The extent to which biotic factors (competition, predation/disease, longevity) regulate lichen community development can be addressed by considering a number of general trends expected in higher plant successions and searching for supporting evidence from lichen studies. Four of the most frequently observed (or predicted) trends during succession are that: (1) superior competitors replace poor competitors; (2) ecologically specialized species replace generalists; (3) chemically well-defended species replace poorly-defended species; (4) long-lived species replace ephemeral species. Available evidence suggests that, for many lichen communities, competitive exclusion rarely occurs once thalli are established. This is especially true for communities that develop on the most stable habitats. An absence of competitive exclusion suggests that lichen successions are driven more by additions of colonists than by species replacements, and replacement trends observed in higher plant successions are therefore observed less frequently inlichen successions.

After a brief historical survey of the developments of vegetation science in general, and of lichen community studies in particular, this paper presents critical comments on some of the main methodological problems in the study of lichen vegetation, with particular regard to the phytosociological approach. It is suggested that in the description and classification of lichen communities lichenologists should avoid the inflation of community names that characterize the present state of the phytosociology of phanerogamic vegetation. This can be accomplished on the basis of a sound methodological approach, where the use of numerical methods can play a major role.

In order to obtain a complete picture of the factors determining spatial pattern in a saxicolous lichen community (or any other plant community) it is important to use a range of analytical techniques at a variety of scales. The Jonas Rockslide study is an example of this, and three analyses from this study are discussed here. Each scale gives a different perspective on the processes acting in the community. The largest scale shows that lichen distributions are correlated with attributes of whole rockfaces such as slope and aspect. The second analysis examines pattern at the scale of 10 cm to 20 cm grid points and shows that within a rockface are many micro-habitats, reflected in the pattern of occurrence of different lichen species at this scale. The smallest scale of analysis is a ‘nearest-neighbour’ analysis, which indicates that lichens sample their environment differently and have different ecological strategies. These processes ultimately act together to give the observed pattern in the community.

The distribution of cryptogams and phanerogams and the microclimate of different habitats have been studied. It is shown that a forest community of phanerogams includes an assemblage of different microhabitats inhabited by different lichens and mosses. The distribution of lichens and mosses is governed by factors that are often of little importance to higher plants.

A new method is described that records the periods of photosynthetic and respiratory activity in lichen thalli in situ by measuring the actinic chlorophyll fluorescence response of the photobiont. The technique is based on a pulse amplitude modulation fluorometer that was specially developed for continuous and unattended measurements in the Antarctic. In Livingston Island, South Shetland Islands, the periods of metabolic activity of Usnea antarctica were recorded simultaneously with the microclimatic parameter in its natural environment. In one series of diurnal measurements the fluorescence response signal was treated as a qualitative on/off signal for photosynthetic activity. In a parallel series the diurnal courses of net photosynthesis and dark respiration of rain-moistened thalli of U. antarctica were measured by means of a CO2/H2O porometer together with gravimetric water content determination. Both methods showed marked coincidence from low water content to water saturation of the thalli. In the laboratory both the actinic fluorescence response and the photosynthetic rate as a function of thallus water content were investigated in Caloplaca regalis. In the laboratory experiments the CO2 exchange cuvette contained a balance for gravimetric water content control as well as the glass-fibre optics of the fluorescence analysis equipment. The experiments revealed a quantitative relationship between actinic fluorescence and photosynthetic rate within the range of optimum to limiting low thallus water content. The method described is suitable for measurements of lichen activity in field microclimate studies and should allow continuous unattended year-round measurements of lichen activity, which are needed in remote regions such as Antarctica.

Temperate rainforests are a poorly researched habitat with respect to lichen ecophysiology in comparison to desert and polar regions. The evergreen, broadleaf forests provide a dim, moist environment that is relatively stable throughout the year. Lichens are abundant in both quantity and species diversity with the large foliose genera Sticta and Pseudocyphellaria normally being dominant, visually and in terms of biomass. These lichens exhibit a great diversity of both form and habitat range. Physiological and morphological adaptation has also been demonstrated. Pseudocyphellaria dissimilis shows changes in thallus water storage capacity with evaporative demand and is also highly shade-adapted. The species has the lowest light saturation and compensation values for photosynthesis yet known for lichens (20 and 1-μmol m−2s−1, PAR, respectively). Unexpectedly it is also highly desiccation-sensitive with some thalli being killed after only 20 h exposure to 15% relative humidity. Photobiont versatility is also a feature of these genera. Photosymbiodemes occur, i.e. a single thallus containing both green algal and cyanobacterial sectors. Because the different sectors have the same fungal partner and grow in the same habitat, it is possible to investigate whether particular physiological traits are photobiont determined. The ability to recover photosynthetic activity in humid air is confined to thalli with green algal photobionts whilst the inability of thalli containing cyanobacterial photobionts to tolerate high light stress may be related to their lack of a protective xanthophyll cycle.

Lichenophagous larvae of a bagworm moth species, associated with endolithic lichens occur in the Golden Gate Highlands National Park, South Africa. Endolithic lichens weather Clarens sandstone in the park by dissolving the cementing material of the sandstone. Bagworm larvae use such loosened quartz crystals to construct the bags in which they live. By erosion of the weathered sandstone, the bagworm larvae contribute to pedogenesis in the park at an estimated rate of 4.4 kilograms per hectare per year. In addition, bagworm larvae utilize endolithic lichens as a food source and contribute to dispersal of the lichens. The larvae further contribute an estimated 200 g of organic matter per hectare per year to the park and partake in the cycling of elements in the micro-ecosystem.

A number of physiological processes relevant to the role of lichens in mineral cycling are discussed. Consideration is given to the cellular location of positively-charged cations, showing (a) the benefits of quantifying intracellular elements for the interpretation of toxic metal stress, and (b) how distribution patterns of physiologically essential elements may be altered by desiccation and rehydration under field and laboratory conditions. The quantitative significance of these dynamic processes associated with metal uptake and loss requires verification under field conditions. A modified sequential elution procedure is proposed that enables quantification of insoluble paniculate mineral matter (acquired by wet and dry deposition) in addition to soluble elements in intercellular, extracellular-exchangeable and intracellular sites.

The nutrient contribution of lichens as litterfall in forests is discussed for a number of different ecosystems and it is hypothesized that lichens are important in capturing nutrients from wet deposition, occult precipitation, sedimentation, impaction and gaseous uptake. Most nutrients captured by these processes represent new nutrient inputs that would otherwise not be intercepted by the ecosystem. Part of these nutrients will be incorporated into lichen biomass and only become available upon death and decomposition, but a portion will be leached by precipitation and become deposited on the soil surface. Although quantifying nutrient sources, fluxes and pool sizes is a potentially complex task, we describe a simplified approach for determining whether lichens significantly affect the mineral cycling of a forest. Preliminary results for an oak woodland in California document that epiphytic lichens may reduce throughfall and alter throughfall chemistry.

Available evidence indicates that basal regions of well-developed lichen mats are composed largely of dead or senescent thallus material. Marked vertical gradients in concentrations of N, P and K in thalli of mat lichens, together with results of N budget studies in Cladonia stellaris and Stereocaulon paschale, suggest that these key elements may be recycled internally to fund new growth at the apices. It is argued that nutrient recycling will increase rates of both apical growth and basal necromass production and that under steady-state conditions this will result in deeper mats and facilitate dominance sensu Grime; thus accumulation of necromass may be an ecological advantage in mat-forming lichens. Basal necromass of the N2-fixing S. paschale contains more N than necromass of the non-N2-fixing C. stellaris; this may be due in part to a greater allocation of N to irrecoverable fungal cell wall components in S. paschale and may underlie a greater requirement for N in this mycobiont.