Adverse Effects of Epiphytic Crustose Lichens upon Stem Photosynthesis and Chlorophyll of Populus tremula L.

Dry cork layer (phellem) in stems of Populus tremula transmitted 35–55 percent of incident irradiation, depending upon moisture content. A cover of crustose Lecanora lichens reduced transmission through phellem to 10 percent or less of incident irradiation. The bark contains photosynthetically active cells. Apparent quantum yield for photosynthetic O 2 ‐evolution was 0.017 in bark covered with dry Lecanora compared with 0.070 in naked bark. The capacity for gross photosynthesis in high light (1090 μmol photons m −2 s −1 ) was reduced by 50 percent in Lecanora‐ covered bark. Lecanora did not reduce the ratio between variable and maximal chlorophyll a fluorescence ( F v /F m ). Chlorophyll content per unit area was similar in leaves and naked bark of Populus tremula . The chlorophyll content in the bark decreased with increasing chlorophyll content in Lecanora . Chlorophyll a/b ratio was 2.5 in the bark compared with 4.0 in leaves and in Lecanora , and the ratio decreased down the stems. The a/b ratio was 2.3 in Lecanora covered bark compared with 2.6 in naked bark. The changes in bark photosynthesis below a Lecanora crust were probably due to acclimation of bark photosynthesis to shade, since the lichen acids in the measured lichens neither suppressed photosynthetic O 2 ‐evolution nor changed the F v /F m in bark disks.