Effect of simulated reindeer grazing on the re-growth capacity of mat-forming lichens

Grazing of mat-forming Cladina-lichens by reindeer (Rangifer tarandus) causes thinning and fragmentation of the carpet. Little is known about the regeneration rate of the lichen mat and how differences in quality of the remaining lichen fragments affect this process. We hypothesized that recovery rate and productivity of mat-forming lichens might be dependent on the wet active time as well as the algal (photobiont) density of the remaining lichens. This was experimentally tested for Cladina stellaris, measuring growth and microclimatic conditions during 4 months in lichen cushions with varying height, density and chlorophyll concentration. The treatments mimicked winter grazing by reindeer, resulting in a mosaic of more or less grazed and trampled patches. Absolute growth was highest (62 g m(-2)) where the photobiont-rich upper part of the Lichen thalli remained and the mat had been thinned to 75% coverage and lowest (3 g m(-2)) where only the lower parts of the thalli remained and the mat had 25% coverage. The relative growth was highest (2.2 mg g(-1) DW day(-1)) in upper thallus parts that had been thinned to 25% coverage and lowest in the control treatment and the two treatments with lower thallus parts (0.1-0.2 mg g(-1) DW day(-1)). The density or thickness of the lichen mat did not affect its wet active time, white productivity increased both with the cushions' photobiont (chlorophyll) density and the irradiance received when the lichens were wet and active. The productivity was tower in the intact cushions compared to those with lower height and density with equal amounts of chlorophyll. The regeneration capacity of mat-forming lichens wilt hence be reduced in dense mats and when reindeer grazing has preferentially removed the photobiont-rich upper half of the thallus. (c) 2005 Gesettschaft fur Okologie. Published by Elsevier GmbH. All rights reserved.