Learning more about lichens -- nitrogen
nutrition and reindeer raiders
In the gardening column of a UK
national newspaper, a reader recently expressed concern about a grey-green
fungus growing on her Ribes bushes. However,
the reader was only half right. The growth was not a fungus but a lichen; its mis-identification representing the general lack of
knowledge about these remarkable composite organisms. Despite their generally
low profile and lack of public appeal, lichens are extremely important. For
example, they are very sensitive to atmospheric pollution, making them good
bio-indicators and, in some parts of the world, they are key components of the
ecosystem. This aspect is discussed by Kytöviita
and Crittenden at Nottingham,
UK (pp. 1537–1545)
in relation to lichens of the sub-arctic forests, where mat-forming lichens can
make up over 90 % of the ground cover and may contain approx. 20 % of the
ecosystem biomass. How then do lichens obtain and conserve N in these
nutrient-poor ecosystems? To answer this question, the authors conducted
N-feeding experiments and carried out N analysis in relation to growth in Stereocaulon paschale
(N-fixing) and Cladonia stellaris
(non-N-fixing). Feeding of nitrate to the basal regions resulted in
translocation towards the growing upper regions, although the amount translocated was much greater in Stereocaulon
than in Cladonia. By contrast, nitrate applied
to the apical regions was not translocated to the
lower parts. These observations correlate well with the predominance of apical
growth in these lichens and with the decrease in N content along the apical to
basal axis. The authors conclude that a ‘physiologically dependent
translocation… follows a sink--source relationship’. In growth experiments
where thalli were cut to different lengths, N
content, especially at the apex, declined during the growth period, again
reflecting the importance of translocation from the basal regions, which were
of course missing in the cut thalli. Finally,
although N-use efficiency and relative growth rate were greater in the non-N-fixing
species, C. stellaris, its abundance in the
field is limited because reindeer prefer it to S. paschale.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
