The influence of competition between lichen colonization and erosion on the evolution of soil surfaces in the Tabernas badlands (SE Spain) and its landscape effects

Abstract

Badlands often contain a mosaic of soil surface types with contrasting hydrological behaviour which drives their short term geomorphic evolution. The Tabernas badlands, in semiarid SE Spain, show a complex mosaic of bare ground, biological soil crusts and plant covered patches, and high variability of covers and morphologies. Previous work has identified the surfaces that act as sources of runoff and sediments and those that act as sinks; the pathways of runoff between surfaces, and the runoff and erosion at catchment scale. However, surfaces without vascular plants, can be quite dynamic, with important effects on geomorphic processes. This work aims to generate hypotheses about the dynamics of both bare soil (34% of the area) and biological soil crusts (33%), and to provide a first estimation of the growth rate of terricolous lichens (as increase of coverage) and their geomorphological implications. The dynamics of ten representative soil surfaces were photographically monitored over 13 years, recording the cover and pattern of bare soil and of the two main kinds of biological soil crust: ‘brown crust’ and ‘white crust’. Erosion/deposition were measured by erosion pins in unbounded plots, and the hydrological and erosional behaviour monitored in bounded plots under natural and simulated rainfall. Biological colonization and crust growth were studied from 2004 within cleared plots in four crust communities. Microclimate was continuously monitored in each community. After 13 years, net erosion was recorded only in previously eroded slopes and divides. Vegetated sites and those covered by biological soil crust remained more or less invariant or recorded sedimentation. In all white crust surfaces, whole crust cover increased by 3% on average, while macrolichen cover increased by nearly 30%. Within the driest brown crust, macrolichen cover increased by 7%, while the whole crust decreased by 3%. According to previous work, lichen cover, particularly of white crust, is often greater in north to east facing orientations because of the higher slope stability and the longer duration of moisture. However, the increase of the whole biological soil crust can depend more on species composition and previous cover and less or not at all on the aspect, although certain species do grow faster in particular orientations. Within the plots cleared in 2004 the greatest increase occurred in the pioneering brown crust, which reached an average of 26% cover after one year, in spite of it receiving the most insolation. Significant microclimatic differences exist among the communities, mainly in radiation, soil moisture and thallus surface temperature. Terricolous lichen crusts, primary colonizers of bare, sufficiently stable areas, can grow quickly and, by increasing soil stability and development and modifying surface hydrology, they can influence surface dynamics and ultimately landform dynamics. Crust growth rate probably decreases as its cover increases and it probably also oscillates with microclimatic oscillations. However, the high potential growth rate recorded allows detection not only of climatic changes but also short term (possibly seasonal) periods of stability. Extensive lichen crusts indicate low erosion rates and particular climatic conditions; variations of crust cover could indicate climatic (or land use) variations.