Objective:
The question addressed by WENNDEx is, How do multiple global change drivers interact to affect the rate at which species reordering occurs, and what changes in ecosystem functioning are associated with this reordering? Answers to this question can elucidate how ecological presses interact to shift species dominance and affect sensitivity of key components of the C cycle to global change drivers and natural, interannual variation in climate. WENNDEx was built in 2006 with funding from NSF-DEB Ecology Program; operational support is now provided by the NSF LTREB program, in addition to LTER.
Multiple global change drivers will impact community and ecosystem structure and function during the next century. Many of these environmental drivers are subtle and persistent presses, such as changing precipitation regimes, increasing temperature, or atmospheric N deposition, with impacts that accumulate over time. These chronic environmental drivers directly and indirectly alter resource availability and interspecific interactions. Predicting how multiple global change drivers will interact is challenging because the effects of co-occurring environmental changes may or may not be additive, and these effects can change or cancel out over decadal time periods. The purpose of WENNDEx is to better understand the potential interactions among environmental changes for grassland community composition, ecosystem processes, and the recruitment of creosotebush seeds and seedlings to estimate the probability of a future state transition to a shrubland ecosystem.
Novelty:
Major changes in terrestrial ecosystems include warming, nitrogen deposition, and altered precipitation regimes. Most field experiments focus on single factors, ignoring the potential for interactions among environmental change factors. Tests of interactions among co-occurring environmental changes are critical to predicting our future and improving management strategies. Over the long-term, we can also evaluate the potential for treatments to interact with increases in both aridity and interannual precipitation variability because this experiment occurs in a highly variable, and drying background climate.
Design:
This multi-factorial experiment has three fully crossed factors: nighttime warming, winter water addition, and nitrogen addition in a completely randomized design for a total of eight treatment combinations. There are five replicates for each of the eight treatment combinations, for a total of 40 plots. Plots are 3 m x 3.5 m. All replicates contain both blue grama and black grama grass. Nighttime warming is imposed on the full plot using lightweight aluminum fabric blankets that are drawn across each warmed plot at night to trap outgoing longwave radiation. Dataloggers controlling shelter movements retract the blankets when wind speeds exceed a threshold (to prevent damage) and when rain or snow occurs. Based on long-term climate records, El Niño rains increase average winter precipitation in our area by 50%, and more intense El Niño events are predicted by climate models. During El Niño winters only, we supplement ambient winter precipitation using an irrigation system and reverse osmosis (RO) water. Rain is added in six experimental events during treatment years (January-March) to mimic actual El Niño winter-storm event size distribution (four 5 mm events, one 10 mm event, and one 20 mm event each winter) and amount (50 mm). Using a backpack sprayer we add 2 g N m-2 y-1 as NH4NO3 prior to the monsoon season because NH4-N (57%) and NO3-N (43%) contribute approximately equally to N deposition at SNWR. Control plots receive the same amount of RO water.
Burn: On August 4, 2009, a lightning-initiated fire began on the Sevilleta National Wildlife Refuge. By August 5, 2009, the fire had reached the WENNDEx site, which was burned extensively though not entirely. Approximately 50% of plots burned on August 5, and those plots which did not burn were burned within three weeks by US Fish and Wildlife Service. Thus, the condition of all plots at the WENNDEx site was comparable by early September 2009.
Responses:
Aboveground net primary production (NPP) and plant species composition are measured allometrically in two 1-m2 permanent subplots in each treatment plot, twice yearly. We measure temperature in all plots at two soil depths (5 cm, 10 cm) and 20 cm aboveground with Campbell Scientific CS107 temperature probes. Soil moisture is measured in each plot using Campbell Scientific CS-616 probes buried at a 45o angle to obtain an integrated measure of moisture in the top 20 cm. Moisture and temperature probes take a reading every 15 minutes. Soil N availability is measured during the monsoon (July – Sept) during some years using Plant Root Simulator probes (WesternAg Industries, Saskatoon, Canada) placed in either blue grama or black grama patches in each plot. Following the fire, we began measurements of soil respiration in a subset of treatments (control, warmed, winter rain, and warmed plus winter rain) measured at 15 min intervals with Vaisala CARBOCAP CO2 sensors (GMM222, Vaisala, Helsinki, Finland) placed at three depths (2, 8, and 16 cm), and with sensors under the canopy of either blue grama or black grama.
We focus on three dominant plant species, all of which are near their range margins, and thus may be particularly susceptible to interacting environmental changes. We hypothesize that warmer summer temperatures and increased evaporation will favor growth of black grama (Bouteloua eriopoda), a desert grass, but that increased winter precipitation and/or available nitrogen will favor the growth of blue grama (Bouteloua gracilis), a shortgrass prairie species. Furthermore, growth and survival of the native shrub creosote (Larrea tridentata, added as seeds and seedlings) may be promoted by heightened winter precipitation, N addition, and/or warmer nighttime temperatures. Treatment effects on limiting resources (soil moisture, nitrogen mineralization), species abundance, and above- and belowground net primary production (NPP) are all being measured to determine the interactive effects of key global change drivers on arid grassland plant community dynamics.
Supporting Documents:
Collins, S. and W. Pockman. 2020. Warming-El Nino-Nitrogen Deposition Experiment (WENNDEx): Soil Nitrogen Data from the Sevilleta National Wildlife Refuge, New Mexico (2006 – 2020). https://portal.edirepository.org/nis/mapbrowse?scope=knb-lter-sev&identifier=307
Warming-El Nino-Nitrogen Deposition Experiment (WENNDEx): Net Primary Production Quadrat Data at the Sevilleta National Wildlife Refuge, New Mexico. https://portal.edirepository.org/nis/mapbrowse?scope=knb-lter-sev&identifier=176
Warming-El Nino-Nitrogen Deposition Experiment (WENNDEx): Soil Temperature, Moisture, and Carbon Dioxide Data from the Sevilleta National Wildlife Refuge, New Mexico. https://portal.edirepository.org/nis/mapbrowse?scope=knb-lter-sev&identifier=305
Collins, S.L., J.E. Fargione, C.L. Crenshaw, E. Nonaka, J.R. Elliott, Y. Xia and W.T. Pockman. 2010. Rapid plant community responses during the summer monsoon to nighttime warming in a northern Chihuahuan Desert grassland. Journal of Arid Environments 74: 611-617. https://doi.org/10.1016/j.jaridenv.2009.10.005
Collins, S.L., L.M. Ladwig, M.D. Petrie, S.K. Jones, J.M. Mulhouse, J Thibault and W.T. Pockman. 2017. Press-pulse interactions: Effects of warming, N-deposition, altered winter precipitation and fire on desert grassland community structure and dynamics. Global Change Biology 23: 1095-1108. https://doi.org/10.1111/gcb.13493