Growth and vitality of epiphytic lichens : II. Modelling of carbon gain using field and laboratory data

Photosynthetic and respiratory CO₂ gas exchange was measured under controlled climate conditions in the laboratory in two epiphytic lichens, Lobaria pulmonaria and Platismatia glauca, with the aim of modelling their net productivity using field microclimate data. For both, the thallus water content (WC) and the light intensity had the greatest impact on photosynthesis. L. pulmonaria had optimum net photosynthesis (NP) at WCs between 75-175% of the thallus dry weight (DW), while P. glauca required a WC of c. 85% for maximal NP without depression at higher WCs. Both species reached light compensation of NP at 5-10 µmol photons m^-2 s^-1 and were saturated at 100-150 µmol photons m^-2 s^-1. Respiratory CO₂ loss corresponded to 35-40% of gross photosynthesis at 85-100% WC and 15° C, in both species. Growth of the two species were followed in transplanted thalli during a 16-month period at two contrasting sites, a forest edge adjacent to a 15 year old clear-cut and within the interior of a mature Picea abies forest. At these sites, the microclimate parameters; light, temperature, relative humidity (RH) and thallus WC were also monitored. Judged from the microclimate data, the lichens were active for 13-19% of the time with thallus WC monitoring, where >60% of the active time occurred in darkness. When photosynthetically active, the edge transplants received a 2-3 times higher light dose and were active for a longer accumulated time compared to the interior transplants. The field microclimate data in conjunction with the laboratory data predicted a 4 times higher DW yield of the edge transplants compared to the interior transplants. However, the DW yield of L. pulmonaria was overestimated at the edge and underestimated for P. glauca in the interior by our model. Possible reasons for these discrepancies and the validity of using laboratory data and microclimate monitoring to predict growth rates of lichens under varying field conditions are discussed.