Begun in fall 2006, this long-term study at the Sevilleta LTER examines changes in net primary production (NPP) caused by increased precipitation variability within a semiarid grassland. Net primary production is a fundamental ecological variable that quantifies rates of carbon consumption and fixation. Estimates of NPP are important in understanding energy flow at a community level as well as spatial and temporal responses to a range of ecological processes. While measures of both below- and above-ground biomass are important in estimating total NPP, this study focuses on above-ground net primary production (ANPP). Above-ground net primary production is the change in plant biomass, including loss to death and decomposition, over a given period of time. Volumetric measurements are made using vegetation data from permanent plots (SEV188, "Monsoon Rainfall Manipulation Experiment (MRME): Net Primary Production Quadrat Data") and regressions correlating species biomass and volume constructed using seasonal harvest weights from SEV157, "Net Primary Productivity (NPP) Weight Data."
The Monsoon Rainfall Manipulation Experiment (MRME) contains three ambient precipitation plots and five replicates of the following treatments: 1) ambient plus a weekly addition of 5 mm rainfall; 2) ambient plus a monthly addition of 20 mm rainfall.
Rainfall is added during the monsoon season (July-Sept) by an overhead (7 m) system fitted with sprinkler heads that produces rainfall-quality droplets. At the end of the summer, each treatment has received the same total amount of added precipitation, delivered in different sized events.
Each plot (9 m x 14 m) includes subplots (2 m x2 m) that receive 50 kg N ha-1 y-1. Measurements include: (1) seasonal (July, August, September, and October through June) soil N; (2) plant species composition and annual net primary production (ANPP); (3) seasonal root and fungal dynamics within mini-rhizotrons, and; (4) soil temperature, moisture, and CO2 fluxes (using in situ solid state CO2 sensors). In addition, soil N2O fluxes, predawn and mid-day (10:00-12:00) water potential, and the mid-day leaf photosynthetic gas exchange and stomatal conductance of black grama are measured prior to and up to five days after scheduled precipitation events.
Derivation of Biomass and NPP:
Data from SEV188 and SEV157 are used to calculate seasonal and annual production of each species in each quadrat for a given year. Allometric equations derived from harvested samples of each species for each season are applied to the measured cover, height, and count of each species in each quadrat. This provides seasonal biomass for winter, spring, and fall.
Seasonal NPP is derived by subtracting the previous season's biomass from the biomass for the current season. For example, spring NPP is calculated by subtracting the winter weight from the spring weight for each species in a given quadrat. Negative differences are considered to be 0. Likewise, fall production is computed by subtracting spring biomass from fall biomass. Annual biomass is taken as the sum of spring and fall NPP.
August 2009 Burn:
On August 4, 2009, a lightning-initiated fire began on the Sevilleta National Wildlife Refuge. The Monsoon site was entirely burned on this date, with all plots subjected to fire of comparable intensity.
02/07/2009 (YX) Metadata created and compiled for 2006, 2007 and 2008.
Other researchers involved with collecting samples/data: Chandra Tucker (CAT; 04/2014-present), Megan McClung (MAM; 04/2013-present), Stephanie Baker (SRB; 09/2010-present), John Mulhouse (JMM; 08/2010-06/2013), Amaris Swann (ALS; 08/2008-01/2013), Maya Kapoor (MLK; 08/2003 - 01/2005, 05/2010-03/2011), Terri Koontz (TLK; 02/2000-08/2003, 08/2006-08/2010), Yang Xia (YX; 01/2005-03/2010), Karen Wetherill (KRW; 02/2000-08/2009); Michell Thomey (MLT; 09/2005-08/2008).
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