| Biological soil crusts, consisting of algae, cyanobacteria, lichens, mosses, fungi, and bacteria, can aggregate soil particles and alter soil nutrients and moisture. They may also affect vascular plants, with consequences for population distributions and community composition. I studied soil crusts and their interactions with four endangered herbs in a xeric, pyrogenic Florida shrubland. Observations along a chronosequence of disturbance by fire and experiments manipulating crusts were used to understand the distribution of crusts, the effects of crusts on seed germination, and the role of crusts in nitrogen cycling. Microalgae, cyanobacteria, diatoms, and chlorophyll a in crusts were heterogeneous in both space and time, and were generally most abundant 10--15 years postfire and away from shrubs. Crusts did affect germination of three species, Eryngium cuneifolium (Apiaceae), Hypericum cumulicola (Hypericaceae), and Paronychia chartacea (Caryophyllaceae), but the effect was variable and lower in magnitude than other factors. More germination occurred in recently burned sites and away from shrubs. Germination of a fourth species, Polygonella basiramia (Polygonaceae), was unaffected by crusts or by site factors. Overall rates of germination were low, however, and may have been driven by rainfall. The distribution of herbaceous roots in the system was studied with Rb, an analog of K. All four species took up Rb at distances of up to 97 cm, with an average root spread of 51 cm. The spatial arrangement of enriched and unenriched plants relative to Rb patches suggested root systems that are radially discontinuous, asymmetric, overlapping, and potentially competing both with each other and with crusts. Delta 15N natural abundances suggested that arbuscular mycorrhizal plants obtain nitrogen primarily from crusts whereas non-mycorrhizal herbs use nitrogen from deeper soils or precipitation. Addition of 15N as a tracer indicated that crusts have more than one function in scrub nitrogen dynamics: they fix nitrogen, compete with plants for nitrogen in rainfall, and retain nitrogen in the system that would otherwise be lost so that it becomes available to plants at a later time. Soil crusts are clearly an important component of the scrub ecosystem and may be in part responsible for the landscape we observe. |
