Developing an Ecosystem Perspective from Experimental Monitoring Programs: I. Demographic Responses of a Rare Geothermal Grass to Soil Temperature |
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Authors: | BRUCE M PAVLIK ANDREW ENBERG |
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Institution: | (1) Department of Biology, Mills College, Oakland, California 94613, USA, US |
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Abstract: | The geysers panic grass Dichanthelium lanuginosum Spellenberg var. thermale (Bol.) Spellenberg or DILA] is exclusively associated with surface geothermal manifestations in Sonoma County, California,
USA (38°46′N, 122°38′W). Steam extraction by power plants could alter the subsurface distribution of heat and water to the
site, potentially impacting subpopulations of this rare plant. The purpose of this study was to use demographic monitoring
to determine: (1) temporal and spatial patterns of soil temperature in relation to the distribution of established DILA individuals
at Little Geysers, (2) in situ response of experimental populations of DILA to spatial variations in soil temperature, and
(3) habitat requirements of DILA as an indicator of its tolerance to variations in surficial geothermal features. Thermocouple
transects and a datalogger provided data for characterizing the spatial and temporal patterns of soil temperature in four
microhabitats (fumarole, DILA stand, Andropogon stand, and cleared). Experimental populations were established by precisely sowing and monitoring DILA seeds in these microhabitats.
The results indicated that spatial and temporal variations in soil temperature had significant effects on the processes of
germination, growth, survivorship, and reproduction, thus producing a readily observed metapopulation patch dynamic in relation
to geothermal activity. Seasonal depressions of soil temperature near the fumaroles by cold air and prolonged rainfall events
also promoted the emergence and survival of DILA seedlings in a microhabitat that was previously too hot to occupy. Over longer
periods of time, DILA metapopulation dynamism reflected climatic and geothermal 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 into transition areas that were once too hot and dry. There have also been shifts
in the underground distribution of steam into areas distant from known geothermal features. The demographic responses of DILA
to spatial and temporal variations in soil temperature indicate that heat is an absolutely essential component of the steam
resource. In its absence, germination, seeding survivorship, growth, and maturation are significantly inhibited even if soil
conditions are favorable and potential competitors are controlled. Ultimately, persistence of the species depends on maintaining
the ecosystem dynamic of colonization and extirpation in response to variations in surficial geothermal features over long
spatial and temporal scales. This should shift management perspective from its narrow focus on individual plants to a wider
focus on monitoring the essential habitat component of steam. |
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Keywords: | : Plant conservation Metapopulation dynamics Monitoring Geothermal Ecosystem management |
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