Bm. Pavlik et A. Enberg, Developing an ecosystem perspective from experimental monitoring programs:I. Demographic responses of a rare geothermal grass to soil temperature, ENVIR MANAG, 28(2), 2001, pp. 225-242
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.