[Latest update: October 31st, 2000]

The Chi-Square-based test on association of species helps to analyse and illustrate the complex,
fragmented communities of foliicolous lichens in the tropical rain forest [63].

[Numbers in brackets relate to relevant publications; underlined phrases without links indicate pages under construction].

The tropical lowland rainforest can be divided into three major microhabitats: (1) The shady understory; (2) natural light gaps caused by fallen trees; (3) the outer canopy. Foliicolous lichens are highly susceptible to the light regime and, since these three compartments are characterized by different light intensities, species assemblies differ markedly between each microhabitat. The lichens typical of each compartment form distinct associations which can be detected by means of a Chi-Square-based test on association of species [6, 56, 61, 63]. These associations or communities share particular features regarding their systematic affinities and their physiognomy [56, 63]:

• The shady understory association is principally composed of the lichen families Arthoniaceae, Opegraphaceae, Strigulaceae (supracuticular species), Trichotheliaceae and Pilocarpaceae. The predominant phycobiont is the green alga Phycopeltis (Trentepohliaceae), and the thalli are usually thin and reduced, with a smooth surface or provided or with small, hydrophobic verrucae. Ascospores are mostly small and produced in high numbers, while asexual reproduction is rare.
  
• The natural light gap association is dominated by the families Strigulaceae (subcuticular species), Gomphillaceae and Ectolechiaceae. The common phycobiont is Trebouxia (Chlorococcaceae), and the thalli are usually well-developed and either encrusted with crystals and then with an irregular surface, or dispersed and whitish. Ascospores are often large and muriform and produced in small numbers, while asexual reproduction is abundant and often highly specialized.
  
• The canopy association is mainly characterized by the family Asterothyriaceae and a few Strigulaceae (subcuticular species), Gomphillaceae and Ectolechiaceae. The phycobiont is mostly Trebouxia (Chlorococcaceae), and the thalli are often dispersed into small, inflated patches encrusted with crystals and provided with a cortex. Apothecia are either immersed in the thallus or provided with dark pigments. Large, thick-walled ascospores are not rare.

While phorophyte specifities are generally absent, some subassociations can sometimes be observed comparing dicot and palm leaves. Palm leaves in the understory and in light gaps support high frequencies of certain Arthoniaceae, Gomphillaceae and in particular Pilocarpaceae, while dicot leaves in light gaps or the canopy are often characterized by almost pure communities of subcuticular Strigula species [6, 56, 63].

Many particular features of the different communities can be interpreted as ecomorphological adaptations. The reduced thallus morphology of shady understory species, which is only possible with a filamentous phycobiont such as Phycopeltis, is an adaptation towards reduced light conditions. On the other hand, hazardous effects of too high irradiation in light gap or canopy species can be mitigated by the whitish, light-reflecting thallus colour, pronounced by the encrustation of crystals or the formation of a cortex. Equally, dark pigments protect the reproductive tissue from too much UV-radiation.

The three main associations characterized above can also be detected when comparing different habitats on a higher scale, e.g. primary forest with secondary vegetation. In the latter, the shady understory association is usually absent while the light gap and canopy associations become dominant. This indicates that, at different scales, community patterns are fractal rather than hierarchical [56]. Within the forest understory, the structure of the shady understory and the light gap association corresponds to the "gap"-dynamics. Light gap associations depend on the presence of gaps caused by fallen trees, and the frequence and abundance of light gaps influence the probability that a particular lichen species is found in a given light gap. Occasional light gaps result in small probability values for particular species, since their dispersal is more difficult, and produce a rather loose association, while abundant light gaps give compact associations, since the species are easily dispersed between different gaps. Thus, the structure of the light gap association can be used to indicate the degree of "gap" dynamics in a particular forest. In fact, forest with high dynamics show well-developed light gap associations [63], while in those with low dynamics, these associations are reduced or completely dissolved [61].

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