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The thermal microenvironment and leaf temperatures of two plants (Saxifraga caespitosa and S. oppositifolia) were studied in a high arctic polar desert over four summers. Surface and plant temperatures exceeded air temperatures by 5 to 15 degrees C during sunny periods, with smaller effects on soil temperature in plant rooting zones. Midday plant temperatures of 20 to 30 degrees C measured during good weather are favorable for physiological activity, but the number of such days is restricted by a short snow-free period (16-45 d).

Surfaces in polar deserts are dominated by sorted stones and bare mineral soil with sparse vascular plant cover (usually <5%). Infrequent snowflush sites are scattered within this landscape, containing greater plant cover and a cryptogamic crust on the mineral soil surfaces. The potential role of this black dust in facilitating greater plant cover by enhancing the thermal relations of plants was investigated in this cold environment. Reduced albedo of crusted surfaces led to higher surface (8-12 degrees C) and soil (4-5 degrees C) temperatures than in noncrusted areas. However, adult plants did not differ in tissue temperature on cryptogamic and bare surfaces. Root growth and survival of seedlings, which were thermally coupled to the surface by their small mass, are more likely to be highly influenced by the presence of crusted surfaces in this high arctic polar desert ecosystem.