The carbon economy of lichens

Growth and survival of photosynthetic carbon autotrophs, such as lichens, are primarily limited by their photosynthetic carbon assimilation minus carbon dioxide (CO2) losses related to growth and maintenance respiration. The absolute, as well as the relative, rates of these two processes will hence determine their capacity to grow. In lichens, both photosynthesis (P) and respiration (R) are strongly constrained by prevailing environmental conditions, particularly water and light. There is also a variation in inherent P and R capacities among species and individuals. Significant progress has been made during the last two decades in understanding how such variations in external conditions and internal capacities affect lichen growth (Boucher and Nash 1990a; Muir et al. 1997; Sundberg et al. 1997, 2001; Hyvärinen and Crittenden 1998b; Palmqvist and Sundberg 2000; Hilmo and Holien 2002; Dahlman and Palmqvist 2003; Hyvärinen et al. 2003; Gaio-Oliveira et al. 2004a, 2006; Gauslaa 2006; Gauslaa et al. 2006b; Palmqvist and Dahlman 2006), including re-establishment of vegetative propagules (Hilmo and Sastad 2001; Hilmo and Ott 2002). This progress has been driven by the development of transplantation techniques in combination with more mechanistically oriented studies, knowledge that has further been used to formulate both conceptual (Palmqvist 2000) and more mechanistically oriented models (Link et al. 1985; Palmqvist and Sundberg 2000; Dahlman and Palmqvist 2003). Such models are useful for the direction of future research towards more explicit hypothesis testing, and could also be adopted to predict how environmental changes may affect particular lichen species, or compare species' abilities to utilize and acclimate to varying environmental conditions.