Some notes on the microclimate associated with Thamnolia
vermicularis var. subuliformis from Snowdon, North Wales
A. PENTECOST and J. H. WILLIAMS
Pentecost, A. & Williams, J. H. 2002. Some notes on the microclimate
associated with Thamnolia vermicularis var. subuliformis from Snowdon,
North Wales. Graphis Scripta 13: 59–63. Stockholm. ISSN 0901-7593.
During 1999 the estimated surface temperature of Thamnolia thalli ranged
from -7.7 to 24.5° C (3-day means). Precipitation occurred on 251 days (69
% of the year) and thalli were snow-covered for 90 days. The Snowdon
(Wales) population is small and declining, perhaps as a result of acid
precipitation which may be exacerbated by summer sheep grazing. Growth
rate was monitored over several years giving annual branch elongation rates
between 0.94 and 5.35 mm.
Allan Pentecost, Department of Life Sciences, King's College London,
Franklin-Wilkins Building, 150 Stamford Street, London SE1 9 NN.
J. H. Williams, School of Agricultural and Forest Sciences, University of
Wales, Bangor, Gwynedd LL57 2UW.
The lichen Thamnolia vermicularis var.
subuliformis is one of about 40 arctic-alpine
species which occur in North Wales
(Pentecost 1987). It is currently known from
three welsh mountains and was first recorded
from Cadair Idris by T. Salwey (1844). It was
first recorded from Snowdon by W. Watson in
August 1915 ’at 3,200 feet’ (BMNH).
Snowdon was visited regularly by
lichenologists in the 19th Century, so the late
recording is a little surprising, suggesting that
as now, it was not a common plant on the
mountain. It is likely to be close to the edge of
its range, as it becomes more frequent further
north on mountains of similar altitude and
does not occur on the mountains of South
The species is currently restricted on
Snowdon to a few isolated patches on Crib-y-
Ddysgl (Nat grid. Ref. 23/60865508, alt. 1015
m) and the rocks above Clogwyn
(23/60085544, alt. 820 m). Since May 1972
when the senior author first observed it on
Snowdon, it appears to have undergone a
decline as it was fairly easy to find at that
time. In Britain the centre of distribution for
Thamnolia is in the Scottish Highlands where
it is widespread, even approaching sea level in
the north (Gilbert 2000). It also occurs on a
few mountains of the English Lake District.
The Cadair Idris site is currently its most
southerly location in Britain (Seaward &
Little is known of the ecology of
Thamnolia vermicularis. It occurs as two
varieties, the var. vermicularis containing
thamnolic acid and the var. subuliformis with
baeomycesic and squamatic acids. Only the
latter is known from the British Isles but both
varieties are known from the adjacent
continent. The varieties are widespread in the
arctic-alpine heaths of Scandanavia where
they are found growing on soil or directly on
limestones (Sheard 1977; Santesson 1993).
Thamnolia is noteworthy among the
macrolichens in having no known sexual
stages, and lacking both soredia and isidia,
presumably reproducing largely by
60 Allan Pentecost & J. H. Williams GRAPHIS SCRIPTA 13 (2002)
fragmentation. Conidia have been described
by Ozenda & Clauzade (1970, p.759) from
France, but are not apparently reported from
the British Isles.
In this paper we have taken advantage of
the fact that Snowdon summit has a remotely
operating continuous weather station
providing extensive data on conditions at the
summit. The station is in close proximity to
the Thamnolia sites permitting a detailed
analysis of conditions over long periods of
time. The summit is also easy to access with
good paths providing year-round monitoring
of the site.
Small patches of Thamnolia were investigated
at the Crib-y-Ddysgyl site at altitudes of 975-
1015 m. Thalli were dispersed among lowgrowing
vegetation or grew directly on soil.
Monitoring began in August 1994 initially on
2 patches with a third added in 1999. Patches
were photographed in colour with a Nikon FT
camera from a set position on 6 occasions up
to April 2000. Close-up photographs were
taken from a fixed position with an adjacent
mm scale and measurements compared
manually. Digital methods were not used due
to the sparsity of cover and clarity of the
pictures obtained. At each site, thallus surfacetemperature
and air temperature was measured
with an Edale type D515 thermistor, the latter
at 1m height in shade. To obtain seasonal
surface temperature estimates a proxy method
was employed by regressing a set of 20 air
temperature measurements onto thallus
temperature. The resulting linear regression
equation (Thallus temp. = 1.51 Air temp. -
0.32) was found to be significant by ANOVA
GRAPHIS SCRIPTA 13 (2002) Thamnolia vermicularis from Snowdon 61
Figure 1. Climatic data for the Snowdon Thamnolia sites during 1999: Surface temperature
estimates (3-day means) of thalli.
have been covered by snow for 90 days (25 %
of the year).
Significant precipitation, defined here as
>=1mm rain per day, fell on 190 days with
less than 1mm on a further 61 days (Figure 2).
Total precipitation for 1999 at Snowdon
summit was 2636 mm. The longest period
without recorded rainfall was 8 days (24-31
July) but there was an instrument breakdown
during August (Figure 2) where only monthly
data were available. Thamnolia was wetted by
precipitation for at least 251 days (69 % of the
year). Kershaw (1975) found a surface
temperature difference of up to 7°C between
wet and dry thalli of Thamnolia, dry thalli
being subjected to higher temperatures. Most
of the surface measurements conducted on
Snowdon were on wet thalli, but where dry
thalli were measured such large differences
were not observed. This was probably due to
exposure to the prevailing westerly winds.
There are few data with which to compare
the growth of Thamnolia, but Lange et al.
(1995) found that under high light intensities,
the temperature for optimal net photosynthesis
was 13-22°C, which coincides with summer
surface values on Snowdon. This suggests that
Thamnolia growth occurs mainly in the
summer. The decline of Thamnolia on
Snowdon is cause for concern. Acid
precipitation from vehicle emissions and
power stations may be responsible since
Wales as a whole is affected by this form of
pollution (Edwards et al. 1990) and the soils
upon which the species grow are acidic and
probably poorly buffered.
The colonies growing on Cadair Idris
appear to be thriving, occurring in
considerable areas (J. Grasse, pers. comm.).
Deposition rates of sulphur- and nitrogenbased
acidity were estimated between 1984
and 1995. The 20 km × 20 km Snowdon
square showed markedly higher rates of wet,
62 Allan Pentecost & J. H. Williams GRAPHIS SCRIPTA 13 (2002)
Figure 2. Climatic data for the Snowdon Thamnolia sites during 1999 Precipitation (3-day
totals) at the Snowdon summit weather station.
dry and total acid deposition than did the 20
km × 20 km Cadair Idris square (Stevens et al.
1997). Another possible explanation is grazing
pressure. During summer, sheep graze the low
turf of Crib y Ddysgl and are bound to disrupt
thalli by trampling. There was clear evidence
of grazing adjacent to the sites so some
damage appears inevitable, but it is not clear
whether the intensity of grazing has varied
historically, or how trampling affects reestablishment
of the lichen fragments. Cooper
et al. (2001) noted damage to High Arctic
lichens and its amelioration by sustained
periods of precipitation. However at our
Snowdon sites there was no evidence of
regeneration of damaged thalli and the
population as a whole appears to be declining
rapidly. Another possible cause of these
changes is the global increase in temperature
over the past 30 years. Since Cadair Idris and
Snowdon receive almost identical weather, but
only Snowdon has suffered a decline in
Thamnolia, such changes are unlikely to be
the cause of decline.
We thank Dr. M. O. Hill for some of the
bryophyte identifications and Jack Grasse for
information on the distribution of Thamnolia
on Cadair Idris. The senior author is grateful
for a small travel grant from the British
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GRAPHIS SCRIPTA 13 (2002) Thamnolia vermicularis from Snowdon 63
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