- In recent years, our ecological knowledge of tropical dry forests has increased
dramatically. However, whole components of the ecosystem, like lichenized fungi,
remain mostly unknown. Crustose lichens in these forests are so abundant, that they are
responsible for the characteristic appearance of a “white bark forest” during the dry
season. The aim of this dissertation is to incorporate lichens into our understanding of the
functioning of tropical dry forests. Prior to this work, lichens in this ecosystem were not
considered at all in ecological studies and only in recent years we started having a better
understanding of what species are present. The thesis is divided in two sections: Chapters
2 and 3 deal with particular cases of lichen systematics, while chapters 4, 5 and 6 deal
with ecological studies of lichens at the ecosystem level and how they interact with other
organisms. All the chapters revolve around lichens of the tropical dry forest of the
Chamela-Cuixmala Biosphere Reserve in Mexico.
In Chapter 2, new collections of the supposedly extinct and doubtfully lichenized
fungus genus Polypyrenula were found. Given that anatomical studies of the fresh
collections were not congruent with its current systematic position, a molecular approach
was followed using the genes ITS, mtSSU and nuLSU. Our molecular analysis
demonstrated that the monospecific genus, previously included in the family
Pyrenulaceae, belongs instead in the Trypetheliaceae, but outside the core genera in the
family. We extend the distribution of Polypyrenula to South America, provided new
information on its phorophytic associations, corroborated that it is a facultatively
lichenized fungus, and reinstated the name Polypyrenula sexlocularis as the correct name
for the species.
In Chapter 3, one new genus and two new species of lichens in the family
Graphidaceae were described based on morphological, chemical and molecular data of
the genes ITS, mtSSU, and nuLSU. The new genus Jocatoa in the subfamily
Graphidoideae is described to accommodate the orphan species Medusulina texana.
While the new species Gymnographopsis corticola and Redonographa parvispora are
described in the subfamily Redonographoideae, together being the only two known
corticolous species in the subfamily. A phylogenetic analysis including all the genera in
the family Graphidaceae, with available sequences, is presented to accommodate the new
genus and to validate for the first time the position of Gymnographopsis. Diagnostic
anatomical and ecological characters are discussed for Redonographoideae.
Gymnographopsis is newly reported to the Northern Hemisphere.
In Chapter 4, we estimate the total lichen biomass at the ecosystem level.
Calculations were based on the bark area of trees, density of different sizes of trees per
hectare, dry mass of lichens per unit area, and the percentage of lichen cover per tree. The
epiphytic lichen biomass in the forest was 1.30 to 1.92 t/ha, of which 180 kg per hectare
were located on the lowest 2.5 m of the main trunk of the trees. Lichen biomass
represents 59 percent of the foliar biomass in the system, suggesting a significant
ecological role that so far is unexplored. To our knowledge, this is the first time that a
lichen biomass estimate is provided for an ecosystem in which crustose lichens are the
dominant growth form.
In Chapter 5, the lichen consumption component of herbivory in the tropical dry
forest was analyzed and compared to the leaf herbivory component. Lichen herbivory
rates were calculated using high definition photographs of permanent microplots across a
four-year period. The annual rate of lichen consumption was 11.5%, with no significant
difference between years, even in the presence of catastrophic events like the category 4
hurricane Patricia. Lichen biomass annual consumption per hectare represents 28.5% of
the biomass lost to total herbivory when considering leaf folivory (chewing) and
lichenivory together. The results show that lichen consumption is an established and
regular process in the forest dynamics of the tropical dry forest. A discussion on the
animals responsible for lichen herbivory is presented.
In Chapter 6, caterpillars of a moth species of the family Psychidae were
discovered living inside mobile bags made from silk and completely covered with small
pieces of lichens. The lichens used as construction material for the caterpillar bags were
identified with molecular techniques and compared to a newly generated database of
genetic barcodes for the lichens in the area. Of the approximately 300 lichen species
expected to occur in the area, only five of them were used by the caterpillars. There was a
strong selectivity for micro-foliose lichens of the family Physciaceae, even though they
represent a small fraction of the mostly crustose lichens present in the forest.
In this dissertation new aspects in the study of tropical dry forest were revealed.
Lichens that were previously ignored were shown to be diverse, abundant and key
components in the dynamics of the ecosystem. Lichens revealed levels of biomass
comparable with the biomass of leaves in the forest and were consumed at similar rates.
Preliminary data from this dissertation points towards a major component of the trophic
web of the ecosystem that is sustained by lichens. Of particular importance is the
potential of lichens to maintain the functionality of the ecosystem during the extended
dry seasons. We suggest that the crustose lichen component should not be underestimated
a priori in ecological studies, especially in areas with significant lichen cover.