INTRATHALLINE AND SIZE-DEPENDENT PATTERNS OF ACTIVITY IN LASALLIA-PUSTULATA AND THEIR POSSIBLE CONSEQUENCES FOR COMPETITIVE INTERACTIONS

1. In dense populations of the saxicolous lichen Lasallia pustulata th e margins of adjacent thalli overlap each other in intraspecific compe tition for space and light, 2. In situ non-destructive monitoring of h ydration-dependent potential photosynthetic activity by modulated fluo rescence systems in different parts of the thallus shows that the acti vity is structured by a centre-to-margin gradient, with the centre of the thallus remaining active for substantially longer periods than the margins when the thalli dry up after being activated by wetting. The pattern reflects the water status of different parts of the thallus: t he margins which are thin and exposed dry up first, 3. The activity pa ttern within individual lichen thalli suggests that marginal overlappi ng between neighbours may have a less detrimental effect on the shadow ed individuals than expected from a pure consideration of the amount o f area shadowed, Because the centre of the lichen thallus is active fo r longer periods, shadowing of this region may possibly be more harmfu l per area unit than an overlap at the less active margins. 4. Larger thalli are active for substantially longer periods than small ones, Ev en the margins of larger thalli tend to be active for a longer period than the centre of small thalli.