Background
The authors write that "biological soil crusts have an intimate association with cyanobacteria, lichens, mosses and green algae that grow on top of or within the top layers of the soil (Evans and Johansen, 1999; West, 1990)," noting that they "are widely distributed in the arid and semi-arid areas all over the world." They also note that these crusts "favor seed germination since they provide a moist microenvironment, and also trap more seeds and supply seeds with safe sites in the field," citing the work of Su et al. (2007).

What was done
Su et al. "investigated the effects of two types of biological soil crusts (moss crust and algal crust) at different ages on the seed germination of Bassia dasyphylla [a perennial grass] and Artemisia ordosica [a perennial semi-shrub]," as well as "the effects of crust conditions (living crust and dead crust) on seed germination of B. dasyphylla, A. ordosica and Eragrostis poaeoides [an annual herb]."

What was learned
The four researchers determined that both algal and moss crusts "significantly enhanced the germination of B. dasyphylla and A. ordosica," and that crust age had no significant effect on the germination of either of the two plants. With respect to the other aspect of their study, they say that "germination was always greatest on the living crust for all species."

What it means
The Chinese scientists say their results suggest that "biological soil crusts, regardless of age or condition, enhance seed germination of three species native to the hyper-arid region of the Tengger Desert in China," which phenomenon spurs the establishment of herbs, grasses and shrubs on such lands. In addition, we note that atmospheric CO2 enrichment has been demonstrated to significantly spur the growth of biological soil crusts themselves (see Deserts: Algae and Lichens in our Subject Index). Hence, the ongoing rise in the atmosphere's CO2 concentration should be promoting a gradual greening of the earth via these means; and a vegetative rejuvenation of this nature has indeed been observed over most parts of the earth over the past several decades.

References
Evans, R.D. and Johansen, J.R. 1999. Microbiotic crusts and ecosystem processes. Critical Reviews in Plant Sciences 18: 183-225.

Su, Y.-G., Li, X.R., Chen, Y.W., Tan, H.J. and Jia, R.L. 2007. Effects of biological soil crusts on emergence of desert vascular plants in North China. Plant Ecology 191: 11-19.

West, N.E. 1990. Structure and function of microphytic soil crusts in wildland ecosystems of arid and semi-arid regions. Advances in Ecological Research 20: 179-223.