biological soil crusts in texas: From Big Bend to the Big Thicket

Often overlooked by even the greater scientific community, biological soil crusts (BSCs) are increasingly recognized for their unique characteristics and contributions to the biosphere.  Composed of a complex association between bryophytes, cyanobacteria, algae, micro fungi, and lichens, BSCs are found on all seven continents.  As such BSCs can be found from the warm and humid tropics to the driest of hot and cold deserts.  Their location in the environment is aptly described as “a delicate layer of simple and more complex non-vascular organisms dancing between the boundaries of the vertical plant community and deeper soil profile.”  These organisms persist in this transitional “boundary layer” as a unique and dynamic ecosystem.   

Biological soil crusts (BSC) are found within the first few millimeters, or immediately on top of, the soil surface. Their development is limited to the interstitial spaces between vascular plans due to their inability to effectively compete with the vertical plant community for light resources. In moist environments this precludes the development of crusts in all but recently disturbed sites. In arid to semi-arid environments, their low water requirements allow them to colonize sites otherwise inhospitable to vascular plants. The diversity of the organisms characterizing these crusts is closely correlated to their relative environments and is primarily controlled by climate, soil type, above ground plant community, and precipitation

Establishment, Maturation, and Recovery

The development and maturation of biological soil crusts follows a somewhat predictable pattern of establishment and succession.  BSC are first colonized by large filamentous cyanobacteria.  As the soil stabilizes, smaller cyanobacteria, micro-fungi, and green algae follow suit. By this point, a vertical plant community has often been established creating considerable competition for light resources. As such bryophytes and lichens will persist in the most suitable spaces.  In tropical and temperate regions, vascular plants tend to outcompete BSC for light resources which then persist only in recently disturbed sites.  However, in arid to semi-arid environments, BSC will continue to mature between plant interspaces. 

Following disturbance, the recovery of BSCs is dependent on a variety of factors to include:

·        continuation/cessation of disturbance regime

·        extant biota at the site

·        long-term environmental conditions

·        ability of non-present historical biota to recolonize the site

·        the presence or absence of a vertical plant community

It is important to remember that the recovery of BSCs to a pre-disturbance state, especially in arid and semi-arid environments, is especially difficult to achieve.

Influencing Factors

A wide variety of BSCs are found in North American from hot and cold deserts, coastal woodlands, to the grasslands of the Great Plains.  Approximately half of all annual rainfall in the northern “cool deserts” (Great Basin and Colorado Plateau) freezes annually.  The latter “hot deserts” receive the precipitation as rain and rarely, if ever freeze.  These two properties (time and form of precipitation), as well as temperature, potential evapo-transpiration, vertical plant community structure, and soil characteristics play considerable roles in the relative species composition of BSC.

Precipitation for BSC environments in North America range from 50- to nearly 750 mm. Annual mean temperatures are also highly variable from 3 to 25 °C. Many species within BSCs are found in all soil crust regions (Bryum argeneteum and Pterygoneurum ovatum).

Biological Soil Crusts of the Chihuahuan Desert

North America is home to three hot deserts- the Mojave, Sonoran, and Chihuahuan.  These deserts are found in the southwestern states (CA, AZ, NM, and TX) and western two thirds of Mexico.  They are characterized by hot summers, mild winters, rare snowfall events, and soils that rarely freeze.

Of the three, the Chihuahuan Desert is the only one to extend into the western-most potion of Texas to include Big Bend National Park and the Davis Mountains.  Most of these soils are of calcareous derision (limestone) with the remaining of volcanic origins.  Precipitation typically occurs as rain in the winter and summer months.  In areas receiving less than 250 mm/year, creosote flats dominate whereas grassland and shrublands dominate in areas receiving greater than 250 mm/year.

The BSC community is this region contains a surprisingly large variety of lichens. Bryophytes can be found in microclimates with greater moisture content such as natural springs, in draws, and other natural depressions. Bryophytes observed in Chihuahuan Desert soil crusts include: Bryum sp., Crossidium aberrans, Tortula atrovirens, Pterygoneurum ovatum, Syntrichia ruralis, Weissia sp., Pseudocrossidium crinitum, Plagiochasma rupestre, and Reboulia hemispaerica.

Literature Cited:

The preceding text was adapted from: Belnap, J., Lange O. 2001. Biological Soil Crusts: Structure, Function, and Management. Springer-Verlag; Berlin, Heidelberg.