Lichenometric dating (lichenometry) and the biology of the lichen genus rhizocarpon:challenges and future directions

Lichenometric dating (lichenometry) involves the use of lichen measurements to estimate the age of exposure of various substrata. Because of low radial growth rates and considerable longevity, species of the crustose lichen genus Rhizocarpon have been the most useful in lichenometry. The primary assumption of lichenometry is that colonization, growth and mortality of Rhizocarpon are similar on surfaces of known and unknown age so that the largest thalli present on the respective faces are of comparable age. This review describes the current state of knowledge regarding the biology of Rhizocarpon and considers two main questions: (1) to what extent does existing knowledge support this assumption; and (2) what further biological observations would be useful both to test its validity and to improve the accuracy of lichenometric dates? A review of the Rhizocarpon literature identified gaps in knowledge regarding early development, the growth rate/size curve, mortality, regeneration, competitive effects, colonization, and succession on rock surfaces. The data suggest that these processes may not be comparable on different rock surfaces, especially in regions where growth rates and thallus turnover are high. In addition, several variables could differ between rock surfaces and influence maximum thallus size, including rate and timing of colonization, radial growth rates, environmental differences, thallus fusion, allelopathy, thallus mortality, colonization and competition. Comparative measurements of these variables on surfaces of known and unknown age may help to determine whether the basic assumptions of lichenometry are valid. Ultimately, it may be possible to take these differences into account when interpreting estimated dates

Nucleation in tropical ecological restoration A nucleação na restauração ecológica de ecossistemas tropicais

Ecological theories of facilitation and nucleation are proposed as a basis for environmental restoration in tropical ecosystems. The main goal of this paper is to present restoration techniques based on the concept of nucleation, in which small nuclei of vegetation are established within a degraded land. The nucleation techniques (artificial shelters for animals, planting of herbaceous shrub life forms, soil and seed bank translocation, seed rain translocation, soil and seed rain translocation's seedling set, artificial perches, planting of native trees in groups, and ecological stepping-stones with functional groups) promote the landscape connectivity on two flows: inward: receiver connectivity and outward: donor connectivity. The nuclei development represents an alternative for restoration by prioritizing the natural processes of succession. This methodology appears to take long to generate vegetation corresponding to tropical climates, but is fundamental in the formation of communities capable of acting, in the future, as a new functional nuclei within the current fragmented landscape. This strategy also encourages greater integration between the theories and projects of ecological restoration for the development of human resources and to benefit the restoration practitioner.<br>As teorias ecológicas da facilitação e nucleação são propostas como base para a restauração ambiental de ecossistemas tropicais. Nesse "Ponto de Vista" apresentam-se técnicas de restauração fundamentadas no conceito de nucleação, onde pequenos núcleos de vegetação são implantados em uma área degradada. As técnicas de nucleação (abrigos artificiais para animais, plantio de espécies herbáceo-arbustivas, transposição de solo e banco de sementes, transposição de chuva de sementes, blocos de mudas procedentes de transposição de solo e chuva de sementes, poleiros artificiais, plantio de árvores nativas em grupos de Anderson, e trampolins ecológicos com grupos funcionais) promovem a conectividade da paisagem sob dois fluxos: interno: conectividade recebedora; e externo: conectividade doadora. O desenvolvimento dos núcleos representa uma alternativa de restauração que prioriza os processos de sucessão natural. A restauração através das técnicas de nucleação pode parecer muito lenta para atingir uma vegetação que corresponda ao clima tropical, porém representa uma base para a constituição de comunidades que, no futuro, poderão potencialmente atuar como novos núcleos funcionais dentro da atual paisagem fragmentada. Encoraja-se maior integração entre teorias ecológicas e projetos de restauração para desenvolvimento de recursos humanos e formação de restauradores ecológicos