Effects of biological soil crusts on seed germination of four endangered herbs in a xeric Florida shrubland during drought

Soil crusts of rosemary scrubs in south-central Florida were examined for effects on seed germination of four herbs that are killed by fire and must recruit from seed: Eryngium cuneifolium (Apiaceae), Hypericum cumulicola (Hypericaceae), Polygonella basiramia (Polygonaceae), and Paronychia chartacea ssp. chartacea (Caryophyllaceae). Biological soil crusts in these sites are dominated by algae, cyanobacteria, fungi, and bacteria. Because crusts can change soil stability, water, and nutrients, they can affect seed germination. A series of greenhouse and field experiments were designed to first examine the effects of crusts in isolation and then to determine their role in the context of other environmental factors – time since fire and distance to the dominant shrub in this system, Ceratiola ericoides. In the greenhouse experiment, germination in autoclaved crusts was dramatically reduced relative to germination in living crusts for all but P. basiramia. In four field experiments where crusts were left intact, disturbed (mechanically or by flaming), or completely removed, the effects of crusts were variable and species-specific, but were significant enough to impact aboveground population sizes. More germination was consistently observed in recently burned sites away from C. ericoides shrubs. Overall rates of germination were generally very low during this study, possibly as a result of seasonal droughts that could have reduced germination, increased seed dormancy, and/or decreased seed viability. The importance of water for germination was confirmed in an experiment with two watering regimes and three crust treatments designed to create a gradient of soil water availability. Germination was significantly greater in the high water treatment and unaffected by soil crust moisture. Dry years are not uncommon in scrub and the results of this study help us to understand how scrub herbs fare during drought and what role biological soil crusts play in germination.