Ecosystem CO2 exchange
During the peak of the growing season (August 2018), we used a Li-Cor 7500 open-path infrared gas analyzer (IRGA; Li-Cor, Lincoln, Nebraska, USA) to measure changes in CO2 concentration and air temperature in the headspace of a chamber placed over the plots. We placed the IRGA inside a 1m3 cube that was made from a polyvinyl chloride (PVC) frame and transparent 6-mil plastic sheeting (Polar Plastics Inc., Oakdale, MN, USA), with internal fans to ensure adequate mixing of air, sealed to the ground with two 1.27 cm diameter × 3.04 m length steel chains wrapped around the base of the chamber. Each measurement cycle began by lowering the chamber, sealing it, and once a consistent rate of CO2 exchange was achieved (typically less than a minute), we began logging a two minute flux measurement in the light at a sampling frequency of 0.5 Hz. At time of flux measurement, we also measured photosynthetically active radiation (PAR) using a MQ-100 Apogee PAR meter (Apogee Instruments, Logan, UT, USA) that was mounted to the top of the IRGA just below upper surface of the chamber. Full light measurements were recorded in a minimum of 900 µmol m-2 s-1 between the hours of 0930 and 1530. For each full light measurement, we calculated net ecosystem exchange (NEE; µmol m s) using Eqn 1 to quantify the maximum continuous exchange of CO2 between the atmosphere, vegetation and soil.
NEE = (ρ*V *(dC/dt)/A) (1)
In Eqn 1, ρ is the air density (mol air m-3), defined as P/RT , where P is the average pressure (Pa),R is the ideal gas constant (8.314 J mol-1 air K-1), and T (K) is the mean temperature.V is the chamber volume (1 m3), dC/dt is the slope of the chamber CO2 concentration against time (µmol CO2 mol air s), and A is the surface area of the ground (1 m2) within the chamber.
In order to calculate standardized NEE values to a fixed light level (PAR = 800 µmol m-2 s-1) and to partition flux measurements between ER and GPP, we used garden shade cloth (Agfabric, Wellco Industries Inc., Corona, CA, USA) to reduce light penetrating our chamber in order to create an NEE light response curve (Lasslop et al. 2010; Williams et al. 2014). Each shade cloth reduced light availability by 50% within chamber. We measured continuous CO2 exchange over two minute intervals with a single layer of garden shade cloth (~50% of ambient light), as well as two layers of shade cloth (~25% of ambient light). Finally, we used a 100% light blocking tarp to measure carbon flux at 0 µmol m-2 s-1 PAR (i.e. Ecosystem Respiration, Re). From the four light measurements (Full sun, half-light, quarter-light, and no light), we fit both hyperbolic and linear functions to predict NEE at a standardized light of PAR = 800 µmol m-2 s-1(NEE800). We compared r-squared values for these two fits and found that the hyperbolic function always fit better. Negative NEE values indicate fluxes from the atmosphere to the ecosystem,i.e. net carbon uptake, and positive values indicate net fluxes to the atmosphere from the ecosystem, i.e. net carbon emissions. NEE values, standardized by light, were used to compare NEE fluxes across subplots and plots. Next, we calculated GPP (Lasslop et al. 2010) by subtracting Re from the standardized NEE values (Eqn 2).
Gross Primary Productivity (GPP) = NEE800 – Re (2)
To account for differences in plant biomass among treatments, we also calculated NEE, Re, and GPP that were standardized per gram of total plant biomass (i.e., sum of above and below-ground biomass). In early August 2018, we estimated above- and below-ground plant biomass in all subplots. To estimate aboveground plant biomass we clipped vegetation in two 0.1 × 1m strips, dried and weighed this vegetation to the nearest 0.001 g; to estimate root biomass, we took 3 soil cores of 5 cm diameter and 30 cm depth, sieved all roots, washed and dried them, and then weighed them to the nearest 0.001 g. By standardizing carbon flux measurements to units of plant biomass, we were able to parse differences in carbon fluxes (NEE, GPP, and Re) between different subplots and plots caused due to differences in plant biomass from those driven by other factors that mediate carbon fluxes (e.g., plant trait shifts, plant community composition, microbial community changes, etc.). Mass-specific NEE (NEEmsp) was calculated as in Eqn 3.
NEEmsp = NEE800 / Total plant biomass (g-1 m-2) (3)