Ambient Plots Research Articles

Performance Evaluation of Three Peanut Cultivars Grown under Elevated CO2 Concentrations

This study explored the performance and physiological responses of three commercially used peanut cultivars in Australian farming systems under ambient and elevated CO2 conditions, aiming to identify the most suitable genotype for dual-purpose (grain and graze) cropping experiments. The experiment utilized an open-top chamber (OTC) facility to regulate CO2 concentrations. The elevated CO2 (EC) treatment targeted approximately 650 ± 50 µmol mol−1, while both ambient CO2 (AC) and control plots operated at a concentration of approximately 400 µmol mol−1. Notably, control plots without chambers served as a reference for current CO2 and environmental conditions. In contrast, despite having the same ambient CO2 concentration, AC plots were enclosed in chambers, allowing for plant growth comparisons with EC plots with the same environmental conditions aside from CO2 levels. The analyses revealed significant effects of CO2 enrichment on peanut plants. In particular, the EC treatment led to enhanced photosynthetic rates (20% in Kairi, 31% in Holt, and 19% in Alloway), alongside reduced stomatal conductance (−55% in Kairi, −32% in Holt, and −40% in Alloway), transpiration, and increased water use efficiency compared to AC conditions. Elevated CO2 levels positively influenced pod yields in Kairi (+41%) and Alloway (+36%). However, CO2 enrichment did not significantly alter the protein content, total phenolic content, cupric-reducing antioxidant capacity, and ferric-reducing antioxidant power of peanut plant material. Furthermore, no significant differences were observed in the phytochemical composition among the three cultivars under ambient or elevated CO2 conditions. On the other hand, analysis of the fibre structure conducted on peanut stover harvested at plant maturity suggested potential declines in feedstock quality. Based on the findings of this research, further investigations and testing, including simulated grazing trials, will be carried out to identify a single breed line suitable for dual-purpose management under future elevated CO2 conditions.

Individual vital rates respond differently to local‐scale environmental variation and neighbour removal

Abstract Understanding how plant fitness varies along natural gradients is critical for predicting responses to environmental change. However, individual vital rates are often used as fitness proxies without knowing how other vital rates vary along the same gradients. We investigated how canopy cover, plant–plant interactions, water availability and soil properties influenced the emergence, survival, seed production and population growth rates of eight annual plant species in semi‐arid Western Australia. We sowed seeds into sun‐exposed and shaded blocks across a reserve, removed all neighbouring plants from half of the interaction neighbourhoods, and used rainout shelters to reduce and increase precipitation relative to ambient plots. Canopy cover had strong negative effects on emergence, but few direct impacts on other vital rates and population growth rates. Direct competitive effects on survival and seed production were rare, although evident for population growth rate for 3/8 species. Competition was stronger in open than shaded plots for half of the species. Canopy cover also interacted with the watering treatment to influence survival of half of the species, but watering alone had few direct impacts on species' vital and population growth rates. We found only positive significant correlations between pairs of rates, and survival and seed production were far more frequently correlated with population growth rate than emergence. Synthesis. Our study illustrates that vital rates can respond to the same local‐scale environmental variation in different ways that are likely not driven by life history trade‐offs. We caution against using emergence as a proxy for population growth rate and emphasise that no single vital rate was a reliable fitness proxy overall. Interactions among abiotic and biotic factors were important drivers of vital and population growth rates for some species, highlighting the need to account for plant–plant interactions when predicting population responses to environmental change.

Carbon dioxide enrichment affected flower numbers transiently and increased successful post-pollination development stably but without altering final acorn production in mature pedunculate oak (Quercus robur L.)

Acorn production in oak (Quercus spp.) shows considerable inter-annual variation, known as masting, which provides a natural defence against seed predators but a highly-variable supply of acorns for uses such as in commercial tree planting each year. Anthropogenic emissions of greenhouse gases have been very widely reported to influence plant growth and seed or fruit size and quantity via the ‘fertilisation effect’ that leads to enhanced photosynthesis. To examine if acorn production in mature woodland communities will be affected by further increase in CO2, the contents of litter traps from a Free Air Carbon Enrichment (FACE) experiment in deciduous woodland in central England were analysed for numbers of flowers and acorns of pedunculate oak (Quercus robur L.) at different stages of development and their predation levels under ambient and elevated CO2 concentrations. Inter-annual variation in acorn numbers was considerable and cyclical between 2015 and 2021, with the greatest numbers of mature acorns in 2015, 2017 and 2020 but almost none in 2018. The numbers of flowers, enlarged cups, immature acorns, empty acorn cups, and galls in the litter traps also varied amongst years; comparatively high numbers of enlarged cups were recorded in 2018, suggesting Q. robur at this site is a fruit maturation masting species (i.e., the extent of abortion of pollinated flowers during acorn development affects mature acorn numbers greatly). Raising the atmospheric CO2 concentration by 150 μL L−1, from early 2017, increased the numbers of immature acorns, and all acorn evidence (empty cups + immature acorns + mature acorns) detected in the litter traps compared to ambient controls by 2021, but did not consistently affect the numbers of flowers, enlarged cups, empty cups, or mature acorns. The number of flowers in the elevated CO2 plots’ litter traps was greater in 2018 than 2017, one year after CO2 enrichment began, whereas numbers declined in ambient plots. Enrichment with CO2 also increased the number of oak knopper galls (Andricus quercuscalicis Burgsdorf). We conclude that elevated CO2 increased the occurrence of acorns developing from flowers, but the putative benefit to mature acorn numbers may have been hidden by excessive pre- and/or post-dispersal predation. There was no evidence that elevated CO2 altered masting behaviour.

Long-term drought promotes invasive species by reducing wildfire severity.

Anthropogenic climate change has increased the frequency of drought, wildfire, and invasions of non-native species. Although high-severity fires linked to drought can inhibit recovery of native vegetation in forested ecosystems, it remains unclear how drought impacts the recovery of other plant communities following wildfire. We leveraged an existing rainfall manipulation experiment to test the hypothesis that reduced precipitation, fuel load, and fire severity convert plant community composition from native shrubs to invasive grasses in a Southern California coastal sage scrub system. We measured community composition before and after the 2020 Silverado wildfire in plots with three rainfall treatments. Drought reduced fuel load and vegetation cover, which reduced fire severity. Native shrubs had greater prefire cover in added water plots compared to reduced water plots. Native cover was lower and invasive cover was higher in postfire reduced water plots compared to postfire added and ambient water plots. Our results demonstrate the importance of fuel load on fire severity and plant community composition on an ecosystem scale. Management should focus on reducing fire frequency and removing invasive species to maintain the resilience of coastal sage scrub communities facing drought. In these communities, controlled burns are not recommended as they promote invasive plants.

Design and Assessment of a Novel Approach for Ecosystem Warming Experiments in High‐Energy Tidal Wetlands

AbstractCoastal salt marshes have an important role in climate change adaptation and mitigation. Direct and indirect responses to warming are expected to vary along the marsh elevation gradient, making ecosystem responses to warming at this marine‐terrestrial ecotone uncertain. The Marsh Ecosystem Response to Increased Temperatures (MERIT) experiment was established in 2018 on the North Sea coast of Germany. Experimental plots are evenly distributed over three elevational marsh zones (pioneer, low marsh, and high marsh) and include three temperature treatments (ambient, +1.5°C, +3.0°C). MERIT's novel design combines active warming (horizontal surface warming cables and vertical soil warming pins) with passive, partially covered domes. For performance assessment, temperature deltas between ambient and warmed plots were calculated and evaluated at seasonal, daily, and diurnal timescales. We used Linear Mixed Models with Residual Maximum Likelihood for evaluating warming treatment effects and constraining environmental factors. MERIT was effective at ecosystem warming in this high‐energy environment both above‐ and belowground. Mixed models show that warming treatment dominates temperature differences belowground and at the soil surface, along with factors such as wind speed, flooding duration, and solar radiation. Aboveground warming was lower than belowground warming, but the dome design minimized issues seen in other open‐top chamber experiments. The combination of passive aboveground warming with feedback‐controlled active surface and belowground heating provides a setup for understanding warming effects on tidal ecosystems without altering the natural impacts of wind, radiation, and tidal inundations at high‐energy coastlines. Our design creates opportunities to expand future warming experiments to remote locations and technically challenging environments.

Soil warming duration and magnitude affect the dynamics of fine roots and rhizomes and associated C and N pools in subarctic grasslands.

The response of subarctic grassland's below-ground to soil warming is key to understanding this ecosystem's adaptation to future climate. Functionally different below-ground plant organs can respond differently to changes in soil temperature (Ts). We aimed to understand the below-ground adaptation mechanisms by analysing the dynamics and chemistry of fine roots and rhizomes in relation to plant community composition and soil chemistry, along with the duration and magnitude of soil warming. We investigated the effects of the duration [medium-term warming (MTW; 11 years) and long-term warming (LTW; > 60 years)] and magnitude (0-8.4 °C) of soil warming on below-ground plant biomass (BPB), fine root biomass (FRB) and rhizome biomass (RHB) in geothermally warmed subarctic grasslands. We evaluated the changes in BPB, FRB and RHB and the corresponding carbon (C) and nitrogen (N) pools in the context of ambient, Ts < +2 °C and Ts > +2 °C scenarios. BPB decreased exponentially in response to an increase in Ts under MTW, whereas FRB declined under both MTW and LTW. The proportion of rhizomes increased and the C-N ratio in rhizomes decreased under LTW. The C and N pools in BPB in highly warmed plots under MTW were 50 % less than in the ambient plots, whereas under LTW, C and N pools in warmed plots were similar to those in non-warmed plots. Approximately 78 % of the variation in FRB, RHB, and C and N concentration and pools in fine roots and rhizomes was explained by the duration and magnitude of soil warming, soil chemistry, plant community functional composition, and above-ground biomass. Plant's below-ground biomass, chemistry and pools were related to a shift in the grassland's plant community composition - the abundance of ferns increased and BPB decreased towards higher Ts under MTW, while the recovery of below-ground C and N pools under LTW was related to a higher plant diversity. Our results indicate that plant community-level adaptation of below ground to soil warming occurs over long periods. We provide insight into the potential adaptation phases of subarctic grasslands.

Carbon starvation following a decade of experimental drought consumes old reserves in Pinus edulis.

Shifts in the age or turnover time of non-structural carbohydrates (NSC) may underlie changes in tree growth under long-term increases in drought stress associated with climate change. But NSC responses to drought are challenging to quantify, due in part to large NSC stores in trees and subsequently long response times of NSC to climate variation. We measured NSC age (Δ14 C) along with a suite of ecophysiological metrics in Pinus edulis trees experiencing either extreme short-term drought (-90% ambient precipitation plot, 2020-2021) or a decade of severe drought (-45% plot, 2010-2021). We tested the hypothesis that carbon starvation - consumption exceeding synthesis and storage - increases the age of sapwood NSC. One year of extreme drought had no impact on NSC pool size or age, despite significant reductions in predawn water potential, photosynthetic rates/capacity, and twig and needle growth. By contrast, long-term drought halved the age of the sapwood NSC pool, coupled with reductions in sapwood starch concentrations (-75%), basal area increment (-39%), and bole respiration rates (-28%). Our results suggest carbon starvation takes time, as tree carbon reserves appear resilient to extreme disturbance in the short term. However, after a decade of drought, trees apparently consumed old stored NSC to support metabolism.

Two decades of fumigation data from the Soybean Free Air Concentration Enrichment facility

The Soybean Free Air Concentration Enrichment (SoyFACE) facility is the longest running open-air carbon dioxide and ozone enrichment facility in the world. For over two decades, soybean, maize, and other crops have been exposed to the elevated carbon dioxide and ozone concentrations anticipated for late this century. The facility, located in East Central Illinois, USA, exposes crops to different atmospheric concentrations in replicated octagonal ~280 m2 Free Air Concentration Enrichment (FACE) treatment plots. Each FACE plot is paired with an untreated control (ambient) plot. The experiment provides important ground truth data for predicting future crop productivity. Fumigation data from SoyFACE were collected every four seconds throughout each growing season for over two decades. Here, we organize, quality control, and collate 20 years of data to facilitate trend analysis and crop modeling efforts. This paper provides the rationale for and a description of the SoyFACE experiments, along with a summary of the fumigation data and collation process, weather and ambient data collection procedures, and explanations of air pollution metrics and calculations.

Microbial response to warming and cellulose addition in a maritime Antarctic soil

AbstractMaritime Antarctic King George Island (South Shetland Islands) has experienced rapid warming in recent decades, but the impacts on soil organic matter (SOM) decomposition remain ambiguous. Most vegetation cover is dominated by bryophytes (mosses), whereas a few vascular plants, such as Deschampsia antarctica and Colobanthus quitensis grow interspersed. Therefore, SOM is mainly enriched with carbohydrates and C‐alkyl, provided by mosses, which lack lignin as a precursor for aromatic compounds and humus formation. However, there is no clear answer to how substrate and temperature increase changes in Antarctic microbial respiration. We determined in what way SOM mineralization changes with temperature and substrate addition by characterizing the temperature sensitivity (Q10) of soil respiration in an open‐top chamber warming experiment. We hypothesized that: (a) cold‐tolerant microorganisms are well adapted to growing in maritime Antarctic soils (~ 0°C), so would not respond to low and moderate temperature increases because they undergo various metabolic mechanism adjustments until they experience increasing temperatures toward optimum growth (e.g., by enzyme production); and (b) cellulose, as a complex carbonaceous substrate of vegetated areas in Maritime Antarctic soils, activates microorganisms, increasing the Q10 of soil organic carbon (SOC) mineralization. Soils (5–10 cm) were sampled after four consecutive years of experimental warming for SOC composition, microbial community structure, and C mineralization at 4, 12, and 20°C with and without cellulose addition. Functional group chemoheterotrophs, represented mainly by Proteobacteria, decomposed more refractory SOC (aromatic compounds), as indicated by nuclear magnetic resonance (NMR) spectroscopy, in ambient plots than in warming plots where plants were growing. The C‐CO2 efflux from the incubation experiment remained stable below 12°C but sharply increased at 20°C. Q10 varied between 0.4 and 4 and was reduced at 20°C, whereas cellulose addition increased Q10. In conclusion, as confirmed during field studies in a climate scenario, cold‐tolerant microorganisms in maritime Antarctic soils were slightly affected by increasing temperature (e.g., 4–12°C), with reduced temperature sensitivity, as summarized in a conceptual model.

Re-visiting soil carbon and nitrogen stocks in a temperate heathland seven years after the termination of free air CO2 enrichment (FACE)

The response of soil carbon to global climate change remains one of the largest uncertainties for future climate projection. In this study, we re-sampled the soil in a long-term, field-scale, multi-factorial climate experiment, CLIMAITE (Free Air CO2Enrichment (FACE), warming and drought in all combinations in a Danish heathland ecosystem) in 2020, seven years after the experiment was terminated. We aimed to study the dynamics of the soil carbon after the cessation of long-term multi-factorial climate manipulation, with special attention to the fate of the additional soil carbon (19% increase) that was sequestered in plots exposed to elevated CO2 concentrations (eCO2). Soil carbon pools in former eCO2 plots, as well as in drought and warming plots, had normalized again by 2020. However, the difference in soil isotopic composition between ambient and former eCO2 plots remained, indicating similar loss fractions from older and newer soil carbon pools in the eCO2 plots as well as stimulation of the decomposition of old soil carbon via priming. Throughout the study period, soil nitrogen dynamics tracked the changes in soil carbon, suggesting that nitrogen from deeper soil layers was transported upwards to meet increasing plant demand during eCO2 but was lost again from the topsoil after termination of the FACE treatment. Our findings show that the soil carbon and nitrogen pools in this ecosystem are highly dynamic and may respond strongly and rapidly to changes in major ecosystem drivers, and that revisiting climate experiments after the cessation of treatments may provide valuable insights into the dynamics, stability and resilience of major element pools in ecosystems.

Prolonged drought causes negative plant-soil feedbacks in grassland species under field conditions

Climate change has been shown to alter plant-soil feedbacks (PSFs, the impacts of plants on future communities via their impacts on soil biotic and abiotic properties), with implications for plant community dynamics. However, few studies have assessed effects on PSFs under field conditions or the mechanisms involved. We assessed drought effects on PSFs of four common grassland species (Lolium multiflorum, Medicago sativa, Plantago lanceolata, and Trifolium repens) in situ in a mesic grassland under ambient rainfall and prolonged drought conditions using a long-term rainfall manipulation system. The four species were first grown in monocultures in sub-plots of each of six ambient and drought plots to condition the soils. Then, each species was grown in a sub-plot conditioned by the same species (‘home’) and a sub-plot conditioned by each of the other three species (‘away’). We found that three species (L. multiflorum, M. sativa and T. repens) produced higher biomass in their ‘home’ soil than away soil under ambient rainfall while they consistently grew better in away soil under drought conditions, indicating drought-induced negative shifts in PSFs. There were strong interaction effects of rainfall treatment and conditioning species on several phospholipid-derived fatty acids markers, including a marker associated with arbuscular mycorrhizal fungi (AMF). We found a significant positive correlation between PSFs and the AMF marker across all treatment combinations, indicating that PSFs were driven by changes in positive plant-soil biotic interactions. Furthermore, drought increased plant parasitic nematode abundances. We found a strong significant negative correlation between PSFs and plant parasitic nematodes across all treatment combinations, indicating that the drought-induced shifts to more negative PSFs were driven by increased negative plant-soil biotic interactions. Drought may therefore negatively impact common grassland species due to stronger negative PSFs but at the same time promote species co-existence by reducing the abundance of competitive species.

Short‐term effects of summer warming on caribou forage quality are mitigated by long‐term warming

AbstractRapid Arctic climate change is leading to woody plant‐dominated ecosystems with potential consequences for caribou foraging and nutritional ecology. While warming has been clearly linked to shrub expansion, the influence of higher temperatures on variables linked to the leaf‐level quality of caribou forage is equivocal. Moreover, warming results in a complex set of ecosystem changes that operate on different timescales such as not only rapidly accelerating phenology, but also slowly increasing thaw depth and plant access to soil resources. Here, we compare changes in leaf nitrogen (N) concentration, digestibility, and protein‐precipitating capacity (PPC) in short‐term (i.e., &lt;1–2 summers) and long‐term (approximately 25 years) experimental warming plots with ambient temperature plots for three species commonly included in caribou summer diets:Salix pulchra(diamond‐leaf willow),Betula nana(dwarf birch), andEriophorum vaginatum(cottongrass). Short‐term warming modestly decreased leaf N concentration inB. nana.Long‐term and short‐term warming slightly increased the digestibility ofS. pulchra, but only short‐term warming increased digestibility inB. nana. Greater dry matter digestibility in both shrubs occurred through reductions in the lignin and cutin quantity in plant cells. Long‐term warming had no impact on PPC and equivocal impact on digestible protein ofB. nana. Overall, we found short‐term warming to be more impactful on forage quality than long‐term warming at Toolik Lake, Alaska. Apart from a long‐term warming reduction of approximately 13% in acid detergent lignin inS. pulchraandB. nana, other differences were only observed in the short‐term warming plots. Hence, our results indicate acclimation of plants to long‐term warming or possible negative feedback in the system to reduce warming effects. We suggest that warming summers may have a lesser effect on caribou forage than changes in winter precipitation or the influence of climate change on the abundance of critical species in the caribou diet.

Radiocarbon Analyses Quantify Peat Carbon Losses With Increasing Temperature in a Whole Ecosystem Warming Experiment

AbstractClimate warming is expected to accelerate peatland degradation and release rates of carbon dioxide (CO2) and methane (CH4). Spruce and Peatlands Responses Under Changing Environments is an ecosystem‐scale climate manipulation experiment, designed to examine peatland ecosystem response to climate forcings. We examined whether heating up to +9 °C to 3 m‐deep in a peat bog over a 7‐year period led to higher C turnover and CO2 and CH4 emissions, by measuring 14C of solid peat, dissolved organic carbon (DOC), CH4, and dissolved CO2 (DIC). DOC, a major substrate for heterotrophic respiration, increased significantly with warming. There was no 7‐year trend in the DI14 C of the ambient plots which remained similar to their DO14 C. At +6.75 °C and +9 °C, the 14C of DIC, a product of microbial respiration, initially resembled ambient plots but became more depleted over 7 years of warming. We attributed the shifts in DI14 C to the increasing importance of solid phase peat as a substrate for microbial respiration and quantified this shift via the radiocarbon mass balance. The mass‐balance model revealed increases in peat‐supported respiration of the catotelm depths in heated plots over time and relative to ambient enclosures, from a baseline of 20%–25% in ambient enclosures, to 35%–40% in the heated plots. We find that warming stimulates microorganisms to respire ancient peat C, deposited under prior climate (cooler) conditions. This apparent destabilization of the large peat C reservoir has implications for peatland‐climate feedbacks especially if the balance of the peatland is tipped from net C sink to C source.

Drainage increases species richness and density of soil macro-invertebrates in the Zoige peatland of eastern Tibetan Plateau

Soil macro-invertebrates play an important role in the decomposition process of soil organic matter. As the world’s largest alpine peatland, the Zoige peatland on the eastern Tibetan Plateau is experiencing the decline in water table level primarily because of historic drainage. However, little is known about the responses of soil macro-invertebrate communities to water table decline. To address this issue, we established a field drainage experimental platform in 2013, which consists of low (50 cm deep drainage ditch) and intermediate (20 cm deep drainage ditch) water table level treatments, and the ambient (no drainage ditch) plots. In July (warm season) and November (cold season) of 2019, we examined the species composition, richness, densities and the vertical distribution of soil macro-invertebrates. Results showed that the species composition differed significantly among treatments. Species richness and densities of overall and different trophic groups (i.e., detritivores, herbivores and predators) of soil macro-invertebrates were significantly higher in the drained treatments compared to the ambient plots. Moreover, soil macro-invertebrates moved down to deeper soil layers in the low water table treatments compared to the ambient plots. In addition, sampling season (warm vs. cold) did not affect the responses of soil macro-invertebrates to water table decline. These results indicate that soil macro-invertebrate community is sensitive to water table decline in the Zoige peatland. Given the recognized importance of soil fauna to ecosystem functioning, more attention should be paid to the conservation of peatlands.

Ethylenediurea (EDU) spray effects on willows (Salix sachalinensis F. Schmid) grown in ambient or ozone-enriched air: implications for renewable biomass production

Ground-level ozone (O3) is a widespread air pollutant causing extensive injuries in plants. However, its effects on perennial energy crops remain poorly understood due to technical difficulties in cultivating fast-growing shrubs for biomass production under O3 treatment on the field. Here we present the results of a two-year evaluation in the framework of which willow (Salix sachalinensis F. Schmid) shrubs were exposed to ambient (AOZ) or elevated (EOZ) O3 in two successive growing seasons (2014, 2015) and treated with 0 (EDU0) or 400 mg L−1 (EDU400) ethylenediurea spray in the second growing season. In 2014, EOZ altered the chemical composition of both top young and fallen leaves, and a novel mechanism of decreasing Mg in fallen leaves while highly enriching it in young top leaves was revealed in shrubs exposed to EOZ. In 2015, EDU400 alleviated EOZ-induced decreases in leaf fresh mass to dry mass ratio (FM/DM) and leaf mass per area (LMA). While EDU400 protected against EOZ-induced suppression of the maximum rate at which leaves can fix carbon (Amax) in O3-asymptomatic leaves, it did not alleviate EOZ-induced suppression of the maximum rates of carboxylation (VCmax) and electron transport (Jmax) and chlorophylls a, b, and a + b in the same type of leaves. In O3-symptomatic leaves, however, EDU400 alleviated EOZ-induced suppression of chlorophylls a and a + b, indicating different mode of action of EDU between O3-asymptomatic and O3-symptomatic leaves. Extensive herbivory occurred only in AOZ-exposed plants, leading to suppressed biomass production, while EOZ also led to a similar suppression of biomass production (EDU0 × EOZ vs. EDU400 × EOZ). In 2016, carry-over effects were also evaluated following cropping and transplantation into new ambient plots. Effects of EOZ in the preceding growing seasons extended to the third growing season in the form of suppressed ratoon biomass production, indicating carry-over effect of EOZ. Although EDU400 protected against EOZ-induced suppression of biomass production when applied in 2015, there was no carry-over effect of EDU in the absence of EDU treatment in 2016. The results of this study provide novel mechanistic understandings of O3 and EDU modes of action and can enlighten cultivation of willow as energy crop.

Resistance and resilience of a semi-arid grassland to multi-year extreme drought

Globally, extreme droughts are likely to become more frequent, more intense, and longer in duration with climate change. Understanding the impacts of drought on ecosystem function (i.e., resistance) and its recovery after drought (i.e., resilience) is critical for long-term sustainability of ecosystem services under climate change. We tested the effects of drought duration on ecosystem resistance by imposing four years of extreme drought (<5th percentile), and examined resilience by imposing two years of extreme drought followed by two years of recovery in a semi-arid grassland in Inner Mongolia, China. We found that the resistance of aboveground net primary productivity (ANPP) decreased as the drought progressed, i.e., ANPP was reduced from 33% in the first treatment year and 60% in the fourth treatment year. Resilience of the ecosystem was such that ANPP of drought plots was 83% of ambient plots one year post-drought. By the second year, ANPP of the drought plots fully recovered to ambient plot levels. Decreasing drought resistance through time was largely driven by forb reductions. High resilience of the system was due to grasses compensating for reductions in forb productivity. Based on these findings, we suggest that droughts may have progressively larger impacts as their duration increases, so single year responses may underpredict the effects of longer droughts likely to occur in the future. Additionally, longer recovery times after extreme drought events may be critical to ensure resilience and recovery of all plant functional types. Overall, we suggest that management should strive to support and maintain the high proportion of high drought resistance and resilience plant functional types in the community may enhance the stability of biomass production in a future climate with longer and more intense extreme droughts.

Plant functional types regulate non‐additive responses of soil respiration to 5‐year warming and nitrogen addition in a semi‐arid grassland

Abstract How climate warming interacts with atmospheric nitrogen (N) deposition to affect carbon (C) release from soils remains largely elusive, posing a major challenge in projecting climate change‒terrestrial C feedback. As part of a 5‐year (2006–2010) field manipulative experiment, this study was designed to examine the effects of 24‐hr continuous warming and N addition on soil respiration and explore the underlying mechanisms in a semi‐arid grassland on the Mongolian Plateau, China. Across the 5 years and all plots, soil respiration was not changed under the continuous warming, but was decreased by 3.7% under the N addition. The suppression of soil respiration by N addition in the third year and later could be mainly due to the reductions in the forb‐to‐grass biomass ratios. Moreover, there were interactive effects between continuous warming and N addition on soil respiration. Continuous warming increased soil respiration by 5.8% in the ambient N plots, but reduced it by 6.3% in the enriched N plots. Soil respiration was unaffected by N addition in the ambient temperature plots yet decreased by 9.4% in the elevated temperature plots. Changes of soil moisture and the proportion of legume biomass in the community might be primarily responsible for the non‐additive effects of continuous warming and N addition on soil respiration. This study provides empirical evidence for the positive climate warming‒soil C feedback in the ambient N condition. However, N deposition reverses the positive warming‒soil C feedback into a negative feedback, leading to decreased C loss from soils under a warming climate. Incorporating our findings into C‐cycling models could reduce the uncertainties of model projections for land C sink and global C cycling under multifactorial global change scenarios. A free Plain Language Summary can be found within the Supporting Information of this article.

BVOC Emissions From a Subarctic Ecosystem, as Controlled by Insect Herbivore Pressure and Temperature

The biogenic volatile organic compounds, BVOCs have a central role in ecosystem–atmosphere interactions. High-latitude ecosystems are facing increasing temperatures and insect herbivore pressure, which may affect their BVOC emission rates, but evidence and predictions of changes remain scattered. We studied the long-term effects of + 3 °C warming and reduced insect herbivory (achieved through insecticide sprayings) on mid- and late summer BVOC emissions from field layer vegetation, supplemented with birch saplings, and the underlying soil in Subarctic mountain birch forest in Finland in 2017–2018. Reduced insect herbivory decreased leaf damage by 58–67% and total ecosystem BVOC emissions by 44–72%. Of the BVOC groups, total sesquiterpenes had 70–80% lower emissions with reduced herbivory, and in 2017 the decrease was greater in warmed plots (89% decrease) than in ambient plots (34% decrease). While non-standardized total BVOC, monoterpene, sesquiterpene and GLV emissions showed instant positive responses to increasing chamber air temperature in midsummer samplings, the long-term warming treatment effects on standardized emissions mainly appeared as changes in the compound structure of BVOC blends and varied with compounds and sampling times. Our results suggest that the effects of climate warming on the total quantity of BVOC emissions will in Subarctic ecosystems be, over and above the instant temperature effects, mediated through changes in insect herbivore pressure rather than plant growth. If insect herbivore numbers will increase as predicted under climate warming, our results forecast herbivory-induced increases in the quantity of Subarctic BVOC emissions.Graphic

Promise and pitfalls of modeling grassland soil moisture in a free-air CO2 enrichment experiment (BioCON) using the SHAW model

Free-air carbon dioxide (CO2) enrichment (FACE) experiments provide an opportunity to test models of heat and water flow under novel, controlled situations and eventually allow use of these models for hypothesis evaluation. This study assesses whether the United States Department of Agriculture SHAW (Simultaneous Heat and Water) numerical model of vertical one-dimensional soil water flow across the soil-plant-atmosphere continuum is able to adequately represent and explain the effects of increasing atmospheric CO2 on soil moisture dynamics in temperate grasslands. Observations in a FACE experiment, the BioCON (Biodiversity, CO2, and Nitrogen) experiment, in Minnesota, USA, were compared with results of vertical soil moisture distribution. Three scenarios represented by different plots were assessed: bare, vegetated with ambient CO2, and similarly vegetated with high CO2. From the simulations, the bare plot soil was generally the wettest, followed by a drier high-CO2 vegetated plot, and the ambient CO2 plot was the driest. The SHAW simulations adequately reproduced the expected behavior and showed that vegetation and atmospheric CO2 concentration significantly affected soil moisture dynamics. The differences in modeled soil moisture amongst the plots were largely due to transpiration, which was low with high CO2. However, the modeled soil moisture only modestly reproduced the observations. Thus, while SHAW is able to replicate and help broadly explain soil moisture dynamics in a FACE experiment, its application for point- and time-specific simulations of soil moisture needs further scrutiny. The typical design of a FACE experiment makes the experimental observations challenging to model with a one-dimensional distributed model. In addition, FACE instrumentation and monitoring will need improvement in order to be a useful platform for robust model testing. Only after this can we recommend that models such as SHAW are adequate for process interpretation of datasets from FACE experiments or for hypothesis testing.

Increased Arctic climate extremes constrain expected higher plant reproductive success in a warmer climate

The low reproductive success of Arctic plants is predicted to increase as the climate warms. However, climate extremes add complexity to these predictions. In the extremely cold year of 2018, multiple Arctic trophic levels experienced reproductive failure. We analysed a unique long-term record of seed viability from experimentally warmed and ambient plots at Alexandra Fiord, Ellesmere Island, which has been running since 1992 and included 2018 and the extremely warm year of 2019. Positive and negative July temperature anomalies and summer temperatures have increased significantly by 0.5, −0.3, and 0.4 °C/decade since 1977, respectively, resulting in greater extremes and increased inter-annual variation. Seed viability of some species has increased with climate warming. Across years, seed viability of woody species was consistently higher in warmed than ambient plots while the opposite was true for forbs. In 2018, seed viability of woody species in ambient plots was significantly lower than normal but comparable with past years for forb species. Not all species returned to normal seed viability levels in 2019. Our study highlights the potential for greater sexual reproductive failure in tundra plants with increasing climate extremes. We suggest that poor seed viability of woody species in cold years could constrain shrub recruitment and may aid forb species establishment on bare tundra.