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Plant responses to high soil temperatures in geothermal areas of Yellowstone National Park.
Germino, Matthew1, Wraith, Jon, 1
ABSTRACT- Microclimate, photosynthesis, and water relations were measured at midsummer for Dichanthelium lanuginosum, a bunchgrass occurring in soils warmed by geothermal activity in Yellowstone National Park. On one of the warmest afternoons in the warmest summer in a 50-year period, all above-ground biomass was dead or senescent on plants in soils warmer than 45°C. Plants in soils cooler than 37°C exhibited no leaf mortality. Soil temperatures for D. lanuginosum at 5 cm became 10°C cooler at night, and leaf temperatures ranged from 2 to 43°C within diurnal periods. Foliage temperatures did not appear influenced by variations in soil temperatures within the geothermal area. Photosynthetic CO2 assimilation measured in the field was 3- and 5-fold greater for plants growing in 30°C compared to 33° or 40°C soils, respectively. Plants had 50% and 75% lower xylem pressure potentials at midday in 30°C compared to 33° or 40°C soils, respectively, indicating greater water stress in warmer soils. Correspondingly, stomatal conductance was 4.4- and 6.8-fold greater in 30°C soils than in 33° and 40°C soils, respectively. No differences in photosynthesis were observed for plants growing in soils ranging from 30° to 42°C in a greenhouse experiment where soil water content and xylem pressure potentials were controlled. Thus, the primary mechanism underlying the high-soil temperature limit of D. lanuginosum in the field appears to involve water stress, possibly due to reduced root water uptake in hot soils.
KEY WORDS: geothermal, heat stress, soil temperatures, multiple stresses