Translocation of 15N indicates nitrogen recycling in the mat-forming lichen Cladonia portentosa

Nitrogen translocation was measured in Cladonia portentosa during 2 yr growth in Scottish heathland. Translocation was predicted to occur if N is resorbed from senescent basal tissue and recycled within the thallus. N-15 was introduced into either the lower (TU thalli) or upper (TD thalli) 25 mm of 50-mm-long thalli as N-15-NH4+, N-15-NO3- or N-15-glycine. Labelled thalli were placed within intact lichen cushions, either upright (TU) or inverted (TD). Vertical distribution of label was quantified immediately following labelling and after 1 and 2 yr. Independently of the form of introduced label, N-15 migrated upwards in TU thalli, with new growth being a strong sink. Sink regions for N-15 during year 1 (including new growth) became sources of N-15 translocated to new growth in year 2. Upward migration into inverted bases was minimal in TD thalli, but was again marked in new growth that developed from inverted apices. Relocation of N to regions of growth could facilitate internal N recycling, a process postulated to explain the ecological success of mat-forming lichens.