Ecological Significance of Necromass Production In Mat-Forming Lichens

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Abstract
Available evidence indicates that basal regions of well-developed lichen mats are composed largely of dead or senescent thallus material. Marked vertical gradients in concentrations of N, P and K in thalli of mat lichens, together with results of N budget studies in Cladonia stellaris and Stereocaulon paschale, suggest that these key elements may be recycled internally to fund new growth at the apices. It is argued that nutrient recycling will increase rates of both apical growth and basal necromass production and that under steady-state conditions this will result in deeper mats and facilitate dominance sensu Grime; thus accumulation of necromass may be an ecological advantage in mat-forming lichens. Basal necromass of the N2-fixing S. paschale contains more N than necromass of the non-N2-fixing C. stellaris; this may be due in part to a greater allocation of N to irrecoverable fungal cell wall components in S. paschale and may underlie a greater requirement for N in this mycobiont.

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