Developing an ecosystem perspective from experimental monitoring programs:I. Demographic responses of a rare geothermal grass to soil temperature

The geysers panic grass [Dichanthelium lanuginosum Spellenberg var, thermal e (Bol.) Spellenberg or DILA] is exclusively associated with surface geothe rmal manifestations in Sonoma County, California. USA (38 degrees 46'N, 122 degrees 38'W). Steam extraction by power plants could alter the subsurface distribution of heat and water to the site, potentially impacting subpopul ations of this rare plant. The purpose of this study was to use demographic monitoring to determine: (1) temporal and spatial patterns of soil tempera ture in relation to the distribution of established DILA individuals at Lit tle Geysers, (2) in situ response of experimental populations of DILA to sp atial Variations in soil temperature, and (3) habitat requirements of DILA as an indicator of its tolerance to variations in surficial geothermal feat ures. Thermocouple transects and a datalogger provided data for characteriz ing the spatial and temporal patterns of soil temperature in four microhabi tats (fumarole, DILA stand, Andropogon stand, and cleared). Experimental po pulations were established by precisely sowing and monitoring DILA seeds in these microhabitats. The results indicated that spatial and temporal varia tions in soil temperature had significant effects on the processes of germi nation, growth, survivorship, and reproduction, thus producing a readily ob served metapopulation patch dynamic in relation to geothermal activity. Sea sonal depressions of soil temperature near the fumaroles by cold air and pr olonged rainfall events also promoted the emergence and survival of DILA se edlings in a microhabitat that was previously too hot to occupy. Over longe r periods of time, DILA, metapopulation dynamism reflected climatic and geo thermal variation. Drought, years inhibited germination for lack of water, but more importantly for the lack of requisite soil temperature depressions in the fumarole microhabitat. Wet years promoted subpopulation expansion i nto transition areas that were once too hot and dry. There have also been s hifts in the underground distribution of steam into areas distant from know n geothermal features. The demographic responses of DILA to spatial and tem poral variations in soil temperature indicate that heat is an absolutely es sential component of the steam resource, in its absence, germination, seedi ng survivorship. growth, and maturation are significantly inhibited even if soil conditions are favorable and potential competitors are controlled. Ul timately, persistence of the species depends on maintaining the ecosystem d ynamic of colonization and extirpation in response to variations in surfici al geothermal features over long spatial and temporal Scales. This should s hift management perspective from its narrow focus on individual plants to a wider focus on monitoring the essential habitat component of steam.