Temperate rainforest lichens in New Zealand: Light response of photosynthesis

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Abstract
The photosynthetic response to photosynthetically active photon flux density (PPFD) was measured in the field for nine New Zealand rain forest lichen species spanning environments from the deep shade within the forest through the forest margin to outside, open ground sites. Light compensation (PPFDcomp) and light saturation (PPFDsat) were much higher for the species of the open sites (over 50 and 500 μmol/m2.s, respectively). Lichens at forest margin or within the forest had PPFDsat of 64.3-29, and 61–162, μmol/m2.s, respectively, but showed no apparent adaptive trends for these parameters. PPFD response of photosynthesis was measured at several thallus water contents (WC) for one species, Pseudocyphellaria coronata, which showed severe depression of photosynthesis at high WC because of increased diffusion resistances, at all PPFD above compensation. PPFDsat was also depressed but not dark respiration, PPFDcomp, or quantum efficiency. Green algal and cyanobacterial lichens showed no relationship between nitrogen and chlorophyll contents but green algal species were always ≤0.7% nitrogen. A third group, cephalodiate (nitrogen fixing) lichens, had nitrogen contents of 1.5–4.4% dry weight with a clear, positive relationship with chlorophyll content. Chlorophyll and nitrogen contents both increased at more shaded sites within the forest. It is suggested that the possession of cephalodia providing a nitrogen source is advantageous within these evergreen forests by allowing a greater contribution to be made to photosynthetic machinery. The PPFDsat of forest lichens seems to be too high for the prevailing PPFD environment and the lichens would rarely be able to photosynthesise at optimal PPFD. It is suggested that this is a protection against transient high light in sunflecks.