Growth in high light conditions might be another environmental factor acting to retain or effect the operation of a CCM on land. This factor has been noted in a study by Smith et al (1998) whom investigated a hypothesis that cyanobiont lichens (with CCM) growing under contrasting microhabitats show inter-specific and intra-specific variation in photosynthetic responses, which could be correlated with the variations in the degree of expression of the biophysical CCM. It was found that populations of Peltigera membranaecea, from exposed crags showed more pronounced CCM activity (by the accumulation of a larger Ci pool) than populations from shaded deciduous oak woodland.
A possible explanation for these observations were that an active CCM will effectively reduce the light-utilization efficiency of photosynthesis (Palmqvist et al 1994c), therefore, increased CCM activity as a strategy of optimising the supply of CO2 to Rubisco, might be most profitable in environments where CO2 is a more limiting resource than light, i.e exposed habitats where lichens are subject to high PAR.
An investigation into the way liverworts (with increasing morphological specialisation, including the hornwort Phaeoecros laevis) deal with excess of light has so far not been in studied. Moreover how the Anthocerotae CCM responses to increased light intensity needs to be evaluated.