B54B-01
Warming and Carbon Dioxide Enrichment Alter Plant Production and Ecosystem gas Exchange in a Semi-Arid Grassland Through Direct Responses to Global Change Factors and Indirect Effects on Water Relations
The Prairie Heating and CO2 Enrichment (PHACE) experiment was initiated in Spring, 2007 to evaluate the combined effects of warming and elevated CO2 on a northern mixed-grass prairie. Thirty 3-m diameter circular experimental plots were installed in Spring, 2006 at the USDA-ARS High Plains Grasslands Research Station, just west of Cheyenne, WY, USA. Twenty plots were assigned to a two-level factorial combination of two CO2 concentrations (present ambient, 380 ppmV; and elevated, 600 ppmV), and two levels of temperature (present ambient; and elevated temperature, 1.5/3.0 C warmer day/night), with five replications for each treatment. Five of the ten remaining plots were subjected to either frequent, small water additions throughout the growing season, and the other five to a deep watering once or twice during the growing season. The watering treatments were imposed to simulate hypothesized water savings in the CO2-enriched plots, and to contrast the influence of variable water dynamics on ecosystem processes. Carbon dioxide enrichment of the ten CO2- enriched plots is accomplished with Free Air CO2 Enrichment (FACE) technology and occurs during daylight hours of the mid-April – October growing season. Warming is done year-round with circularly-arranged ceramic heater arrays positioned above the ring perimeters, and with temperature feed-backs to control day/night canopy surface temperatures. Carbon dioxide enrichment began in Spring, 2006, and warming was added in Spring, 2007. Results from the first year of CO2 enrichment (2006) confirmed earlier reports that CO2 increases productivity in semi-arid grasslands (21% increase in peak seasonal above ground biomass for plants grown under elevated CO2 compared to non-enriched controls), and that the response was related to CO2- induced water savings. Growth at elevated CO2 reduced leaf carbon isotope discrimination and N concentrations in plants compared to results obtained in control plots, but the magnitude of changes were highly species specific. Ecosystem-level gas exchange measurements indicated that interactions between watering and CO2 enrichment increased C cycling over a range of soil moisture conditions, although watering had a greater relative impact on C fluxes than CO2 enrichment. Results from the combined warming and CO2 enrichment experiment in 2007 indicate soil fluxes of CO2 increased with elevated CO2 and warming, but decreased with warming later in the year compared to un-heated controls. Soil CH4 uptake was enhanced by elevated CO2 but reduced by warming, particularly later in the year. Soil fluxes of N2O were unaffected by treatment. These preliminary results indicate potentially strong feedbacks between carbon cycling and warming are mediated by ecosystem processes in this semiarid rangeland.
B54B-02
CLIMAITE – a three factor climate change ecosystem manipulation experiment.
The Danish multi factorial climate change effects on vegetation experiment (Climaite) have now been conducted for two years on semi-natural grassland. The day time [CO2], night time temperature and precipitation (drought) have been altered, according to a regional climate change model for the year 2075, in a full factorial split plot design. The manipulated area for each treatment is 7 m2 and it is replicated 6 times. The CO2 and temperature treatments have been conducted continuously except for periods with snow cover. The CO2 is enhanced to 510 ppm via a FACE system based on concentrated CO2 released upwind under pressure. The control of the [CO2] varies with wind speed and irradiation, but during 50 percent of the fumigation period the target concentration was kept within +/-5 percent. The temperature treatment is conducted via infrared reflective curtains covering the plots during night time, and the warming of plants and soil depends of the day irradiation, night time wind speed and factors related to seasonality. In general, the air temperature is increased during night time with 1-2 C° and negligible during the day. The soil temperature in 5 cm depth is enhanced to 0.3 – 0.6 C° during night and day. The artificial summer droughts lasted about one month and differences in soil water content were developed over time. By the end of the treatment the èv content in the soil was as low as 0.06 m3 m-3 compared to 0.20 m3 m-3 in the control. Numerous physical and biological parameters in the grassland ecosystem have been measured and several are responding to the changed environment. After 9 months of exposure enhanced [CO2] stimulated the net photosynthesis (based on dry weight) in both of the domination plant species Calluna Vulgaris and Deschampsia flexuosa. When the plants were exposed to short term saturated [CO2] during gasexchange measurements the long term CO2 treated plants also had the highest photosynthesis rate, meaning that the plants were not physiological down regulated. Calluna Vulgaris exposed to both enhanced [CO2] and temperature in combination with drought showed a lowered photosynthesis during short term saturated [CO2] gasexchange measurements. After one and a half year of manipulation the xylem water potential in the plants were measured during drought treatment. Drought treatment generally decreased the xylem water potential, and the enhanced [CO2] diminished the drought effect for both of the species except for Calluna exposed to all treatments in combination, here the xylem water potential dropped to the lowest level measured. http://www.climaite.dk
B54B-03 INVITED
Linking the Response of Annual Grasslands to Warming and Altered Rainfall Across Scales of Gene Expression, Species, and Ecosystem
Climate change can influence terrestrial ecosystems at multiple biological levels: gene expression, species, and ecosystem. We are studying California grassland mesocosms with seven annual species (five grasses, two forbs) that were started in 2005. In the 2006-2007 growing season, they were exposed to three rainfall treatments (297, 552, and 867 mm y-1) and soil and air temperature (ambient and elevated +4oC) in replicated greenhouses. This presentation will combine plant and ecosystem level results with transcript level analyses associated with key enzymes, such as rubisco and glutamine synthetase (GS). Because rainfall is the dominant climate variable for most processes in this Mediterranean ecosystem, the effect of warming was strongly mediated by rainfall. In fact, we saw significant interactions between temperature and rainfall treatments at all three biological levels. For example, at the ecosystem level, warming led to a decrease in aboveground and total NPP under low rainfall, and an increase under high rainfall. For the dominant species, Avena barbata, warming had no effect under high rainfall, but suppressed Avena NPP in low rainfall. At the same time, warmer, wetter conditions accelerated Avena flowering by almost 15 days. This shift in phenology was presaged by observations at the transcript level. Specifically, in the high temperature, high rainfall treatment, the levels of mRNAs for RbcS and GS2 (encoding the small subunit of rubisco and the chloroplastic isoform of GS, respectively) declined while GS1 (encoding the cytosolic isoform of GS) was upregulated several weeks before heading. The transcript level response (along with soil and plant nitrogen data) indicated the leaf had switched from a carbon and nitrogen sink to a source - consistent with more mature plant function and earlier flowering. Soil CO2 respiration also showed strong rain-by-temperature interactions that were due mainly to changes in root response (respiration and/or exudates) rather than in microbial respiration. Overall, the pervasive rain-by-temperature interactions mean that it may be very difficult to predict the effect of warming alone, without accounting for changes in precipitation (in our Mediterranean system). While predictions of warming of 3-6°C in the next 100 years are fairly certain, changes in precipitation are much more uncertain, with some forecasts drier and others wetter for a given location. We suggest that uncertainty about future precipitation and the interacting influences of temperature and moisture on ecosystems are currently key limitations in predicting ecosystem response to climate change, particularly in Mediterranean ecosystems such as the one studied here.
B54B-04
Effects of long-term increases in atmospheric CO2 concentration, precipitation, temperature and N deposition on gross N mineralization and nitrification rates in California annual grassland soil
We have studied the effects of long-term increases in atmospheric CO2 concentration, precipitation, temperature and N deposition on soil inorganic N content, and gross and net N transformation rates in Californian grassland. We sampled soil (0-10 cm) from the Jasper Ridge Global Change Experiment plots in February and April 2005, and determined the gross N mineralization and nitrification rates using the 15N isotope dilution methodology. Statistical analysis revealed significant interactions between multiple climate change factors, making mechanistic interpretations difficult. Consistent effects of N deposition and an interaction between atmospheric CO2 concentration and temperature treatments on N cycling were observed. These results demonstrate the importance of studying interactions between multiple climate change factors in order to provide realistic predictions of C and N cycling under realistic future environmental conditions.
B54B-05
Patterns of Nonlinearity in Ecosystem Carbon and Water Dynamics in response to Gradual Changes in Temperature, CO2, and Precipitation: Modeling Analysis
It is commonly acknowledged that ecosystem responses to global climate change are nonlinear. However, patterns of the nonlinearity have not been well characterized on ecosystem carbon and water processes. We used a terrestrial ecosystem (TECO) model to examine nonlinear patterns of ecosystem responses to changes in temperature, CO2, and precipitation individually or in combination. The TECO model was calibrated against experimental data obtained from a grassland ecosystem in central USA and ran for 100 years with gradual change at 252 different scenarios. We primarily used the 100th-year results to explore nonlinearity of ecosystem responses. Variables examined in this study are net primary productivity (NPP), heterotrophic respiration (Rh), net ecosystem carbon exchange (NEE), runoff, and evapotranspiration (ET). Our modeling results show that nonlinear patterns were parabolic, asymptotic, and threshold-like in response to temperature, CO2, and precipitation anomalies, respectively, for NPP, NEE, and Rh. Runoff and ET exhibited threshold-like pattern in response to both temperature and precipitation anomalies but were less sensitive to CO2 changes. Ecosystem responses to combined temperature, CO2, and precipitation anomalies considerably differed from the responses to individual factors in terms of response patterns and/or critical points of nonlinearity. Our results suggest that nonlinear patterns in response to multiple global change factors were diverse and were considerably affected by combined climate anomalies on ecosystem carbon and water processes. The diverse response patterns in nonlinearity have profound implications for both experimental design and theoretical development.
B54B-06
Ocean Acidification Consequences of Stabilization of Atmospheric Carbon Dioxide
We investigate ocean chemistry changes that would result from the stabilization of atmospheric carbon dioxide concentrations at different levels. To determine the fate of ocean chemistry after atmospheric carbon dioxide is stabilized, we perform a suite of simulations using the UVic Earth system model in which atmospheric CO2 is stabilized at levels ranging from 280 ppm to 5000 ppm. Atmospheric carbon dioxide is absorbed by the ocean, and makes the ocean more acidic (lowers ocean pH), decreasing carbonate-ion concentrations. These changes in ocean chemistry have the potential to significantly affect marine organisms. For example, a decrease in the saturation state of calcium carbonate minerals (aragonite and calcite) associated with the decrease in carbonate ion concentration will pose a great threat to the growth of calcifying organisms such as reef-building corals and pteropods. Before the industrial revolution, over 99 per cent of warm water coral reefs were bathed with open ocean waters with aragonite saturation greater than 3.25. If atmospheric carbon dioxide concentrations stabilize at 550 ppm, only 2 per cent of existing coral reefs will be in such environments. Even with atmospheric CO2 stabilization at 450 ppm, parts of the Southern Ocean will become undersaturated with respect to aragonite, causing the shells of pteropods to dissolve. At 450 ppm, about 10 per cent of the global ocean will have experienced a pH reduction greater than 0.2 units, violating US EPA water quality criteria for pH changes in open ocean waters. These changes in ocean chemistry are largely independent of the amount of climate change. Thus, consideration of biological consequences of ocean chemistry changes may favor lower atmospheric CO2 stabilization targets than might be selected based on consideration of climate change consequences alone.
B54B-07
Thresholds in conservation effectiveness under climate change
We describe a new approach to conservation that derives from the effect of energy on ecosystem properties and then evaluate potential threshold responses of conservation effectiveness under climate change. Many tests of species energy theory provide evidence that species richness varies with measures of energy such as net primary productivity (NPP). Across continents, this relationship is most often unimodal, with species richness peaking in intermediate energy places and decreasing at higher NPP levels. NPP also influences ecosystem response to habitat fragmentation, recovery after disturbance, and trophic relationships. We have developed a topology for conservation whereby conservation priorities and management effectiveness differ among low, intermediate, and high energy ecosystems. We evaluated the projected change in NPP and conservation topology of ecoregions under future climate change scenarios. We found that projected NPP under climate change caused a subset of ecoregions to shift across the peak of the unimodal species energy curve, suggesting dramatic changes in conservation effectiveness are possible.