ISSN 1188-603X

No. 251 June 10, 2000 Victoria, B.C.
Dr. A. Ceska, P.O.Box 8546, Victoria, B.C. Canada V8W 3S2


From: Patrick Williston [ ]

Microbiotic crusts are assemblages of lichens, bryophytes (mosses and liverworts), algae, fungi, bacteria, and cyanobacteria that are important contributors to the ecology and species diversity of terrestrial ecosystems in semiarid regions. Unfortunately they are also inconspicuous and taxonomically challenging, and are often overlooked in ecological studies and vegetation inventories. In many regions, the importance of these crusts is poorly understood and their basic floristics completely unknown. Indeed, only in the last 30 years have research efforts focussed upon how human caused disturbances, such as trampling by domestic livestock, affect crust diversity and function.

Over the past three years I have spent some time examining the successional patterns of microbiotic crusts in ponderosa pine forests, one of British Columbia's most restricted ecosystems. This article summarizes some observations made along a disturbance gradient reflecting past grazing activity by livestock (Williston 1999). Early successional sites were sampled in recently heavily grazed (as evidenced by the presence of dung and consumed forage) while later successional sites had not been used by livestock for a period of time. The precise dates of use were not determined.

Study Area

Sites were situated in ponderosa pine forests on a silt terrace along the South Thompson River near Kamloops. These silt terraces are residual lakebed sediments created by glacial lakes that formed during the melting of the Pleistocene glaciers. They occur in many inland valleys in southern British Columbia and tend to support an unusually rich community of ground-dwelling lichens and bryophytes.


While vascular plants have long been used to evaluate successional patterns and rangeland condition, microbiotic crust species are now recognized as being equally valuable indicators of succession. In the Kamloops area, Nuttall's pussytoes (Antennaria parviflora) and pasture sage (Artemisia frigida) are associated with the crustose lichen Diploschistes muscorum, and the mosses Bryum caespiticium and Ceratodon purpureus in areas of recent disturbance. Stipa comata and Poa secunda are associated with the lichens Physconia muscigena, Cladonia pyxidata, Placythium uliginosum, and the haircap moss Polytrichum piliferum in moderately disturbed, or mid-successional sites. Bluebunch wheatgrass (Pseudoroegneria spicata), junegrass (Koeleria macrantha), yarrow (Achillea millefolium), and umber pussytoes (Antennaria umbellata) are associated with the pelt lichen Peltigera rufescens, and the cup lichens Cladonia borealis and C. chlorophaea in late successional sites. The latest succession associates include rough fescue (Festuca campestris), the moss Brachythecium albicans, and the cup lichen Cladonia gracilis.

Positive feedback

In general, disturbance-tolerant species tend to be drought-tolerant and are often small. These include crustose and squamulose lichen life forms, and diminutive (often < 1mm tall) bryophytes. Late successional species tend to be larger, and among lichens, structurally more complex. This includes fruticose and foliose lichens, and mainly acrocarpous, or creeping mosses. In late successional sites, pleurocarpous mosses eventually replace most lichens which are poorer competitors. Mosses have faster growth rates than lichens, and have been shown to be more effective at retaining moisture (West 1990; Atwood 1998).

The patterns of microbiotic crust succession suggest a positive feedback mechanism. A developed microbiotic crust absorbs and retains a greater amount of water than does bare soil (Atwood 1998). Crust-covered microsites with greater moisture encourages the germination of native perennial bunchgrasses (St. Clair et al. 1984; Reitkerk and van der Koppel 1997). Shade provided by bunchgrasses favors mosses and taller, broader lichens. These larger, broader species further retard water loss due to evaporation. Disruption of the crust by grazing livestock results in a loss of moisture and a change in the species composition to smaller, drought-tolerant species. These observations mirror those by Reitkerk and van der Koppel (1997) who described how positive feedback loops within a semiarid vascular plant community were vulnerable to disruption by grazing livestock, causing a loss of moisture and nutrients.


The microbiotic crusts of ponderosa pine forests on silt terraces near Kamloops, British Columbia, support an unusually rich assemblage of ground-dwelling lichens and bryophytes. Among them are numerous rare or seldom collected species. The crusts in this region form successional assemblages that change in species composition over time. Early successional sites are often dominated by diminutive, xerophytic bryophytes, and crustose and squamulose lichens, while late successional sites also support foliose lichens, fruticose lichens, and larger bryophytes. It is hypothesized that this trend from smaller to larger species relates to increased moisture retention, and denotes a positive feedback mechanism. Referring to vascular plants, Rietkerk and van de Koppel (1997, p. 74) assert that "plant-soil interactions serve as one of the most influential positive feedback loops in semi-arid grazing systems." My research suggests that the same mechanism applies to the microbiotic crust community and may be of equal importance.


Atwood, L. 1998.
Ecology of the microbiotic crust of the antelope-brush (Purshia tridentata) shrub steppe of the south Okanagan, British Columbia. M.Sc. thesis, University of British Columbia, Vancouver, Canada. 130p.
Lloyd, D., Angove, K., Hope, G., and C. Thompson. 1990.
A Guide to Site Identification and Interpretation for the Kamloops Forest Region. B.C. Ministry of Forests, Victoria, Canada. 399p.
Reitkerk, M. and J. van der Koppel. 1997.
Alternate stable states and threshold effects in semi-arid grazing systems. Oikos 79: 69-76.
St. Clair, L.L. Webb, B.L., Johansen, J.R., and G.T. Nebeker. 1984.
Cryptogamic soil crusts: enhancement of seedling establishment in disturbed and undisturbed areas. Reclamation and Revegetation Research 3: 129-136.
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.
Williston, P. 1999.
Floristics and successional patterns of microbiotic crusts in ponderosa pine forests of southern inland British Columbia. M.Sc. thesis, University of British Columbia, Vancouver, Canada. 115p.


From: Dirk Albach [ ]

The ice age might have had different effect on alpine species in Europe and North America. I am a Ph.D. student at the university of Vienna studying this hypothesis using the sister taxa Veronica alpina (Europe) and Veronica wormskjoldii and Veronica cusickii (North America). Due to shortage of funding for travel expenses I will not be able to collect all necessary populations myself. Can you, please, help me collect specimens (3-5 individuals/population preferably dried over silicagel + a voucher herbarium specimen) from various populations across the U.S. and Canada? Thank you for your help!


From: Adolf Ceska [ ]

The Oregon State University Press recently published two important contributions to our knowledge of local flora of Mount Rainier, Washington, and the Steens Mountain, Oregon. Both are similar in format, layout and depth of taxonomical treatment. They both cover all species of vascular plants known from the areas and are excellent field guides. They both contain a short introduction with geographical and historical information on the area, followed by keys for identification and descriptions of the species. Both use illustrations from Hitchcock et al. Vascular Plants of the Pacific Northwest; those in the Flora of Mount Rainier are pasted in the text, and in the Steens Mountain flora are collected in plates at the end of the book. Both books have eight centre pages filled with colour photographs. I cannot say which format I prefer, as they are both equally excellent.

Biek, David. 2000.
Flora of Mount Rainier National Park. [Washington] Oregon State University Press, Corvallis, Oregon. 506 p. ISBN 0-87071-470-8 [soft cover] Price: US$29.95
"Flora of Mount Rainier National Park provides a complete listing of the 866 species of native and introduced plants found in the Park, with keys, line drawings, and descriptions for identification, as well as a guide to plant location."
Mansfield, Donald H. 2000.
Flora of Steens Mountain. [Oregon] Oregon State University Press, Corvallis, Oregon. 410 p. ISBN 0-87071-471-6 [soft cover] Price: US$29.95

"A significant contribution to Oregon and Great Basin flora, this comprehensive field guide identifies plants of Steens Mountain and surrounding areas in southeastern Oregon, including Malheur national Wildlife Refuge, Diamond Craters, and the Alvord Desert." Altogether 871 species.

One nice detail that I noticed in Mansfield's Flora was that the author did not sweep the taxonomical problems of Poa secunda s.l. under the carpet, but mentioned all the formerly recognized species, such as "Poa ampla, P. canbyi", etc. that occur in the area. This follows the suggestion which Dr. Kellogg (the author of the much simplified Poa secunda treatment) gave in her 1985 paper (Kellogg, E.A. 1985. Variation and names in the Poa secunda complex. Journal of Range Management 38: 516-521). This will enable guide users to understand the variation within the broad Poa secunda complex better.

Both books are available from:


The new International Code of Botanical Nomenclature (St. Louis Code) has been printed:
Greuter, W., J. McNeill, F.R. Barrie, H.M. Burdet, V. Demoulin, T.S. Filgueiras, D.H. Nicolson, P.C. Silva, J.E. Skog, P. Trehane, N.J. Turland, D.L. Hawksworth [Editors and Compilers]. 2000.
International Code of Botanical Nomenclature (Saint Louis Code) adopted by the Sixteenth International Botanical Congress St. Louis, Missouri, July-August 1999. (Regnum Vegetabile, 138). xviii + 474 p. ISBN 3-904144-22-7 [hard cover] Price 80.00DM or US$44.44.
Available from: Please note: IAPT members must indicate their membership in IAPT in order to obtain the membership discount! [ IAPT = International Association of Plant Taxonomists]

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