Global Change Scenarios Research Articles

Globally limited individual and combined effects of multiple global change factors on allometric biomass partitioning

AbstractAimPlant biomass allocation reflects the distribution of photosynthates among different organs in response to changing environmental conditions. Global change influences plant growth across terrestrial ecosystems, but impacts of individual and combined multiple global change factors (GCFs) on plant biomass allocation at the global scale are unclear.LocationGlobal.Time periodContemporary.Major taxa studiedPlants in terrestrial ecosystems.MethodsWe conducted a meta‐analysis of data comprising 4,180 pairwise observations to assess individual and combined effects of nitrogen addition (N), warming (W), elevated CO2(C), irrigation (I), and drought (D) on plant biomass allocation based on the ‘ratio‐based optimal partitioning’ and ‘isometric allocation’ hypotheses.ResultsWe found that (a) ratio‐based plant biomass fractions of different organs were only minimally affected by individual and combined effects of the studied GCFs; (b) combined effects of two‐factor pairs of GCFs on plant biomass allocation were commonly additive, rather than synergistic or antagonistic; (c) moderator variables influenced, but seldom changed the direction of individual and combined effects of GCFs on plant biomass allocation; and (d) neither individual nor combined effects of the studied GCFs altered allometric relationships among different organs, indicating that patterns of plant biomass allocation under the environmental stress conditions exerted by the multiple GCFs were better explained by the isometric allocation rather than the ratio‐based optimal partitioning hypothesis.Main conclusionsOur results show consistent patterns of allometric plant biomass partitioning under effects of multiple GCFs and provide evidence of an isometric plant biomass allocation trajectory in response to global change perturbations. These findings improve our understanding and prediction of terrestrial vegetation responses to future global change scenarios.

Planting Season Impacts Sugarcane Stem Development, Secondary Metabolite Levels, and Natural Antisense Transcription.

To reduce the potentially irreversible environmental impacts caused by fossil fuels, the use of renewable energy sources must be increased on a global scale. One promising source of biomass and bioenergy is sugarcane. The study of this crop’s development in different planting seasons can aid in successfully cultivating it in global climate change scenarios. The sugarcane variety SP80-3280 was field grown under two planting seasons with different climatic conditions. A systems biology approach was taken to study the changes on physiological, morphological, agrotechnological, transcriptomics, and metabolomics levels in the leaf +1, and immature, intermediate and mature internodes. Most of the variation found within the transcriptomics and metabolomics profiles is attributed to the differences among the distinct tissues. However, the integration of both transcriptomics and metabolomics data highlighted three main metabolic categories as the principal sources of variation across tissues: amino acid metabolism, biosynthesis of secondary metabolites, and xenobiotics biodegradation and metabolism. Differences in ripening and metabolite levels mainly in leaves and mature internodes may reflect the impact of contrasting environmental conditions on sugarcane development. In general, the same metabolites are found in mature internodes from both “one-year” and “one-and-a-half-year sugarcane”, however, some metabolites (i.e., phenylpropanoids with economic value) and natural antisense transcript expression are only detected in the leaves of “one-year” sugarcane.

A framework linking biogeography and species traits to plant species vulnerability under global change in Mediterranean-type ecosystems

We review the roles that plant species traits and biogeography play in species’ exposure and vulnerability to decline or extinction under global change, focusing on separate and combined impacts of multiple threats – climate change, land-use change, and altered disturbance regimes. We establish a conceptual framework and research agenda for identifying the spatial characteristics of species ranges, as well as the life history and functional traits, that are associated with extinction risk for plant species with functional attributes emblematic of fire-prone, winter-precipitation Mediterranean-type ecosystems (MTEs). MTEs worldwide are characterized by their high plant diversity and unique floras, historical and contemporary high rates of land use change, and strong interactions between climate, fire, and land use. We focus on the California Floristic Province (CFP), an MTE that is a global plant diversity hotspot, and show how our framework can be used to understand the relationships between vulnerability to multiple global change drivers, species traits, and biogeography. Vulnerability can be assessed across species using linked distribution and population models that forecast plant responses to global change scenarios. Our overarching hypothesis is that species-specific vulnerability to global change in MTEs is a function of interactions between species and spatial traits: the nature of this interaction will depend on the type of global change process.

Urban surface uses for climate resilient and sustainable cities: A catalogue of solutions

In the current scenario of massive urbanization and global climate change, the urban surfaces and their characteristics have a key role, as they significantly influence the quality of life in urban areas, as well as their environmental conditions. To shed light on the role of urban surfaces in fostering climate resilient and sustainable cities, this paper proposes a catalogue of solutions for the urban surface use. The catalogue presents the main surface uses suitable for the built environment, and discusses the potential conflicts and synergies among them in the view of a multiple and integrated utilization of urban surfaces. Reviewing studies published in the last 15 years, this study aims to answer three major questions: (i) which solutions do exist, (ii) where can these be applied, and (iii) which benefits do they provide. The discussion demonstrates that the use of urban surfaces might lead the development of multiple opportunities for improving the existing urban environments and supporting not only environmental, but also social and economic resilience. Finally, it emphasizes the need for specific quantitative and qualitative approaches to address the multi-disciplinary challenges posed by the design and implementation of surface uses, and the evaluation of their contribution to site-specific objectives.

Linking leaf traits to the temporal stability of above- and belowground productivity under global change and land use scenarios in a semi-arid grassland of Inner Mongolia

The biotic drivers for the temporal stability of aboveground net productivity (ANPP) in natural ecosystems are well understood. However, knowledge gaps still exist regarding the relative importance of biotic and abiotic drivers regulating the temporal stability of aboveground productivity (ANPP), belowground net productivity (BNPP), and community net productivity (NPP) under global change and land use scenarios. Thus, in this study, we aimed to study the effects of increased water and nitrogen availability on temporal stability of ANPP, BNPP, and NPP and underlying mechanisms at sites with different long-term grazing histories in typical grasslands of the Inner Mongolia. The results suggested that resource addition affected the ANPP stability, but it did not change the stability of BNPP and NPP, which were all mediated by grazing histories. Most importantly, our study further indicated that species asynchrony, primarily contributed to the stability of ANPP and NPP by weakening their variation, and species asynchrony was regulated directly by plant diversity-related variables and indirectly by soil variables which were affected by resource addition and grazing history. In addition, an increase of ANPP stimulated under resource addition was a secondary contributor to ANPP stability. Specifically, the community-weighted mean of specific leaf area (CWM SLA) regulated the ANPP stability indirectly by promoting species asynchrony, while functional diversity of leaf area and SLA both directly controlled the BNPP stability. Findings of our study demonstrate that different mechanisms drove temporal stability of above- and belowground productivity. Our study has important implications for maintaining the temporal stability of community productivity and for establishing sustainable management practices of semi-arid grasslands under global change and land use scenarios.

Exploring relationships between drought and epidemic cholera in Africa using generalised linear models

BackgroundTemperature and precipitation are known to affect Vibrio cholerae outbreaks. Despite this, the impact of drought on outbreaks has been largely understudied. Africa is both drought and cholera prone and more research is needed in Africa to understand cholera dynamics in relation to drought.MethodsHere, we analyse a range of environmental and socioeconomic covariates and fit generalised linear models to publicly available national data, to test for associations with several indices of drought and make cholera outbreak projections to 2070 under three scenarios of global change, reflecting varying trajectories of CO2 emissions, socio-economic development, and population growth.ResultsThe best-fit model implies that drought is a significant risk factor for African cholera outbreaks, alongside positive effects of population, temperature and poverty and a negative effect of freshwater withdrawal. The projections show that following stringent emissions pathways and expanding sustainable development may reduce cholera outbreak occurrence in Africa, although these changes were spatially heterogeneous.ConclusionsDespite an effect of drought in explaining recent cholera outbreaks, future projections highlighted the potential for sustainable development gains to offset drought-related impacts on cholera risk. Future work should build on this research investigating the impacts of drought on cholera on a finer spatial scale and potential non-linear relationships, especially in high-burden countries which saw little cholera change in the scenario analysis.

Grazing influences biomass production and protein content of alpine meadows

Alpine grasslands are essential for carbon sequestration and food supply for domestic and wild herbivores inhabiting mountainous areas worldwide. These biomes, however, are alternatively threatened by the abandonment of agricultural and livestock practices leading to a fast-growing shrubification process while other mountain grasslands are suffering from the impacts of overgrazing. The functioning of alpine meadow ecosystems is primarily driven by climatic conditions, land-use legacies and grazing. However, although it is critically important, the role of large herbivores on the aboveground biomass and protein content of palatable plants is poorly understood for most alpine meadows. In this work, we explore the effects of grazing on grassland vegetation at two different spatial and temporal scales in the Eastern Pyrenees, Spain. Remote sensing was used to assess the effect of high and moderate grazing (HG and MG respectively) on grass biomass using the leaf area index (LAI) at the meso-scale (patches between 2.3 and 38.7 ha). We also explored the impact of null (NG), overgrazing (MO, mimicked overgrazing) and high (HG) grazing intensities at local scale setting eighteen 1 m2 exclusion boxes in six meadows (three boxes each) commonly used by domestic and wild ungulates. Historical satellite data showed that LAI values are greater in high than in low grazed areas (HG, mean = 0.66, LG, mean = 0.55). Along the same lines, high and moderate grazing pressures improved biomass production at the local-scale (HG, mean = 590.3 g/m2, MO, mean = 389.3 g/m2 and NG, mean = 110.8 g/m2). Crude protein content reached higher values under MO pressure than under HG pressure. Our results confirm that grazing intensity exerts significant changes on the above-ground biomass production and the protein content of plants consumed by domestic (cattle and horses) and wild ungulates (Southern Chamois, Rupicapra pyrenaica). We can conclude that ungulates sustain biomass and nutritive values of grass exerting a negligible effect on biomass and protein content of woody vegetation. Our results will inform management guidelines to support profitable grazing activities and promote conservation of the open landscapes in the alpine ecosystems under the current global change scenario.

Optimising Artificial Moss Growth for Environmental Studies in the Mediterranean Area.

Bryophytes are poikilohydric organisms that play a key role in ecosystems, while some of them are also resistant to drought and environmental disturbances but present a slow growth rate. Moss culture in the laboratory can be a very useful tool for ecological restoration or the development of urban green spaces (roof and wall) in the Mediterranean region. Therefore, we aim to: (i) determine the optimal culture conditions for the growth of four moss species present in the Mediterranean climate, such as Bryum argenteum, Hypnum cupressiforme, Tortella nitida, and Tortella squarrosa; (ii) study the optimal growth conditions of the invasive moss Campylopus introflexus to find out if it can be a threat to native species. Photoperiod does not seem to cause any recognisable pattern in moss growth. However, temperature produces more linear but slower growth at 15 °C than at 20 and 25 °C. In addition, the lower temperature produced faster maximum cover values within 5–8 weeks, with at least 60% of the culture area covered. The study concludes that the culture of moss artificially in the organic gardening substrate without fertilisers is feasible and could be of great help for further use in environmental projects to restore degraded ecosystems or to facilitate urban green spaces in the Mediterranean area. Moreover, this study concludes that C. introflexus could successfully occupy the niche of other native moss species, especially in degraded areas, in a future global change scenario.

How COVID-19 Exposed Water Supply Fragility in Florida, USA

Healthcare demand for liquid oxygen during the COVID-19 pandemic limited the availability of oxygen needed for ozone disinfection of drinking water in several urban areas of Florida. While the situation reduced the state’s capacity to provide normal drinking water treatment for millions of people, calls for water conservation during the emergency period resulted in virtually no change in water consumption. Here, we point out that 38–40% of the potable water produced by one of the major utilities in Florida is not used for drinking water but instead is used for outdoor landscape irrigation. This suggests that emergency-level calls for reduced water use could have been made if outdoor irrigation was limited, but we present data showing that there was little change in public behavior, and the state was unable to meet necessary water use reductions during the emergency. This inability to meet short-term emergency water conservation needs foretells a long-term lack of resilience against other global change scenarios and suggests that much work is still needed to build resilience into Florida’s water future. We conclude this Viewpoint paper by calling for more urgent sociohydrological research to understand the coupled human-natural drivers of how water supplies respond to global change.

Multiple Stress Reduces the Advantage of Pesticide Adaptation.

Under global change scenarios, multistress conditions may occur regularly and require adaptation. However, the adaptation to one stressor might be associated with the increased sensitivity to another stressor. Here, we investigated the ecological consequences of such trade-off under multiple stress. We compared the pesticide tolerance of the crustacean Gammarus pulex from agricultural streams with populations from reference streams. Under optimum temperature, G. pulex from agricultural streams were considerably more tolerant to pesticides as compared to the reference populations. Here, we assume that the increased tolerance in agricultural populations is the combination of acclimation, epigenetic effect, and genetic evolution. After experimental pre-exposure to very low concentration (LC50/1000), reference populations showed increased pesticide tolerance. In contrast, pre-exposure did not further increase the tolerance of agricultural populations. Moreover, these populations were more sensitive to elevated temperature alone due to the hypothesized fitness cost of genetic adaptation to pesticides. However, both reference and agricultural populations showed a similar tolerance to the combined stress of pesticides and warming due to stronger synergistic effects in adapted populations. As a result, pesticide adaptation loses its advantage. The combined effect was predicted well using the stress addition model, developed for predicting the synergistic interaction of independent stressors. We conclude that under multistress conditions, adaptation to pesticides reduces the general stress capacity of individuals and trade-off processes increase the sensitivity to additional stressors. This causes strong synergistic effects of additional stressors on pesticide-adapted individuals.

Global cultivation of wheat crops induces considerable shifts in the range and niche of species relative to their wild progenitors

Species’ range and niche play key roles in understanding ecological and biogeographical patterns, especially in projecting global biotic homogenization and potential distribution patterns of species under global change scenarios. However, few studies have investigated the ability of crop cultivation to influence potential range sizes and niche shifts of species. Wheat and its wild progenitors share the same origin and evolutionary history, and thus provide an excellent system to explore this topic. Using ensembled ecological niche models and niche dynamic models, we studied the potential range sizes of wheat and its wild progenitors, as well as their niche dynamics. Our results showed that wheat had larger range size and niche breadth than its wild progenitors, suggesting that wheat cultivation is a more powerful driver of range and niche expansion than natural niche evolution. Additionally, wheat and its wild progenitors occupied different niche positions, and the former did not conserve the niches inherited from the latter, implying that wheat cultivation considerably induces niche shifts. The niche dynamics between wheat and its wild progenitors were not only closely associated with cultivation but were also modified by the niche conservatism of its wild progenitors. In contrast to most invasive plants, wheat, as a global staple crop species, did not conserve the niche space inherited from its wild progenitors, suggesting that compared with most plant invasions, cultivation may have a stronger effect on niche shifts. Therefore, global niche shifts induced by crop cultivation need much more attention, though the underlying mechanisms require further study.

Fast climatic changes place an endemic Canary Island macroalga at extinction risk

Global warming and ultraviolet radiation (UVR) affect marine organisms worldwide. However, we still lack knowledge about the consequences of these environmental changes to range-restricted macroalgae species, such as Gelidium canariense, a habitat-forming endemic of the Canary Islands. The aim of this study is to provide useful data for analyzing the conservation status of this endemic species. To do this, (1) we compared distributional data from 1987 with the data collected in 2008 and 2019 along the northern coast of Tenerife; (2) we assessed the variation in macroalgae assemblages dominated by G. canariense over the last decade on this same cost; and (3) we explored the correlation of these results with the actual trends of sea surface temperature (SST), air temperature at 2 m above the sea surface (T2m), UVR, and downward solar radiation (DSR). The results showed significant changes in the distribution of G. canariense in the last 30 years at Tenerife. More than 90% of its populations have declined. The endemic macroalgae have been replaced by the globally distributed species Pterocladiella capillacea at nearly all the sites studied. The cover of G. canariense and Gelidium arbuscula decreased significantly with increasing SST and T2m. Additionally, G. canariense is negatively related to the rise in UVR. These results highlight the vulnerability of both species of Gelidium to global warming and UVR, but especially of G. canariense. The drastic decline of this species reveals the need to upgrade its conservation status and implement protection measures to reduce the damage that predicted global change scenario may have to their relict populations.

Stem Photosynthesis Affects Hydraulic Resilience in the Deciduous Populusalba but Not in the Evergreen Laurus nobilis

Stem photosynthesis has been suggested to play relevant roles to cope with different biotic and abiotic stress factors, including drought. In the present study, we performed measurements of stem hydraulic conductance and non-structural carbohydrate content in the evergreen Laurus nobilis L. and the deciduous Populusalba L., subjected to inhibition of stem photosynthesis and successive exposure to a drought-recovery cycle in order to check if stem photosynthesis may be involved in allowing hydraulic recovery after drought stress relief. Stem shading affected the growth of L. nobilis but not of P. alba saplings. By contrast, inhibition of stem photosynthesis was coupled to inhibition of hydraulic recovery following embolism build-up under drought in P. alba but not in L. nobilis. The two study species showed a different content and behavior of nonstructural carbohydrates (NSCs). The differences in NSCs’ trend and embolism reversal ability led to a significant relationship between starch content and the corresponding hydraulic conductance values in L. nobilis but not in P. alba. Our findings suggest that stem photosynthesis plays a key role in the maintenance of hydraulic functioning during drought especially in the deciduous species. This, in turn, may increase their vulnerability under current global climate change scenarios.

Long‐term litter removal rather than litter addition enhances ecosystem carbon sequestration in a temperate steppe

Abstract Global change can greatly affect plant productivity and subsequently litter input to soil, with potential impacts on soil carbon (C) fluxes. However, the effects of litter layer in mediating C cycling and budget at an ecosystem scale are still not clear. As part of a long‐term litter fall manipulation experiment in a temperate steppe on the Mongolian Plateau, this study was conducted to explore effects of litter removal and addition on ecosystem C budget and the associated mechanisms. Overall, litter removal enhanced photosynthetic active radiation (PAR) at soil surface by 31.0%, but litter addition reduced it by 26.5%. Litter removal decreased soil inorganic nitrogen (SIN) content at the depth of 0–10 cm by 13.6%, but litter addition enhanced it by 14.1%. Litter removal increased legume abundance by 131.1%, whereas litter addition enhanced grass abundance by 30.7% but decreased forb abundance by 33.1%. Litter removal increased gross ecosystem productivity (GEP) through enhancing PAR and legume abundance, whereas litter addition elevated GEP by changing plant community composition mediated by decreasing PAR and increasing SIN. Litter removal did not affect ecosystem respiration (ER), but litter addition stimulated ER by 14.9%. Therefore, while litter addition did not affect net ecosystem productivity (NEP), litter removal surprisingly enhanced NEP by 23.9% over the later 7 years. Our findings highlight that more above‐ground litter input derived from biomass production may not necessarily result in a larger ecosystem C sink in grasslands under global change scenarios in the future. On the contrary, proper litter removal may be an effective way to increase the ecosystem C sink in grassland management. A free Plain Language Summary can be found within the Supporting Information of this article.

Present and Future Climate-Related Distribution of Narrow- versus Wide-Ranged Ostrya Species in China

The niche breadth–range size hypothesis states that geographic range size of a species is positively correlated with its environmental niche breadth. We test this hypothesis and examine whether the correlation varies with climate change and among taxa through modeling (processing Maximum entropy (Maxent)) potential distributions in present and future climate scenario of four sympatric Ostrya species in China and with different geographic range sizes, including extremely rare O. rehderiana. Potential geographical distributions of narrow- versus wide-ranged Ostrya species were predicted based on their niche breadths. Niche equivalency and similarity tests were performed to examine niche overlap between species pairs. Potential distribution areas of wide niche breadth species (O. japonica and O. trichocarpa) were significantly wider than those of narrow niche breadth species (O. multinervis and O. rehderiana) although niche divergence was hardly observed among them. In the future scenarios of global climate change, wide-ranged O. japonica would have wider potential distribution than in the current scenario, even expanding their geographic range. Conversely, suitable habitats of narrow-ranged O. multinervis and O. rehderiana would be reduced strikingly in future scenarios compared to in the current scenario, and they might be subjected to a high risk of extinction. Potential distribution range sizes of the Ostrya species would positively correlate with their niche breadths in future scenarios, and their niche breadths would determine their distribution variation with climate change. The Ostrya species having broader niche currently would be further widespread in future scenarios while narrowly distributed Ostrya species having narrower niche currently would further reduce their distribution range under changed climate and might be subjected to a high risk of extinction in future scenarios. Our results support the range size–niche breadth hypothesis both at present and future climate scenarios, and they provide useful reference for conservation of rare species like O. rehderiana.

Coupled modes of projected regional change in the Bering Sea from a dynamically downscaling model under CMIP6 forcing

Three different global earth system models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) were used to explore anticipated changes in the Bering Sea under high (SSP126) and low (SSP585) carbon mitigation scenarios (i.e. low and high emission scenarios), via dynamical downscaling. A multivariate pattern analysis, based on Empirical Orthogonal Functions applied to monthly time series, reveals strong coupling of changes across several biophysical variables and the global forcing itself, on both yearly and multidecadal time scales. Rising air and ocean temperatures from the global models are strongly coupled with rising regional temperatures and reduced ice cover/thickness, as well as strong changes to the phenology of the plankton food chain, including reduced biomass of large zooplankton in the fall. This method ultimately provides a compact way to estimate the changes to many regional attributes under a variety of global change scenarios. Application of this method to a broad ensemble of the CMIP6 global model air temperatures suggests that compared to present conditions, the Bering Sea shelf bottom temperatures in July will warm by an average of ∼4 degrees C by the end of the 21st century under SSP585, as compared with ∼1 degrees C under SSP126, with greatest warming focused on the outer northern shelf.

Hermaphroditism in fish: incidence, distribution and associations with abiotic environmental factors

The distribution of hermaphroditism in fishes has traditionally been mainly explained by its dependence on biotic factors. However, correlates with major abiotic factors have not been investigated on a quantitative basis and at a global scale. Here, we determined the incidence of hermaphroditism in fish at the family and species level, tested the hypothesis that evolutionary relationships account for the poor presence of hermaphroditism in freshwater species, and tested the association of sexual systems with latitude, habitat type and depth. Functional hermaphroditism is reported in 8 orders, 34 families and 370 species of fishes, all teleosts. Sequential hermaphroditism predominates over simultaneous hermaphroditism at a ratio ~ 5:1 and protogyny (female-to-male sex change) predominates ~ 6:1 over protandry (male-to-female). We found 12 hermaphroditic species that can live in freshwater. However, seven of these species are from four primarily marine families while there are only five species from two mostly freshwater families. Protogynous and bi-directional sex changers have a tighter association with reef-associated tropical and subtropical habitats when compared to protandrous species, which tend to be more plastic in terms of distribution requirements. Finally, simultaneous hermaphrodite species live both in the deep sea and shallow waters in similar proportions. This study can be the basis for further research in specific groups for different purposes, including ecological and evolutionary issues as well as conservation and management of exploited species. Understanding the environmental correlates can help to forecast changes in the distribution or phenology of hermaphrodites in a global change scenario.

Climate Change Effects on Fish Passability across a Rock Weir in a Mediterranean River

Climate change represents a major challenge for the management of native fish communities in Mediterranean rivers, as reductions in discharge may lead to a decrease in passability through small barriers such as weirs, both in temporary and perennial rivers. Through hydraulic modelling, we investigated how discharges from a large hydropower plant in the Tagus River are expected to affect the passability of native freshwater fish species through a rock weir (Pego, Portugal), equipped with a nature-like fish ramp. We considered not only mean daily discharge values retrieved from nearby gauging stations (1991–2005) for our flow datasets, but also predicted discharge values based on climatic projections (RCP) until the end of the century (2071–2100) for the Tagus River. Results showed that a minimum flow of 3 m3 s−1 may be required to ensure the passability of all species through the ramp and that passability was significantly lower in the RCP scenarios than in the historical scenario. This study suggests that climate change may reduce the passability of native fish species in weirs, meaning that the construction of small barriers in rivers should consider the decreases in discharge predicted from global change scenarios for the suitable management of fish populations.

The future of temporary wetlands in drylands under global change

ABSTRACT The Andalusian International University held a workshop entitled Temporary wetlands’ future in drylands under the projected global change scenario in March 2020 in Baeza, Spain, with 26 participants from 10 countries. The workshop objectives were to promote international cooperation and scientific exchange on the conservation and protection of temporary wetlands. The participants highlighted the extreme conditions that temporary and permanent wetlands, ponds, and shallow lakes are currently facing and predicted a dismal future for these systems due to climate change. To foster a holistic view of these ecosystems, the workshop included wetland watersheds. It was concluded that the main threats are those affecting water quality and quantity as well as egg-seed banks, species population dynamics, and food webs. The inherent characteristics of waterbodies in drylands, including high resilience and resistance to harsh conditions, are already negatively impacted by direct human actions and climate change. Another threat is the time lag between scientific warnings about threats and the social and political concern leading to mitigating actions. Thus, more effective actions to protect and conserve temporary wetlands are essential. Research networks could help stimulate the necessary conservation actions, but the global recession due to the COVID-19 pandemic will pose a challenge as economies are burdened with urgent expenditure. This special issue of the journal Inland Waters is the outcome of the workshop presentations and is composed of the ensuing papers on wetlands in drylands.

Magnitude and mechanisms of nitrogen‐mediated responses of tree biomass production to elevated CO2: A global synthesis

Abstract Elevated atmospheric CO2 concentration (eCO2) typically stimulates tree growth, which is mediated by nitrogen (N) availability; but how N regulates tree biomass responses to eCO2 remains uncertain, which limits our prediction of forest carbon (C) cycling under future global change scenarios. A meta‐analysis of a global dataset including 3,399 observations from 283 papers published from 1980s to February 2021 was conducted with the aim of elucidating N‐mediated responses of tree biomass production to eCO2 and the underlying mechanisms. We found that eCO2 stimulated tree biomass production (+32.0%), while it induced accumulation of non‐structural carbohydrates in leaves rather than in woods and roots, suggesting that the production may be C‐limited but depend on the sink strength of organs. Biomass responses to eCO2 of N‐fertilized trees (+39.6%) were 68.4% greater than those of non‐fertilized trees (+23.5%), confirming that tree growth is also N‐limited. Such N limitation was alleviated by the eCO2‐induced increases in N uptake and N‐use efficiency (NUE), with the former being more important. Increases in tree N pool arose from the enhanced production of fine roots with a lower specific root length, whereas increases in NUE resulted from the flexibility in tissue C:N ratios instead of N resorption efficiency. The positive responses of tree biomass production to eCO2 were greater for ectomycorrhizal trees and conifers than for arbuscular mycorrhizal trees and angiosperms, respectively. Synthesis. Our findings suggest that eCO2 stimulates tree biomass production by increasing C availability, and alleviating N limitation in a feedback way via enhancing N uptake and NUE, and they improve our mechanistic understanding of responses of forest productivity and C sequestration to eCO2 under global change.