|
PLANT ECOLOGY
LAB: Biological Soil Crusts Archbold Biological Station, P.O. Box 2057, Lake Placid, Florida 33862 USA Phone: 863-465-2571 FAX: 863-699-1927 send e-mail |
|
home | index | research | plant lab home | species directory |
|
Invisible Soil Organisms and Scrub Conservation [reprinted, with permision, from Ridge Rangers, Volunteer News from Central Florida, Spring 2002, pp.1-2] Scientists are still uncovering some of the themes and patterns that govern the Florida scrub. So look closely the next time you are walking in the scrub, the allure of this habitat is in the details. Soils, even the seemingly sterile sands of central Florida, teem with microscopic life. In a single gram of soil, there are as many as a billion organisms. And though invisible to the naked eye, soil microbes throughout the world are critical to decomposition and nutrient cycling that help maintain the plant communities we see. Biological soil crusts. In drier regions of the world, soil microorganisms form what are known as biological soil crusts. Because of their hidden nature, these crusts have been termed cryptogamic, cryptobiotic and microbiotic. Crusts are created when soil organisms cause the uppermost layer of soil to solidify into a single, cohesive layer. The "glue" that holds the soil together is made up of the living organisms themselves together with the sticky substances they excrete and leave behind as they move through the dry soil. Biological soil crusts are a well-known feature of deserts in the southwestern United States and have only recently been recognized in the southeast. Crusts in Florida scrub are unique because they occur in a wet climate where dry conditions are caused by rapid drainage of water through sandy soils. What organisms create soil crusts and where do they occur? Florida scrub soil crusts are found primarily in openings or gaps in the vegetation. The scrub crusts are unusual in that they are indistinguishable from uncrusted sands until you feel the aggregation and look closely. To feel soil crusts, poke your finger into an undisturbed area and you will find resistance compared to loose sand. To see the soil crusts, gently scrape away the surface sand grains on a wet morning and you will discover a green layer a few millimeters deep. This green color comes from the algae and cyanobacteria that dominate the scrub crusts. Algae and cyanobacteria are photosynthetic, using sunlight as a source of energy; they move up and down in the sand in response to changing levels of light and water. There are also fungi, bacteria, lichens and occasional mosses living in and on the soil crusts. Fungi and bacteria are heterotrophic, feeding on other organisms and dead material in the soil. Lichens are created by a symbiotic association of fungi and algae (or cyanobacteria) and can become dominant on top of rosemary scrub soils 15 to 30 years after a fire. Crusts are not uniform across the landscape. They are extremely vulnerable to disturbances, including fire and trampling, resulting in differences across sites based on fire history and within sites based on local disturbances such as animal and hiking trails. For example, after a fire, it takes algae and cyanobacteria 10 to 15 years to reach their peak in abundance. Total recovery may take from one to 100 years depending on crust species composition, size of the disturbance and distance to intact crust. Soil crusts and endangered scrub herbs. Seed germination of several endangered scrub herbs is improved by the presence of soil crusts. Seeds of the Highlands scrub St. John’s wort, Hypericum cumulicola, germinated far more in intact soil crusts particularly at long unburned sites when compared to crusts that had been disturbed or removed. Two other scrub herbs, wedge-leaf button snakeroot, Eryngium cuneifolium, and papery whitlow-wort, Paronychia chartacea, also had increased germination when seeds were in intact crusts in the greenhouse. Crusts can affect the fate of seeds in several ways. Small seeds that get wedged between crusted grains of sand may be hidden from insects that would like to eat them. Additionally, some of the microorganisms in crusts produce fungal and bacterial inhibitors that may reduce microbial attacks on seeds. And perhaps most important of all, crusts typically have higher levels of soil moisture because the sticky gel-like substances excreted by algae and cyanobacteria can rapidly absorb and retain water from rain, fog or dew. Soil crusts and scrub insects. Mark Deyrup of Archbold Biological Station and Thomas Eisner of Cornell University discovered a species of cricket called the pygmy mole cricket that is only a quarter inch in length. This species is limited to scrub habitats where the crickets burrow in the stable sand of soil crusts. From their burrows, pygmy mole crickets dine on the algae and cyanobacteria that live in the crusts, leaving behind elevated burrow trails that reveal their passing. Little is known about the interactions of these crickets and the soil crusts, but we can imagine that their selective grazing may affect the species composition of crusts and thus, modify the role of crusts in the scrub community. [see also: Deyrup, M. and T. Eisner, T. 1996. Description and natural history of a new pygmy mole cricket from relict xeric uplands of Florida (Orthoptera: Tridactylidae). Memoirs Entomological Society Washington 17: 59-67; and Deyrup, M. and T. Eisner. 1996. Photosynthesis beneath the sand in the land of the pygmy mole cricket. Pacific Discovery 49: 44-45.] Soil crusts and the nitrogen cycle. Nitrogen is generally the nutrient most limiting to plant growth in terrestrial ecosystems around the world. Nitrogen can enter systems in one of two ways: biological fixation and atmospheric deposition. In Florida scrub, soil crusts are important in both of these processes. Cyanobacteria and some bacteria in soil crusts fix nitrogen. That is, they take gaseous nitrogen from the atmosphere and transform it to ammonium, which can be used by soil microbes and plants for growth. In nitrogen-poor systems like Florida scrub, nitrogen fixed by crusts can be the major source nitrogen for plants. Crusts in Florida fix substantial amounts of nitrogen as long as they have not been disturbed — after a fire or in trampled areas, their ability to fix nitrogen is reduced by a factor of 50. Though fire is natural in the scrub landscape, understanding its impacts helps land managers use the appropriate frequency, intensity and extent of disturbance. Nitrogen is also entering Florida scrub through atmospheric deposition. In this case, the nitrogen returns to land with rain or dust. Because of the sandy soils in scrub, most of the nitrogen in rainfall is lost because of rapid drainage. Crusts act as a sieve through which nitrogen entering the system must pass, and crusts immediately sequester a portion of the deposited nitrogen thus making it unavailable to plants. Within 15 days, however, the nitrogen in crusts becomes available to plants, probably as a result of microbial death. An ecosystem perspective. The greatest danger to the Florida scrub is habitat loss, as the last intact areas are converted to agriculture, housing and commercial building. For the scrub that remains, management of this fire-dependent habitat will be necessary to avoid rapid extinction of the many endangered plants and animals that exist only here. The recognition that even the seemingly invisible soil crusts are important for nutrient cycling and the survival of plant and insect communities makes it clear that scrub conservation will require an ecosystem perspective. Christine Hawkes studied biological soil crusts at Archbold Biological Station as part of her graduate work at the University of Pennsylvania. Christine now holds a David H. Smith Postdoctoral Fellowship at the University of California, Berkeley. Bibliography
|
Hawkes, C.V., 9 May 2002.