Do vehicle track disturbances affect the productivity of soil-growing lichens in a fog desert?

Abstract

1. The re-establishment of soil-crust lichens following disturbances may be enhanced or discouraged by human-altered microtopographies. This research asks whether such microtopographic changes alter microclimates significantly, and whether changes affect metabolically active periods of recolonizing lichens. 2. In the fog-driven Namib Desert, the water relations of five lichen species were investigated and photosynthetic activity (PSII) was assessed via chlorophyll a fluorescence under natural and altered conditions, based on measured microclimates in vehicular disturbances. 3. Surface temperatures in vehicle tracks were significantly higher than controls by up to 2°C. Fog deposition was significantly lower in tracks, and evaporation rates were higher. 4. Foliose and fruticose lichens retained fog moisture for long periods, and carried out PSII at low relative moisture contents. Desiccation rates of lichens were estimated to increase by 36% in tracks, which reduces PSII durations by as much as 50% in the critical hours following sunrise. Lecidella crystallina was least affected due to a pre-existing low water-holding capacity. Extreme temperatures (>45°C) halted PSII in certain species. 5. These results illustrate the importance of temperature-moisture ratios on desert soil surfaces. Reduced periods of photosynthesis caused by human-induced microtopographic changes, as predicted here, may have important ecological implications by reducing productivity in lichen communities. © 2006 British Ecological Society.