Rosette scrub occurrence and fog availability in arid mountains of Mexico

Large succulent leaf rosettes are a characteristic life form in many deserts. In certain areas they become the dominant life form, creating a vegetation type indicated as rosette scrub. The large number of rosette species suggests a close relationship between form and environment. Rosettes are excellent harvesters of low-intensity rains and fogs. We propose that some rosette-dominated formations of the Mexican mountains, namely the montane rosette scrub, occur in altitudinal belts around mountains where fog is abundant. We sampled four altitudinal gradients in mountains with different flora recording the abundance and richness of plants. At one site, the Tehuacán Valley, we also measured the quantity of fog along the gradient, below, above and in the rosette scrub for one year. We found that the abundance and richness of succulent rosette species are strongly associated with altitude, showing maximum values in the well-defined elevational belts where the montane rosette scrub occurs. Other life forms, such as stem succulent cacti or woody shrubs, do not show this mid-elevation pattern. The altitudinal ranges where the montane rosette scrub occurs usually coincide with areas where clouds and fog form. Our micrometeorological measurements indicate that rosette plants growing within a cloud belt can increase their water supply by 10–100% by harvesting fog. Outside these belts fog harvest is negligible. Desert rosettes constitute one of the most common fog-harvesting growth forms