Lichens and higher plants on stone: a review - AseanBiodiversity.info

Fig. 1. Block diagram of the fate of a building in relation to human <strong>and</strong>
environmental intervention.
several towns in the south of France, Italy <strong>and</strong> Tunisia as far
as vascular <strong>plants</strong> are concerned (Lisci <strong>and</strong> Pacini, 1993a,
b). Since these sites range around the Mediterranean, they
re ect the situation of much of the Mediterranean Basin
<strong>and</strong> the monuments of forty centuries of history.
Since lichens <strong>and</strong> vascular <strong>plants</strong> establish in di erent
ways, cause di erent types of damage <strong>and</strong> require di erent
methods of control, they are dealt with separately in the
pages that follow.
2. <strong>Lichens</strong>
2.1. Types of lichens: structure <strong>and</strong> distribution
<strong>Lichens</strong> are fungi (Ascomycetes or seldom Basidiomycetes)
that live in symbiosis with photosynthesizing
organisms (cyanobacteria or green algae) (Hale, 1974).
The partners of the symbiosis, the fungus (mycobiont) <strong>and</strong>
the alga (phycobiont), give rise to a simple structure known
as a thallus. The lichen thallus grows where the alga <strong>and</strong>
fungus, alone, could not survive. This is why lichens colonize
a large variety of substrates: trees (epiphytes), the
ground (terricolous lichens), stone (epilithic lichens) <strong>and</strong>
even glass.
On stone objects, the pH of the substrate performs the
rst selection of lichen ora. Calcicolous species develop
on neutral <strong>and</strong> alkaline substrates <strong>and</strong> silicicolous species on
acid substrates. The forms of growth of thalli that colonize
stone (Fig. 2) may be:
• crust-like: the thallus adheres closely to the surface <strong>and</strong>
penetrates it, forming a kind of crust;
• foliose: the thallus penetrates the substrate only with its
rhizines (thread-like anchorage devices) <strong>and</strong> can easily
be removed from the surface of the stone;
• fruticose: the thallus develops in three dimensions, taking
on a hanging, rami ed form, generally anchored to the
substrate by a type of button;
• endolithic: these only occur on calcareous stone: the thallus
is completely immersed in the substrate <strong>and</strong> is di cult
to distinguish, being white in color. It is generally noticed
when it reproduces because the fruiting bodies emerge
from the stone, leaving behind small pits.
Lichen reproduction occurs by germination of spores or
multiplication of the cells of the soredia <strong>and</strong> isidia. Spores
arising from fruiting bodies (Fig. 3c) are formed by sexual
reproduction of the fungal partner only. Dispersed in the
environment, they reconstitute a symbiosis if they encounter
a suitable phycobiont. The soredia, balls of fungal laments
around an algal cell (Fig. 3a), <strong>and</strong> isidia, appendages of the
upper cortex of the thallus that envelop several algae (Fig.
3b), are structures of vegetative reproduction in which both
partners of the symbiosis take part.
The reproductive structures are dispersed in the atmosphere
by wind or transported by birds <strong>and</strong> insects. If they
fall into cracks, pores or cavities that retain water, new
growthmay arise (Garty, 1992).
It is not a coincidence that di erent types of lichens
grow on monuments. Eachspecies can only live in a de -
nite range of pH, humidity, luminosity <strong>and</strong> nitrogen supply;
some species can only live in a very narrow range of values
(stenoic) whereas other species grow over a wider interval
of conditions (euroic).
Wirth (1980) devised indices of pH, hygrophytism,
nitrophytism <strong>and</strong> photophytism on the basis of detailed
knowledge of the ecology of the various species <strong>and</strong> their
relationships. These indices can be used to characterize
the pH range of substrates, their humidity, solar radiation
<strong>and</strong> eutrophication on the basis of the species identi ed
(Monte, 1991a). This information is useful for establishing
existing gradients of humidity values <strong>and</strong> solar radiation so
that alterations can be interpreted (Monte, 1991b; Corball,
et al., 2001).
<strong>Lichens</strong> behave as natural sensors of atmospheric pollution
because the symbiosis between fungus <strong>and</strong> alga has its
weak points. Highpollution, particularly by sulfur dioxide,
damages the lichen thallus, rst leading to retarded growth
<strong>and</strong> then to death. The number of species tends to decrease
drastically from the periphery to the center of urbanized areas,
with a decrease in the number of species <strong>and</strong> of the
surface area colonized (Seaward, 1976).