Determining how interactions of nighttime warming, winter precipitation and N deposition affect shrub establishment and the abundance of dominant grasses in grassland ecosystems.
Globally, average temperatures have increased by 0.6 ºC and N fixation has more than doubled since pre-industrial times. Also, regional climate is becoming more variable, including the possibility of more intense, longer-lasting heat waves. Warming and N deposition have already altered plant communities, decreasing diversity and changing species phenology and ranges. Temperature and N-deposition increases are expected to have dramatic impacts on aridland ecosystems over the next century. Although much work has been conducted on individual effects of these changes, research in our LTER IV will address the interactive effects of these drivers on community dynamics and ecosystem processes.
Climate change coupled with increased resource availability alters plant growth and potentially drives long-term shifts in community composition and ecosystem processes. The primary goal of our climate and resource manipulation experiment is to determine how interactions of nighttime warming, winter precipitation and N deposition affect shrub establishment and the abundance of dominant grasses in grassland ecosystems. We will conduct a multi-factorial, fully crossed (N=6 plots per treatment) field experiment combined with simulation model analyses to determine the direct and interactive effects of increased nighttime temperatures, N deposition, and winter precipitation on grassland community composition and the growth of experimentally introduced creosote seeds and seedlings. This research will directly address our mechanistic model of patch structure and dynamics, with treatments designed to mimic expected conditions in the latter half of this century. Although warming should favor southern species (black grama, creosote), N deposition and increased winter precipitation may favor northern species (blue grama). How these contrary forces will interact to determine community composition is unknown. Ultimately, this field experiment will investigate how climate change and resource availability will affect the transition of C4-dominated grassland to C3-dominated creosote shrubland.
The nighttime warming treatment is imposed using lightweight, aluminum-fabric shelters mounted on rollers similar to a window shade (Fig) that are drawn across the warming plots each night to reduce outgoing longwave radiation. Data loggers control shelter movement 1) at night and in the morning, 2) when wind speeds exceed a threshold value (to prevent damage to shelters), and 3) when rain or snow is detected by a leaf-wetness sensor (to prevent an unintended rainout effect). Air and soil temperature are measured in each plot with thermocouples at 10 and 0 cm above and 5 cm below the soil surface in grass-covered and bare soil areas. Initial funding for this project comes from an NSF Ecology award (PIs: Fargione, Collins, Pockman).
Based on long-term climate records, El Niño rains increase winter precipitation in our area by an average of 50%. We will impose an El Niño-like rainfall regime using an irrigation system and RO water (however, this year with intense El Nino activity, we are holding off on this treatment until next year). Similar irrigation systems have been installed in two other water addition experiments at the Sevilleta. El Niño rains will be added in 6 experimental storm events that mimic actual El Niño winter-storm event size and frequency. During El Niño years, we will use ambient rainfall and not impose experimental rainfall events. Soil moisture in each plot is measured at 5 and 15 cm beneath grass and bare soil using soil moisture probes buried horizontal to the surface. Predawn leaf water potential of blue grama, black grama, and creosote is measured in spring and summer of each year using a Scholander pressure chamber to provide a measure of plant water status that should reflect the integrated effect of our treatments on each species. To assess whether nighttime transpiration is altering predawn water potential, we are making spot measurements of transpiration by each species around the time of water potential sampling using a LiCor LI-6400 portable gas exchange system.
For N deposition, we add 20 kg ha-1 yr-1 of N in the form of NH4NO3 because NH4 and NO3 contribute approximately equally to N deposition at SNWR (57% NH4 and 43% NO3; Baez et al., 2006b). To mimic current patterns of N deposition, N is applied in two pulses of equal magnitude, one at the beginning of each growing season (late February and early July). We selected this amount because it is a plausible rate of N deposition for many arid systems by the end of this century, and because preliminary results suggest that it is sufficient to induce a vegetation response in at least some species. Soil N is measured using ion-exchange resins placed under grass and bare areas in each plot and collected at the end of the monsoon season (Sept.) and at the end of spring (June).
Biomass of herbaceous species is measured allometrically three times each year in two 1-m2 permanent subplots in each treatment plot. These measurements also yield species composition data. To assess the effect of our treatments on the ability of creosote to establish among grassland vegetation, we measure the germination of seeds placed in our experimental plots as well as the survival and growth of greenhouse-germinated seedlings planted in a small sub-plot in each treatment plot. Together, these measures will allow us to determine how environmental change will affect dominance, patch structure, community dynamics, and ecosystem processes in this desert grassland. We also will use our ECOTONE model to simulate the direct and interactive effects of increased nighttime temperatures, N deposition, and winter precipitation on grassland community composition and the invasion by creosote. We will parameterize ECOTONE using data from our field experiment. Model results will be tested by simulating changes in species composition along our long-term transects for historic time periods with different amounts and seasonality of rainfall. We will then use ECOTONE to predict long-term consequences of changes in nighttime temperatures, N deposition, and seasonality of precipitation to plant community composition and creosote invasion. We also will examine the effects of alternative rainfall, temperature, and N deposition scenarios. For example, we expect that increases in monsoonal rainfall will favor grasses compared with increases in winter precipitation that should favor creosote. Furthermore, increases in spring rainfall and N deposition should favor blue grama. Increases in summer rainfall with no change in N deposition should favor black grama. Sensitivity analyses will be conducted to determine the relative importance of different abiotic (e.g., fire) and biotic drivers (e.g., creosote invasion) to species composition.