), representing T2, continued to cool
to −1◦C until 05:30LT without any jumps. Further exper-
iments confirmed these results. The simultaneity of temper-
ature jumps for different hair-ice woods was observed again,
but exceptions occurred as well.
Figure 12. Temperature variation during the second experiment:
heat-treated piece (1), untreated piece (2) with hair ice, air temper-
ature close to wood samples (3), and further away (4).
3.1.5 Experiments of 13–15 January 2012
In the second year we tried to find out if the fungus activ-
ity is a prerequisite of the observed temperature jumps, in
other words, if the jumps are signatures for hair-ice forma-
tion. We selected a beechwood sample with abundant hair-
ice growth. After wetting it and cutting it in half, both pieces
still showed abundant hair-ice growth during the night from
13 to 14 January 2012, with jumps similar to Fig. 11. Then
one piece (1) was held in hot water for 5min to kill the fun-
gus. Both pieces were exposed to the cold again during the
following night. As shown in Fig. 10, the untreated sample
(2) produced dense hair ice again, but no hair ice was found
on the heat-treated sample (1); instead, a thin ice crust cov-
ered this wood. The temperature variations are presented in
Fig. 12. All sensors started slightly above 0◦C, and all values
were near −4.3◦C at 09:00 LT the following morning. Due
to colder weather, the cooling was faster and reached lower
values than in Fig. 11. Both wood samples - with and without
hair ice - showed similar and almost simultaneous increases,
indicating the onset of freezing at 22:35 LT. Air temperatures
at the centre (3) and edge (4) of the container indicate forced
cooling with minor modulations.
3.2 Discussion
Before the onset of freezing, the temperature of hair-ice
branches cannot be distinguished from the environmental
temperature. The small difference (0.01◦C) found during the
first night in 2011 is representative of the measurement error.
This means that heating by fungal metabolism is much more
marginal than estimated in Sect. 3.1.1. In other words, the
combustion rate of nutrients in our test sample is clearly less
than 1 mg/h.
Before freezing begins, the wood temperatures, shown in
Fig. 11 and 12, decrease to supercooled conditions below
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D. Hofmann et al.: Evidence for biological shaping of hair ice 4269
0◦C. A sudden increase marks the onset of freezing by the
release of latent heat. The onset in one branch triggers the on-
set in nearby branches usually within a short time. Ice nucle-
ation appears as an explosive event distributing microscopic
ice particles. The temperature increase is similar for samples
with hair-ice formation and for samples with killed fungus.
During the phase of ice growth, the wood temperature
(−0.1 to −0.4◦C) is nearly constant. In addition to the data
of Figs. 11 and 12, this range was found in all other measure-
ments that we made (Mätzler et al., 2013). These values are
signatures of premelting and thus of ice segregation. The sta-
bilisation is based on the heat of fusion. With falling ambient
temperature the freezing rate increases and thus the transfer
of latent heat to the wood increases, too. The associated tem-
perature increase compensates for the decrease of the ambi-
ent temperature, keeping the branches close to melting point.
The stabilisation acts as long as there is sufficient liquid wa-
ter in the wood rays. The continued freezing dehydrates the
branch. When the liquid supply is exhausted, ice production
comes to an end, the heat production stops, and the wood
temperature converges to the ambient one. This transition is
shown near the end in Fig. 12.
The temperature difference between the pieces with and
without hair-ice production is small, indicating that the freez-
ing rates are similar. The result means that the fungus activity
hardly plays a role for the rate of ice formation. This finding
may appear as a surprise when looking at the difference be-
tween the branches (1, 2) of Fig. 10. But, note that hair ice
is extremely fine, its density being extremely small. On the
other hand the ice crust on the heat-treated piece (1) mainly
consists of bulk ice. To show that its mass is indeed simi-
lar to the hair-ice mass, we compare two situations with the
same ice volume, one consisting of Nhairs per branch sur-
face area, the other one consisting of an ice crust covering
the branch surface with a constant thickness d. If we denote
the hair diameter by dhand the hair length by lh, it is easy to
show
d=π
4Nlhd2
h.(2)
To get an estimate of dwe need the parameters on the
right-hand side of Eq. (2). From Fig. 3 and similar obser-
vations we find: dh≈0.014mm, N≈20mm−2, and for the
hair length we choose lh=50 mm to get d=0.15mm for the
crust thickness. The comparison is consistent with the two
branches of Fig. 10.
With regard to Fig. 12, minor differences between
Curves (1) and (2) can be noted. Firstly, after freeze onset,
Curve (1) shows small oscillations, possibly due to recrys-
tallisation. On the other hand, the temperature variation of
Curve (2) is very smooth. Secondly, towards the end, when
the strongest cooling occurs, the temperature decrease is
slower for Curve (2) with hair ice than in Curve (1) due to
thermal insulation by the ice wool.
The similarity of the temperature and thus of the rate of ice
growth means that ice segregation is the common mechanism
for ice production on the wood surface. The role of the fun-
gus is in shaping the ice as hairs and to prevent it from recrys-
tallisation. This is the main result from the physics of hair
ice. Furthermore it indicates that temperature and pressure
enhancements associated with fungal metabolism in hair-ice
wood (Wagner and Mätzler, 2009) do not appear to be no-
ticeable.
4 Chemistry of melted hair ice
4.1 Organic carbon
First investigations of non-filtrated and filtrated meltwater
samples from hair ice, by means of a total organic carbon
analyser (TOC-VCPH, Shimadzu, Japan), show similar and
significant amounts of organic carbon (> 200mg L−1)as well
as small amounts of (total) nitrogen (> 10mgL−1). Ion chro-
matography revealed that the main part (70%) of nitrogen
consists of ammonium ions. We conclude that melting of hair
ice provides us with a real organic carbon solution, mostly
free of wood and aerosol particles.
4.2 Mass spectrometry
For the elucidation of the unknown components we ap-
plied mass spectrometry (MS) aiming at a complete spec-
trum of organics with regard to molecular size and polar-
ity. Representative screening was achieved by various ion-
isation techniques in combination with appropriate sepa-
ration techniques. First, we performed gas chromatogra-
phy, coupled with electron-impact mass spectrometry (GC–
EIMS) of filtrated aqueous solutions according to Turska
et al. (1997), followed by the more sensitive headspace-
GC–EIMS (Seto, 1994, Snow and Slack, 2002). For small
carboxylic acids (known as e.g., plant exudates) and pep-
tides/proteins (possible degradation products and/or anti-
freeze proteins), we used capillary electrophoresis and elec-
trospray Fourier-transform ion cyclotron resonance mass
spectrometry (ESI-FTICR-MS, Marshall et al., 1998). None
of these methods yielded any significant peak.
In the second step, we applied our most sensitive mass
spectrometer equipped with an electrospray source coupled
to ultra performance liquid chromatography (UPLC-ESI-
MS, Swartz, 2005; Guillarme et al., 2010), resulting in first
(non-resolved) chromatograms. The electrospray full-scan
spectra in positive and negative ionization mode and differ-
ent retention times are highly complex – ranging over the
whole mass range, but similar among themselves. Such spec-
tra – Gaussian in shape with emerging odd-numbered peaks –
look similar to fulvic/humic acids in dissolved organic mat-
ter (DOM) of terrestrial/marine water, soil/sediments, peat
bog, kerogen, and eventually crude oil (Kujawinski, 2002;
Sleighter and Hatcher, 2007; Hertkorn et al., 2008).
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4270 D. Hofmann et al.: Evidence for biological shaping of hair ice
Figure 13. Detail of a high-resolution mass spectrum of melted hair
ice.
Therefore, in the third step, we simultaneously desalted
and concentrated hair-ice samples by solid-phase extraction
(SPE). The methanolic eluates, mixed with 20% water for
better ionisation yield, were introduced again by flow in-
jection in the ultrahigh-resolving ESI-FTICR-MS. In con-
trast to our former investigation, we arranged the measur-
ing conditions towards complex samples (averaging seven
spectra of 50 transients each) to improve the statistics. The
high-resolution spectrum in the mass range 200–1000Da [1
Dalton (Da)=1.660539 ×10−27 kg, corresponding to 1/12
of the mass of a C-12 atom] show a bimodal distribution
(maxima at 500 and 610Da, not shown). The detailed evalu-
ation of these spectra revealed the typical presence of molec-
ular families containing ions that differ from each other in
the number of CH2groups, the degree of saturation (grey
arrows in Fig. 13), and the functional group substitution
(blue arrows in Fig. 13), with a significantly higher inten-
sity of the odd-numbered masses (description of such spec-
tra in Leinweber et al., 2009). The reason for the latter phe-
nomenon is the mass spectrometric nitrogen rule: under elec-
trospray conditions the ionisation is by proton transfer to [M-
H]−or [M+H]+ions, therefore nitrogen-free organics like
CHO and CHOS compounds are odd-numbered, while or-
ganics with one (or three or five) nitrogen atom(s) are even-
numbered.
In Fig. 13, ions with the mass space of 0.0364 Da (blue ar-
rows) were derived from the mass difference between CH4
and O, representing the most important formal functional
group substitution, giving the typical clusters within a nomi-
nal mass. Furthermore, expanded sections of successive, ran-
domly selected nominal masses show that every single (in-
tensive) odd-numbered mass peak has a corresponding lower
abundant even-numbered mass peak higher in one m/z – the
exact mass space of 1.0034 Da is thereby attributed as its 13C
isotopic peak (olive-coloured arrows). Still, the mostly one-
nitrogen-containing peaks in hair ice are less intense. This re-
sult shows the same qualitative tendency as our first organic
Figure 14. Van Krevelen plot of the CHO compounds of hair ice
(for peak intensities >600 counts), with ellipses indicating typical
ranges of selected biopolymer components, adapted from Sleighter
and Hatcher (2007).
analyses in terms of the organic nitrogen to organic carbon
ratio of only approximately 0.014 :1 – but with the consider-
ation that complex ESI spectra are never quantitative because
of the quench effects of easily to poorly ionising substances.
We developed a post-processing to formula assignment
of all compounds based on Scilab routines. The resulting
mass lists are transformed to Excel tables for sorting and/or
preparation of graphical figures describing the characteris-
tics of DOM. The van Krevelen diagram – H/C versus
O/C for all detected compounds – is mostly used to clas-
sify the compounds in terms of polarity and aromaticity.
Furthermore, Sleighter and Hatcher (2007) and Hockaday
et al. (2007) published ranges of biogeochemically impor-
tant compound classes in such plots. The [O/C, H /C] val-
ues found for melted hair ice, which range between [0.40–
0.72, 1.52–0.95], correspond to Sleighter’s lignin and tannin
ranges, indicated by ellipses in the van Krevelen plot of hair
ice and presented in Fig. 14.
4.3 Discussion
Mass spectrometry has shown that hair-ice water contains
fragments of lignin and tannin. Lignin is a main compo-
nent (20 to 30 % of dry mass) of wood, stabilising the cells
against compression. In contrast to cellulose, it is more diffi-
cult to decompose. Lignin is indigestible by animal enzymes;
only some fungi (causing white rot) and bacteria are able to
secrete lignase and thus biodegrade the polymer. Lignin is
an irregular biopolymer with molecular mass in excess of
10000 Da, consisting of various types of substructure, that
are repeated in a haphazard manner. Tannins are similarly ir-
regular, with molecular weights between 500 and 3000Da
and up to 20000 Da (proanthocyanidins). Lignin and tannin
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D. Hofmann et al.: Evidence for biological shaping of hair ice 4271
consist of hydrophobic skeletal structures with numerous po-
lar functional groups. The hydrophobic lignin/tannin macro-
molecules may act as crystallisation nuclei.
The lignin and tannin classification was obtained by
atomic ratios, shown in Fig. 14. By the application of
electrospray, the most gentle mass spectrometric ionisation
technique, covalent bonds stay intact whereas the weaker
ones (van der Waals bonds and electrostatic bonds) are de-
stroyed. Consequently, molecules in the mass range between
200 and 800Da result, in general, as fulvic acids do. This
means that we are unable to distinguish between the original
lignin/tannin macromolecules and their (partial) degradation
products.
We also detected sulphur- and nitrogen-containing com-
pounds, but with very small intensities in contrast to the CHO
peaks and in accordance with our TOC and TN measure-
ments. We assume that we detected marginal leftovers from
the total degradation of more easily degradable (and nutri-
tious) wood compounds, like proteins, which had already be-
gun the process of mineralisation. This would explain the sig-
nificant ammonium concentration in melted hair ice.
5 Conclusions
Our investigations shed light on the mystery of hair ice that
can grow on certain branches of dead wood. The forma-
tion is related to a winter-active fungus. Comparatively dense
mycelium was observed in the superficial wood cells of hair-
ice-bearing sections of the investigated branches. If the fun-
gus activity is stopped, either by a fungicide or by hot water,
the production of hair ice ceases as well.
Hair-ice shape and direction are influenced by the geome-
try at the mouth of the wood rays where the hairs are formed
when the water is freezing. The extreme ratio of hair diameter
to hair length with an order of 1: 10 000 is most surprising.
In spite of the fact that surface tension tries to reduce this
ratio, the shape is maintained over many hours, and some-
times several days at temperatures close to melting point. A
recrystallisation inhibitor must be responsible for stabilising
the hairs. It may be contained in the thread-forming fibre that
appears when hair ice starts to melt, and it could be related
to lignin, the main organic component that we found.
With respect to the origin of water, hair ice is a basicryo-
gen, meaning that the ice originates from water in a porous
substrate, in our case, the wood. Inside the substrate, melt-
ing point is lowered by intermolecular forces in the interface
between ice and the substrate, called premelting. When the
external temperature is sufficiently low, water freezes on suit-
able nuclei on the substrate surface and transfers latent heat
of fusion to the substrate. Once ice nuclei have formed, ice
segregation starts to extract additional water from the sub-
strate, leading to the growth of ice and to the dehydration
of the substrate. We found that this effect occurs under the
same conditions with similar freezing rates in wood with and
without hair-ice production.
The fungus provides decomposed lignin and tannin as or-
ganic materials. The notion that they may act as recrystalli-
sation inhibitors is indicated by properties of lignosulfonate
(waste of cellulose production) found in Sandermann and
Dehn (1951), namely that it delays the hardening of cement.
The fungus activity plays a minor role with regard to the
rate and amount of ice formation. Hair-ice branches with
active and with killed fungus undergo similar temperature
curves, indicating that the freezing rates are very similar and
that fungal metabolism is too weak to cause a measurable
temperature enhancement. The difference must be in shaping
the ice. Whereas the untreated branches produce hair ice, the
heat-treated ones produce crusty ice sticking to the wood sur-
face. Although the fungus effect is still a mystery, it must be
directly related to the hair-ice shape because the suppression
of hair-ice growth acts immediately after the fungus activity
is stopped.
Our findings not only confirm Wegener’s hypothesis that
fungal activity plays an important role, but they unravel the
mystery about the whitish covering on hair-ice-producing
wood, described by Wegener (1918) and Wagner (2005).
There is clear evidence to suggest a causal relationship be-
tween the fungus, Exidiopsis effusa, and the growth of hair
ice. This fungus is known to cause white rot. Indeed, the
brightness of hair-ice wood increases with age, and its dry
density decreases more and more, indicating wood decom-
position. The chemical analysis confirms and specifies the
degradation. Explaining further details of the connection be-
tween the fungus and hair ice will be tasks for future re-
search.
Author contributions. For several years, C. Mätzer has been work-
ing (in collaboration with Gerhart Wagner) on hair ice, its observa-
tion in nature, reproduction, and the test of a fungus hypothesis. In
the presented paper he is mainly responsible for the physical mea-
surements and interpretations, described in Sect. 3. G. Preuss inves-
tigated the fungi and the wood samples by microscopic techniques,
described in Sect. 2.2, leading to the identification of Ee. D. Hof-
mann performed and interpreted the chemical analyses presented in
Sect. 4. The paper is a strong collaboration of scientists from differ-
ent fields with one object of collective discussion.
Acknowledgements. We thank Gerhart Wagner for contributing
Sect. 2.1 including Fig. 5. We regret his wish not to act as
a co-author. We thank the Institute of Applied Physics, Uni-
versity of Bern for support with the temperature measurements.
Niklaus Kämpfer and Andreas Hasler first indicated possible re-
lationships between hair ice and ice-segregation processes to us.
Bernhard Steffen, JSC, Forschungszentrum Jülich, developed the
Scilab-based evaluation program.
www.biogeosciences.net/12/4261/2015/ Biogeosciences, 12, 4261–4273, 2015
4272 D. Hofmann et al.: Evidence for biological shaping of hair ice
The article processing charges for this open-access
publication were covered by a Research
Centre of the Helmholtz Association.
Edited by: J. Kesselmeier
References
Bavendamm, W.: Erkennen, Nachweis und Kultur der holzverfär-
benden und holzzersetzenden Pilze, in: Handbuch der biologis-
chen Arbeitsmethoden, Herausgeber: Abderhalden, E., Abt. XII,
27–1134, Urban & Schwarzenberg, Berlin und Wien, 1936.
Dash J. G.: Thermomolecular Pressure in Surface Melting: Motiva-
tion for Frost Heave, Science, 246, 1591–1593, 1989.
Dash, J. G., Fu, H., and Wettlaufer, J. S.: The premelting of ice and
its environmental consequences, Rep. Prog. Phys., 58, 115–167,
1995.
Dash, J. G., Rempel, A. W., and Wettlaufer, J. S.: The physics
of premelted ice and its geophysical consequences, Rev. Mod.
Phys., 78, 695–741, 2006.
Gminder, A.: Handbuch für Pilzsammler, Frankh-Kosmos,
Stuttgart, 2008.
Griffith, M. and Yaish, M .W. F.: Antifreeze proteins in overwinter-
ing plants: a tale of two activities, Trends Plant Sci., 9, 400–405,
2004.
Guillarme, D., Ruta, J., Rudaz, S., and Veuthey, J. L.: New trends
in fast and high-resolution liquid chromatography: a critical
comparison of existing approaches, Anal. Bioanal. Chem., 397,
1069–1082, 2010.
Haller, B. and Probst, W.: Botanische Exkursionen im Winterhalb-
jahr, Gustav Fischer, Stuttgart, New York, 1983.
Heilmann-Clausen J. and Christensen, M.: Fungal diversity on de-
caying beech logs – implications for sustainable forestry, Bio-
divers. Conserv., 12, 953–973, 2003.
Hertkorn, N., Frommberger, M., Witt, M., Koch, B. P., Schmitt-
Kopplin, P., and Perdue, E. M.: Natural Organic Matter and the
Event Horizon of Mass Spectrometry, Anal. Chem., 80, 8908–
8919, 2008.
Hockaday, W. C., Grannas, A. M., Kim, S., and Hatcher, P. G.: The
transformation and mobility of charcoal in a fire-impacted water-
shed, Geochim. Cosmochim Ac., 71, 3432–3445, 2007.
Jülich, W.: Basidiomyceten, in: Kleine Kryptogamenflora, Heraus-
geber: Gams, H., Band IIb, 1. Teil, Gustav Fischer, Stuttgart,
New York, 1984.
Knoll, F.: Untersuchungen über den Bau und die Funktion der Zys-
tide und verwandter Organe, Jahrbuch wiss. Bot., 50, 453–501,
1912.
Krieglsteiner, G. J. (Hrsg.): Die Großpilze Baden-Württembergs,
Band 1: Allgemeiner Teil: Ständerpilze: Gallert-, Rinden-,
Stachel- und Porenpilze, Ulmer, Stuttgart, 2000.
Kujawinski, E. B.: Electrospray ionization Fourier transform ion cy-
clotron resonance mass spectrometry (ESI FT-ICR MS): charac-
terization of complex environmental mixtures, Environ. Foren-
sics, 3, 207–216, 2002.
Laux, H. E.: Der große Kosmos Pilzführer, Frankh-Kosmos
Verlags-GmbH & Co. KG, Stuttgart, 2010.
Leinweber, P., Jandl, G., Eckhardt, K. U., Schulten, H. R., Schlicht-
ing, A., and Hofmann, D.: Analytical pyrolysis and soft-
ionization mass spectrometry, in: Biophysico-chemical processes
involving natural nonliving organic matter in environmental sys-
tems, Herausgeber: Senesi, J., Xing, B., and Huang, P. M., 1st
Edn., Wiley, Hoboken, 539–588, 2009.
Lenggenhager, K.: Zur Frage der Haareis-Bildung, Arch. Meteor.
Geophy. B., 36, 371–379, 1986.
Marshall, A. G., Hendrickson, C. L., and Jackson, G. S.: Fourier
Transform Ion Cyclotron Resonance Mass Spectrometry: A
Primer, Mass Spectrom. Rev., 17, 1–35, 1998.
Mätzler, C., Wagner, G., Preuss, G., and Hofmann, D.: Enlighten-
ing the Mystery of Hair Ice, IAP Research Report 2013-01-MW,
Institute of Applied Physics, University of Bern, Bern, 2013.
Moser, M.: Ascomyceten, in: Kleine Kryptogamenflora, Herausge-
ber: Gams, H., Band IIa, Gustav Fischer Stuttgart, 1963.
Mühleisen, R. and Lämmle, A.: Neue Untersuchungen von Kam-
meis (Haareis), Meteorol. Rundsch., 28, 55–60, 1975.
Ozawa, H.: Thermodynamics of frost heaving: A thermodynamic
proposition for dynamic phenomena, Phys. Rev. E, 56, 2811–
2816, 1997.
Ozawa, H. and Kinosita, S.: Segregated ice growth on a microp-
orous filter, J. Colloid Interf. Sci., 132, 113–124, 1989.
Petersen, J. H. and Læssøe, T.: MycoKeyTM – the mycological
information site, www.mycokey.com (last access: 17 January
2013), 2003.
Rempel, A. W., Wettlaufer, J. S., and Worster M. G.: Premelting
dynamics in a continuum model of frost heave, J. Fluid Mech.,
498, 227–244, 2004
Richter, H. G. and Dallwitz, M. J.: Commercial timbers: descrip-
tions, illustrations, identification, and information retrieval, in
English, French, German, and Spanish, Version May 04, www.
delta-intkey.com/wood/ (last access: 17 January 2013), 2000.
Riggenbach , A: Untersuchungen über den Eschenpilz, Promotion-
sarbeit Nr. 2561, ETH Zürich, Phytopatologische Zeitschrift,
Band 27, Heft 1, 1–40, Verlag Paul Parey, Berlin, Hamburg,
1956.
Robert, V., Stegehuis, G., and Stalpers, J.: The MycoBank engine
and related databases www.mycobank.org (last access: 17 Jan-
uary 2013), 2005.
Rothmaler, W. (Begr.): Exkursionsflora von Deutschland, Band 1:
Niedere Pflanzen, 3 Auflage, Elsevier, München, 1994.
Sandermann, W. and Dehn, U.: Einfluß chemischer Faktoren auf
die Festigkeitseigenschaften zementgebundener Holzwolleplat-
ten, Holz Roh. Werkst., 9, 97–101, 1951.
Schoch, W., Heller, I., Schweingruber, F. H., and Kienast, F.: Wood
anatomy of central European Species, www.woodanatomy.ch
(last access: 17 January 2013), 2004.
Schweingruber, F. H.: Anatomie europäischer Hölzer, Paul Haupt,
Bern and Stuttgart, 1990.
Seto, Y.: Determination of volatile substances in biological samples
by head space gas-chromatography, J. Chromatogr., 674, 25–62,
1994.
Sleighter, R. L. and Hatcher, P. G.: The application of electrospray
ionization coupled to ultrahigh resolution mass spectrometry for
the molecular characterization of natural organic matter, J. Mass
Spectrom., 42, 559–574, 2007.
Snow, N. H. and Slack, G. C.: Head-space analysis in modern
gas chromatography, TRAC-Trends, Anal. Chem., 21, 608–617,
2002.
Biogeosciences, 12, 4261–4273, 2015 www.biogeosciences.net/12/4261/2015/
D. Hofmann et al.: Evidence for biological shaping of hair ice 4273
Sprecher, E.: Über die Guttation bei Pilzen, Planta, 53, 565–574,
1959.
Swartz, M. E.: UPLC (TM): An introduction and review, J. Liq.
Chromatogr. R. T., 28, 1253–1263, 2005.
Thielke, C.: Membranaggregate und Filamente in den Zystiden von
Volvariella bombacina, Zeitschr. Mykologie, 49, 257–264, 1983.
Turska, M., Wolska, L., Zygmunt, B., and Namiesnik, J.: Direct in-
jection of aqueous samples into gas chromatographic columns,
Chemia Analityczna, 42, 787–797, 1997.
Vignes, M. and Dijkema, K. M.: A Model for the Freezing of Wa-
ter in a Dispersed Medium, J. Colloid Interf. Sci., 49, 165–172,
1974.
Wagner, G.: Haareis – eine seltene winterliche Naturerscheinung,
Schweiz Zeitschrift für Pilzkunde, November 2005, 269–271,
2005.
Wagner, G. and Mätzler, C.: Haareis – Ein seltenes biophysikalis-
ches Phänomen im Winter, Naturwissenschaftliche Rundschau,
62, 117–123, 2009.
Wegener, A.: Haareis auf morschem Holz, Die Naturwis-
senschaften, 6, 598–601, 1918.
www.biogeosciences.net/12/4261/2015/ Biogeosciences, 12, 4261–4273, 2015
