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

  • Published source details Hawkes C.V. (2004) Effects of biological soil crusts on seed germination of four endangered herbs in a xeric Florida shrubland during drought. Plant Ecology, 170, 121-134.


Biological soil crusts (dominated by algae, cyanobacteria, fungi, and bacteria) of rosemary scrublands in Florida were examined for effects on seed germination of four herbs that are killed by fire and which can only regenerate from seeds: wedgeleaf eryngo Eryngium cuneifolium, highlands scrub St. Johnswort Hypericum cumulicola, Florida jointweed Polygonella basiramia and paper nailwort Paronychia chartacea ssp. chartacea. Soil crusts influence soil stability, water and nutrients, and may affect seed germination of higher plants. As part of a series of experiments, field studies where crusts were left intact, disturbed by raking or by flaming, or completely removed, were undertaken.

Study site: The study was undertaken in xeric Florida rosemary Ceratiola ericoides scrub sites at Archbold Biological Station, Highlands County (27º11'N, 81º 21'W), Florida, southeast USA. The area has hot, wet summers and mild, dry winters; average annual precipitation is 1,333 mm with almost half falling in June to August.

Experimental design: To test the effect of disturbance of biological soil crusts on germination (combined with time elapsed since the last fire and distance to dominant C.ericoides shrubs, see below), three crust treatments were applied:

i) crusts left intact;

ii) flamed - torched with a propane blowtorch for 20 seconds;

iii) disturbed - raking to 6-8 cm depth and pounding with a trowel until no large lumps remained and the crust was well mixed.

The disturbance treatments mimicked common disturbances (i.e. fire and trampling). The experiment was a factorial design (3 x since fire x 2 shrub distances x 3 crust treatments x 4 species) with 15 replicates (0.5 m² plots) of each.

Three categories of time since fire were assigned: recently burned (8 years post-fire), intermediate (10-15 years post-fire) and long unburned (30 years post-fire). Effects of the dominant shrub on germination in the crust treatments were tested by sowing seeds at 0 and 1 m from the shrub edge.

Seed was locally collected (Aug-Dec 1998) and refrigerated until sown in January 1999, by sprinkling onto the soil surface and covering lightly with sand. For P.basiramia, five seeds were sown; 10 seeds of the three species were sown.

Additional experiments were undertaken in 1998-99 and 1999-2000, set up as above but E. cuneifolium was excluded, plot size was larger (4 m²), the flamed treatment was replaced by crust removal, and shrub treatment and long-unburned sites were not included in 1999-2000. Germination rates were extremely low in both years so only some overall trends are presented.

Germination and seedling survival: Germination and seedling survival were assessed monthly.

Overall rates of germination were low, possibly as a result of droughts; E.cuneifolium failed to germinate at all.

Highlands scrub St. Johnswort: Most H.cumulicola germination occurred away from shrubs in sites < 8 years post-fire (intact - 10%; disturbed -11%; flamed - 6%). However, in the older post-fire sites away from shrubs, germination was 3-7% (equivalent to 3 to 6 six times greater) in intact crusts compared to the disturbed and flamed crusts; of only six seeds that germinated near shrubs, five were in intact crusts. Germination was consistently lower near to shrub under all treatment and environmental factor combinations. There were no differences in germination across time since fire in intact crusts. Intact crusts appeared more favourable for germination of this species under the prevailing weather conditions. Overall rate of seedling survival was highest for H.cumulicola (surviving 78% of the time), but as so few seeds germinated, the other two species had more seedlings alive at the end of the experiment. Seedlings survival trends are not possible due to the very small sample sizes (6 seedlings or less/treatment combination).

Paper nailwort: P.chartacea seeds germinated best in sites of time since fire < 8 years and away from C.ericoides shrubs (intact - 48%; disturbed - 67%; flamed - 38%), followed by < 8 years and near to C.ericoides shrubs (intact - 25%; disturbed - 14%; flamed - 26%). Germination tailed off in the older post-fire sites, particularly where seeds had been sown near to shrubs. Overall seedling survival rate was 38%. Seedlings surviving about equally well in sites > 30 years and < 8 year post-fire, but dropping substantially in 10-15 year post-fire sites. However, sample sizes were small (10 or less/ treatment combination), and no trend was apparent.

Florida jointweed: P.basiramia germination was unaffected by the crust treatments and the two environmental factors tested, although there was a trend for better germination in time since fire < 8 years and away from C.ericoides shrubs in intact (30%) and disturbed (45%) crusts. Overall seedling survival rate was 61%. Seedlings survived best in sites > 30 years post-fire, followed closely by < 8 year, then 10-15 year post-fire sites. However, again sample sizes were small (9 or less/ treatment combination), and no trend was apparent.

Conclusions: Where crusts were left intact, disturbed by raked or by flaming, or completely removed, the effects were variable and species-specific. Higher germination rates were consistently observed in recently burned sites away from C.ericoides shrubs. Other than this it is not possible to draw firm conclusions from the experiment because, possibly due to prevailing drought conditions in the study years, germination rates were so low.

The importance of water for germination was confirmed in a watering experiment, germination being significantly greater in the high water treatment. Drought years are not uncommon in Ceratiola ericoides scrublands, and water availability is clearly one of the overriding factors in governing germination and seedling survival of these rare herbs.

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