PARENT SESSION
Wednesday, August 9, 8:00-11:30 am
COS 53 - Ecophysiology II: respiration, transpiration, and primary production
L-4, Lobby Level, Cook Convention Center
Presiders: M Coleman and M Fuller
Altered precipitation regimes and aboveground net primary production in a semi-arid grassland.
Heisler, Jana*,1, Knapp, Alan1, Kelly, Eugene*,1, 1 Colorado State University, Fort Collins, CO, USA
ABSTRACT- Water availability is a primary factor limiting aboveground net primary productivity (ANPP) in grasslands, and many aspects of ecosystem structure and function are known to be sensitive to alterations in both the temporal distribution of rainfall events as well as the seasonal quantity. General circulation models forecast more extreme weather patterns in the central US, characterized by fewer, but larger rainfall events during the growing season and subsequently longer intervening dry intervals between storms. In mesic grasslands, such changes in the temporal distribution of rainfall results in reductions in ANPP and CO2 uptake by the dominant grasses. To determine whether similar impacts on ecosystem function would be observed in more arid grasslands, we manipulated the frequency of growing season rainfall in 15 rainout shelters in the shortgrass steppe of northeastern Colorado. We kept growing season precipitation quantity constant (30-year mean) for all treatments, but altered the size and frequency of individual events such that they occurred at 10-, 20-, or 30-day intervals (n=5 shelters per interval frequency). In contrast to responses in mesic grasslands, frequent rainfall events resulted in reduced ANPP (127±47 g/m2), likely due to consistently low soil moisture and high evapotranspiration. ANPP was greatest when rainfall occurred at 20- (178±38 g/m2) and 30-day intervals (162±48 g/m2). In these treatments, soil moisture peaked following large rain events and remained elevated for ≥15 days. Larger rainfall event size also allowed for water to reach greater depths in the soil profile where it was less susceptible to the high evaporative demand of this ecosystem. While the dominant shortgrass steppe grasses can remain productive when subjected to a high frequency of small events, we conclude that rain-use-efficiency may be increased in this grassland under some more extreme precipitation regimes.
Key words: precipitation variability, ANPP, grasslands