John A. Arnone

Divergence of reproductive phenology under climate warming

Because the flowering and fruiting phenology of plants is sensitive to environmental cues such as temperature and moisture, climate change is likely to alter community-level patterns of reproductive phenology. Here we report a previously unreported phenomenon: experimental warming advanced flowering and fruiting phenology for species that began to flower before the peak of summer heat but delayed reproduction in species that started flowering after the peak temperature in a tallgrass prairie in North America. The warming-induced divergence of flowering and fruiting toward the two ends of the growing season resulted in a gap in the staggered progression of flowering and fruiting in the community during the middle of the season. A double precipitation treatment did not significantly affect flowering and fruiting phenology. Variation among species in the direction and magnitude of their response to warming caused compression and expansion of the reproductive periods of different species, changed the amount of overlap between the reproductive phases, and created possibilities for an altered selective environment to reshape communities in a future warmed world.

Prolonged suppression of ecosystem carbon dioxide uptake after an anomalously warm year

Terrestrial ecosystems control carbon dioxide fluxes to and from the atmosphere through photosynthesis and respiration, a balance between net primary productivity and heterotrophic respiration, that determines whether an ecosystem is sequestering carbon or releasing it to the atmosphere. Global and site-specific data sets have demonstrated that climate and climate variability influence biogeochemical processes that determine net ecosystem carbon dioxide exchange (NEE) at multiple timescales. Experimental data necessary to quantify impacts of a single climate variable, such as temperature anomalies, on NEE and carbon sequestration of ecosystems at interannual timescales have been lacking. This derives from an inability of field studies to avoid the confounding effects of natural intra-annual and interannual variability in temperature and precipitation. Here we present results from a four-year study using replicate 12,000-kg intact tallgrass prairie monoliths located in four 184-m3 enclosed lysimeters. We exposed 6 of 12 monoliths to an anomalously warm year in the second year of the study and continuously quantified rates of ecosystem processes, including NEE. We find that warming decreases NEE in both the extreme year and the following year by inducing drought that suppresses net primary productivity in the extreme year and by stimulating heterotrophic respiration of soil biota in the subsequent year. Our data indicate that two years are required for NEE in the previously warmed experimental ecosystems to recover to levels measured in the control ecosystems. This time lag caused net ecosystem carbon sequestration in previously warmed ecosystems to be decreased threefold over the study period, compared with control ecosystems. Our findings suggest that more frequent anomalously warm years, a possible consequence of increasing anthropogenic carbon dioxide levels, may lead to a sustained decrease in carbon dioxide uptake by terrestrial ecosystems.

The responses of alpine grassland to four seasons of CO2 enrichment: a synthesis

Abstract Alpine grassland at 2 470 m altitude in the Swiss Central alps was exposed to elevated CO2 by using open top chambers (16 ambient, 16 elevated CO2). Some plots received mineral fertilizer at a rate of N-deposition commonly measured in low altitude parts of Europe. Here we present a summary of results and data from the final harvest. Above-ground biomass measured after the completion of growth in the fourth season of treatment was not affected by CO2 enrichment as was found by previous biometric estimates, but mean below-ground biomass was slightly stimulated (+ 12%, n.s.). In contrast, net CO2 uptake per unit land area was strongly stimulated by CO2 enrichment at the beginning of the experiment, and during the early part of each season. However, the CO2 stimulation decreased during the later part of each growing season. By year four, also mid-season differences in CO2 uptake per unit land area had disappeared. Neither microbial biomass, soil respiration in the laboratory, nor in situ land-area-based CO2 evolution during the 10 week growing season increased under elevated CO2. The total biomass N-pool and free soil nitrate and ammonium (capture by ion exchange resin bags) remained unaffected, whereas leaf nitrogen concentration was reduced and nonstructural carbohydrate concentration increased under elevated CO2 in forbs. These differences in tissue composition largely disappeared during senescence and litter formation. Despite low CO2 responsiveness at ecosystem level, species responses differed in terms of nitrogen, carbohydrates, tillering and flowering, suggesting the possibility for long-term changes in community structure. Addition of NPK equivalent to 40 kg N ha−1 a−1 had massive effects on all plant traits studied, but did not enable stimulated growth under CO2 enrichment. However, when fertilizer and CO2 enrichment were provided jointly, soil microbes were stimulated, indicating a co-limitation by carbon and nutrients (most likely nitrogen). Since responses to elevated CO2 were absent in both warm and cold growing seasons, we conclude that this late successional plant community is carbon saturated at current atmospheric CO2 concentrations for reasons not directly related to nutrient supply and climate. Perhaps, contrary to our expectation, evolutionary adjustments of this “old” ecosystem to the life conditions at high altitudes caused carbon to become a surplus resource today.

Stimulated N2O flux from intact grassland monoliths after two growing seasons under elevated atmospheric CO2

Abstract Long-term exposure of native vegetation to elevated atmospheric CO2 concentrations is expected to increase C inputs to the soil and, in ecosystems with seasonally dry periods, to increase soil moisture. We tested the hypothesis that these indirect effects of elevated CO2 (600 μl l−1 vs 350 μl l−1) would improve conditions for microbial activity and stimulate emissions of nitrous oxide (N2O), a very potent and long-lived greenhouse gas. After two growing seasons, the mean N2O efflux from monoliths of calcareous grassland maintained at elevated CO2 was twice as high as that measured from monoliths maintained at current ambient CO2 (70 ± 9 vs 37 ± 4 μg N2O m−2 h−1 in October, 27 ± 5 vs 13 ± 3 μg N2O m−2 h−1 in November after aboveground harvest). The higher N2O emission rates at elevated CO2 were associated with increases in soil moisture, soil heterotrophic respiration, and plant biomass production, but appear to be mainly attributable to higher soil moisture. Our results suggest that rising atmospheric CO2 may contribute more to the total greenhouse effect than is currently estimated because of its plant-mediated effects on soil processes which may ultimately lead to increased N2O emissions from native grasslands.

Activity of surface-casting earthworms in a calcareous grassland under elevated atmospheric CO2

Abstract Earthworms make up the dominant fraction of the biomass of soil animals in most temperate grasslands and have important effects on the structure and function of these ecosystems. We hypothesized that the effects of elevated atmospheric CO2 on soil moisture and plant biomass production would increase earthworm activity, expressed as surface cast production. Using a screen-aided CO2 control facility (open top and open bottom rings), eight 1.2-m2 grassland plots in Switzerland have been maintained since March 1994 at ambient CO2 concentrations (350 μl CO2 l−1) and eight at elevated CO2 (610 μl CO2 l−1). Cumulative earthworm surface cast production measured 40 times over 1 year (April 1995–April 1996) in plots treated with elevated CO2 (2206 g dry mass m−2 year−1) was 35% greater (P<0.05) than that measured in plant communities maintained at ambient CO2 (1633 g dry mass m−2 year−1). At these rates of surface cast production, worms would require about 100 years to egest the equivalent of the amount of soil now found in the Ah horizon (top 15 cm) under current ambient CO2 concentrations, and 75 years under elevated CO2. Elevated atmospheric CO2 had no influence on the seasonality of earthworm activity. Cumulative surface cast production measured over the 7-week period immediately following the 6-week summer dry period in 1995 (no surface casting) was positively correlated (P<0.05) with the mean soil water content calculated over this dry and subsequent wetter period, when viewed across all treatments. However, no correlations were observed with soil temperature or with annual aboveground plant biomass productivity. No CO2-related differences were observed in total nitrogen (Ntot) and organic carbon (Corg) concentration of surface casts, although concentrations of both elements varied seasonally. The CO2-induced increase in earthworm surface casting activity corresponded to a 30% increase of the amount of Ntot (8.9 mg N m−2 vs. 6.9 mg N m−2) and Corg (126 mg C m−2 vs. 94 mg C m−2) egested by the worms in one year. Thus, our results demonstrate an important indirect stimulatory effect of elevated atmospheric CO2 on earthworm activity which may have profound effects on ecosystem function and plant community structure in the long term.

Concurrent and lagged impacts of an anomalously warm year on autotrophic and heterotrophic components of soil respiration: a deconvolution analysis.

*Partitioning soil respiration into autotrophic (R(A)) and heterotrophic (R(H)) components is critical for understanding their differential responses to climate warming. *Here, we used a deconvolution analysis to partition soil respiration in a pulse warming experiment. We first conducted a sensitivity analysis to determine which parameters can be identified by soil respiration data. A Markov chain Monte Carlo technique was then used to optimize those identifiable parameters in a terrestrial ecosystem model. Finally, the optimized parameters were employed to quantify R(A) and R(H) in a forward analysis. *Our results displayed that more than one-half of parameters were constrained by daily soil respiration data. The optimized model simulation showed that warming stimulated R(H) and had little effect on R(A) in the first 2 months, but decreased both R(H) and R(A) during the remainder of the treatment and post-treatment years. Clipping of above-ground biomass stimulated the warming effect on R(H) but not on R(A). Overall, warming decreased R(A) and R(H) significantly, by 28.9% and 24.9%, respectively, during the treatment year and by 27.3% and 33.3%, respectively, during the post-treatment year, largely as a result of decreased canopy greenness and biomass. *Lagged effects of climate anomalies on soil respiration and its components are important in assessing terrestrial carbon cycle feedbacks to climate warming.

Interactions between plant species and earthworm casts in a calcareous grassland under elevated CO2

We tested the hypothesis that the spatial proximity of a plant species to nutrient-rich earthworm casts (e.g., 100% more ammonium and 30% more phosphate than in adjacent soil) is an important determinant of a plant’s responsiveness to elevated atmospheric CO2. In 1995 we mapped the location of both earthworm surface casts and plants in each of 16 1.2-m2 plots in a species-rich calcareous grassland in northwestern Switzerland. Eight plots have been maintained under current ambient CO2 concentrations (350 μL CO2/L), and eight have been maintained at elevated CO2 (600 μL CO2/L) since March 1994. In addition, total ramet production of each species, as a measure of performance, and cumulative cast production at each location (cell) were recorded at peak community biomass in 1995. Plant species within functional groups (graminoids, non-legume forbs, and legumes) differed markedly in their degree of association with casts; however, after two growing seasons elevated CO2 had no effect on plant species or functional group associations with casts. No statistically significant relationship could be demonstrated between plant-species response (i.e., ramet production) to elevated CO2 and the degree of association with casts within any of the functional groups. However, a positive relationship was observed between the mean response of graminoid species to elevated CO2 (measured as the percentage change in mean total ramet production of graminoid species, relative to mean total ramet production at ambient CO2) and their mean degree of association (%) with surface casts at ambient CO2. Thus, graminoid species more frequently associated with casts (e.g., Anthoxanthum odoratum and Carex caryophyllea) produced more ramets per square meter at elevated CO2 than those less frequently associated with casts (e.g., Agrostis tenuis and Danthonia decumbens). These results, along with the strong and significant positive correlations observed between ramet production and associated cumulative cast mass across CO2 treatments for most plant species in all functional groups demonstrate: (1) that plant species differ significantly in their degree of association with nutrient-rich earthworm surface casts, regardless of the relative abundance of plant species in the community; (2) that graminoid species that are more highly associated with casts may respond more strongly to rising CO2 than those less highly associated with casts; and (3) that nutrient-rich earthworm casts stimulate the growth (ramet production) of most plant species in these grassland communities, even at current levels of atmospheric CO2. The data further suggest that these species-specific relationships between plants and casts have helped define the current structure of these highly diverse grassland communities and will likely influence their future structure as global CO2 levels continue to rise.

Earthworm and soil moisture effects on the productivity and structure of grassland communities

Abstract The objectives of this study were (1) to evaluate the effect of earthworm activity on aboveground plant biomass production of native calcareous grassland communities in NW-Switzerland and (2) to determine which plant functional types (graminoids, non-legume forbs and legumes) are most responsive as indicators of potential effects on plant community structure. Earthworm activity was manipulated in the field by creating three earthworm densities (low: 37, ambient: 114, high: 169 worms m−2) and two soil moisture conditions (ambient and 280 mm yr−1 additional rain) in 30 1×1 m2 trenched plots (to a depth of 45 cm with nylon screening). Earthworm density was censused and readjusted in the spring and autumn of 1996 and again in the spring of 1997 using the Octet electro-sampling method. Earthworm activity, measured as cumulative surface cast production, was significantly different among worm density treatments (low worm density: 591±49, ambient: 991±87, high: 1469±120 g cast d.m. m−2 yr−1), P

Quantifying the effects of phenology on ecosystem evapotranspiration in planted grassland mesocosms using EcoCELL technology

Use of plant phenological variables in models predicting evapotranspiration (ET) has largely relied on relatively simple (e.g., linear) relationships which may not be sufficiently accurate to predict small—yet ecologically significant—changes in plant phenology that are expected to occur in response to global climate change. A dearth of experimental data reflects the difficulties in quantifying these relationships against the background of large environmental variability that occurs in the field. Our main objective was to quantify how plant phenology (leaf area index [LAI] and root length density [RLD]) affect ET and its components during an entire vegetation cycle in large-scale model grassland (Bromus tectorum) ecosystems using the Ecologically Controlled Enclosed Lysimeter Laboratory (EcoCELL)—a unique open flow and mass balance laboratory. We also aimed to compare the three methods employed by the EcoCELL laboratory to measure ecosystem ET (whole-ecosystem gas exchange, weighing lysimetry, and weighing lysimetry combined with time domain reflectometry [TDR]) in order to independently confirm the performance of the unique gas exchange technology. Cumulative ET during the 190 days of the experiment measured with the three different methods compared very well with each other (mean errors <1%). We found that ET reached maximum levels at relatively low LAI (2–3), but as LAI increased beyond this value, small increase in transpiration were more than offset by decreases in soil evaporation, thereby causing declines in ET. A combined rectangular hyperbola (effects on transpiration) and linear (effects on soil evaporation) function between LAI and ET accounted for almost 90% of all variability in measured daily ET. RLD showed relationships to ET similar to those observed for LAI due to high covariance between RLD and LAI, but root length densities did not explain any additional variability in daily ET beyond that explained by LAI under the well-watered conditions of the experiment. Taken together, our results show that: (i) the EcoCELL mesocosm laboratory can precisely and accurately quantify hydrologic processes of large soil–plant monoliths under controlled environmental conditions; (ii) plant canopy phenological changes affect ecosystem ET, and the contribution of transpiration, in non-linear ways; (iii) these non-linear responses must be accounted for when assessing the consequences of changes in plant phenology—e.g., due to global environmental change—on ecosystem hydrology.

Atmospheric mercury exchange with a tallgrass prairie ecosystem housed in mesocosms.

This study focused on characterizing air-surface mercury Hg exchange for individual surfaces (soil, litter-covered soil and plant shoots) and ecosystem-level flux associated with tallgrass prairie ecosystems housed inside large mesocosms over three years. The major objectives of this project were to determine if individual surface fluxes could be combined to predict ecosystem-level exchange and if this low-Hg containing ecosystem was a net source or sink for atmospheric Hg. Data collected in the field were used to validate fluxes obtained in the mesocosm setting. Because of the controlled experimental design and ease of access to the mesocosms, data collected allowed for assessment of factors controlling flux and comparison of models developed for soil Hg flux versus environmental conditions at different temporal resolution (hourly, daily and monthly). Evaluation of hourly data showed that relationships between soil Hg flux and environmental conditions changed over time, and that there were interactions between parameters controlling exchange. Data analyses demonstrated that to estimate soil flux over broad temporal scales (e.g. annual flux) coarse-resolution data (monthly averages) are needed. Plant foliage was a sink for atmospheric Hg with uptake influenced by plant functional type and age. Individual system component fluxes (bare soil and plant) could not be directly combined to predict the measured whole system flux (soil, litter and plant). Emissions of Hg from vegetated and litter-covered soil were lower than fluxes from adjacent bare soil and the difference between the two was seasonally dependent and greatest when canopy coverage was greatest. Thus, an index of plant canopy development (canopy greenness) was used to model Hg flux from vegetated soil. Accounting for ecosystem Hg inputs (precipitation, direct plant uptake of atmospheric Hg) and modeled net exchange between litter-and-plant covered soils, the tallgrass prairie was found to be a net annual sink of atmospheric Hg.