Four communities or sites dominated by blackbrush (Cokoflne rumosissima Torr.) were studied in the Kaiparowits Basin of southern Utah and northern Arizona. One site has been heavily grazed yearlong for about 100 years; the second has been lightly to moderately grazed in winter for 3 years; the third has had 10 years of recovery from heavy grazing; and the fourth is a relic, ungrazed blackbrush ecosystem. Soils were 87 to 99% sand and gravel with mean pH's 8.2 to 8.5. The relic site had significantly more herbaceous vegetation cover (composed primarily of perennial grasses) and total cover than all other sites. The relic site also had significantly more shrub and cryptogamic cover than the heavily grazed and recovery sites. The recovering site showed no significant differences than the heavily grazed site for any of the measured parameters.

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SUMMARY ']^he relationship between carbon dio.\ide e.xcliange oi Microcoleiis-and Scytoiieina-dom'urMcd cryptogamie crusts and resaturation time was measured in the laboratory with a modilied discrete sampling technique and infrared gas analysis. Maximum net photosynthetic rate of Alicrocolciis was 187 nmol CO., m"" s"' and of Scytonciiia was 111 nmol CO., m's ' tor rehydration to lOO",, soil saturation. Botli crust types demonstrated a slow rise in resaturation respiration and took 2 days to become fully acti\e after the lirst rehydration to 100",, soil saturation after long-term dryness, and only one day to become active after the second rehydration cycle.

SUMMARYTlie carbon dioxide exchange of Mkrocoleus-and Scytonema-dom'mated cryptogamic crusts as related to dehydration was measured in the laboratory with a modified discrete sampling technique and infrared gas analysis. The dehydration curves predicted that carboxylation and dark respiration rates for both crust types would become zero at from 4 to 5 % water content (W) (approximately 16-23 % soil saturation), with the water contents at which the rates became zero significantly lower in the second treatment cycle than the first. The dehydration curves predicted that net photosynthesis rates would become zero at 6-7-11-2 "^ W (27-44 °' o soil saturation), with the water contents at which the rates became zero significantly higher in the second cycle of treatment than the first.

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