The effect of changes in timing and magnitude of precipitation on carbon and water fluxes: Scaling C3, C4 and CAM to ecosystem.
Tissue, David*,1, Patrick, Lisa1, Griffith, Alden2, Alpert, Holly2, Ignace, Danielle3, Cable, Jessie3, 1 Texas Tech University, Lubbock, TX, USA2 University of California, Santa Cruz, Santa Cruz, CA3 University of Arizona, Tucson, AZ
ABSTRACT- A precipitation experiment was initiated in the sotol-grassland of Big Bend National Park, Texas in January 2002. This system is primarily comprised of C3 shrubs, C4 grasses and CAM succulents. Water was added to this desert ecosystem in the summer as three separate storm events each year representing a 25% increase in precipitation above that received as ambient rainfall. Annual precipitation in 2002 was average (357 mm), while precipitation in 2003 (410 mm; 14% above average) and 2004 (567 mm; 53% above average) was higher than average. In September 2004, soil-, plant- and ecosystem-level fluxes of carbon dioxide and water were measured over a 24-h period from control plots (C; natural rainfall only) and plots watered in the summer (S). Ecosystem water flux was unaffected by the precipitation treatment. However, ecosystem carbon dioxide fluxes were greater during the daytime (but not the nighttime) in the S plots leading to greater carbon dioxide uptake over the 24-h period. Greater ecosystem carbon dioxide uptake in S plots was due in part to decreased soil respiration and greater photosynthesis in C4 grasses. Increased rates of photosynthesis in C4 grasses in S plots were due to higher leaf water potential and increased stomatal conductance. There was no indication that C3 shrubs had increased photosynthesis in S plots. Interestingly, spectral indices, such as NDVI (healthy green vegetation) and Water Band (plant water content), were lower in S plots. Despite indications of increased ecosystem carbon dioxide uptake, only CAM plants have shown an increase in above-ground growth in S plots over the 3-year precipitation treatment period. Above-ground annual net primary productivity (ANPP) in S plots has not been significantly affected, although trends indicate that it has increased. Future detailed measurements at different spatial and temporal scales following precipitation pulses of different sizes may help further define the role of soil, root and microbial activity and functional group characteristics in regulating carbon and water fluxes at the ecosystem level.
Key words: desert, photosynthesis, respiration, precipitation
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