ABOVE AND BELOWGROUND PRODUCTION PATTERNS IN ALASKAN TUSSOCK TUNDRA: PLASTICITY IN RESPONSE TO CLIMATE CHANGE
SULLIVAN, PATRICK F. Natural Resource Ecology Laboratory, Colorado State University.
Welker, Jeffrey M. Natural Resource Ecology Laboratory, Colorado State University.
Fahnestock, Jace T. Natural Resource Ecology Laboratory, Colorado State University.
Understanding system nutrient and water balance under current and future climate regimes requires a thorough knowledge of plant resource allocation. With recent improvements in optic and electronic technology, minirhizotron camera systems have become a useful tool for measuring root production dynamics in some natural systems. This study examined the patterns of shoot growth relative to root production and the influence of warming, fertilization and increased winter snow on differential resource allocation. Shoot growth measures and minirhizotron images were taken on a mean interval of 7.7 days during the 2001 growing season, with an additional fall sampling date to capture late season root production.
Results from our control plots suggest an inverse relationship between aboveground growth rate and belowground production rate in the moist tussock system. During periods of peak aboveground growth rate, belowground production rate was at a relative minimum. Conversely, during periods of peak belowground production rate, aboveground growth rate was at a relative minimum. In contrast with control plots, warming and fertilization treatments produced periods of high concurrent above- and belowground production early in the growing season, but resumed the pattern of mutual exclusivity by mid-July. Allocation patterns in plots treated with increased winter snow displayed a magnified case of mutual exclusivity in aboveground growth and belowground production. Upon emergence from the snow, nearly all growth was concentrated aboveground. With progression of the growing season, belowground production rate increased steadily, while aboveground growth rate decreased steadily, such that the aboveground minimum and the belowground maximum were coincidental in late August.
These patterns, derived from treatment and control plots, suggest a trade-off in resource allocation, where environmental constraints necessitate partitioning of carbon and nutrient acquisition. We hypothesize that, in addition to nutrient limitation, conditions specific to the respective microclimates of roots and shoots are important determinants of the observed patterns. Patterns in aboveground growth may closely correspond with variables such as photosynthetically active radiation and ambient air temperature, while patterns in belowground production, in addition to the indirect influence of aboveground variables, may correspond closely with soil temperature, depth of thaw and microbial biomass. Based on these differences, we hypothesize that the apparent mutual exclusivity in aboveground growth and belowground production is a consequence of the buffered soil environment, which delays the onset and extends the terminus of the growing season, and the balance between cumulative microbial metabolism and plant available nutrients. These hypotheses will be tested in a path analysis framework.
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