Linking interdecadal changes in British river ecosystems to water quality and climate dynamics

Macroinvertebrate communities in Western European rivers have changed substantially in recent decades. Understanding the causes is challenging because improvements in water quality have coincided with climatic variations over this period. Using data covering >2300 rivers and 21 years (1991–2011) across England and Wales, we analysed family‐level distributions and nationwide trends in prevalence (proportion of sampling locations where an organism was present) to diagnose the causes of ecological change. Our aims were to: (i) reveal the taxa driving assemblage‐level trends; (ii) identify the main changes in family‐level prevalence and distribution patterns; and (iii) test whether changes were accounted for by improving water quality, increasing temperatures or variations in discharge. While previous analyses revealed increasing richness among British river invertebrates, a partial turnover of taxa is now evident. Two distinct components of temporal trend have comprised: (i) overall increases or decreases in taxon prevalence over 21 years, which correlated with pollution sensitivity and discharge; and (ii) short‐term variations in prevalence that correlated primarily with temperature and nutrient concentrations. The longer‐term changes in prevalence were reflected in expansions or contractions in families' distributions linked to water quality, with little evidence of shifts consistent with increasing temperatures. Although these monitoring data had limitations (e.g., family‐level data, few headwaters), they provide no clear evidence of long‐term climate effects on invertebrates; the one feature consistent with climate warming – a small northward expansion of the range of many taxa – was accounted for by large improvements in water quality in northern England. Nevertheless, changes linked to discharge and temperature over the shorter‐term (<2 years) point to the climatic sensitivity of invertebrate communities. It is therefore likely that any long‐term climatic changes since 1990 have been outweighed by the strength and geographical extent of the recovery from poor water quality.     

Land‐use legacy and the differential response of stream macroinvertebrates to multiple stressors studied using in situ experimental mesocosms

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Distribution of skates and sharks in the North Sea: 112 years of change

How have North Sea skate and shark assemblages changed since the early 20th century when bottom trawling became widespread, whilst their environment became increasingly impacted by fishing, climate change, habitat degradation and other anthropogenic pressures? This article examines long‐term changes in the distribution and occurrence of the elasmobranch assemblage of the southern North Sea, based on extensive historical time series (1902–2013) of fishery‐independent survey data. In general, larger species (thornback ray, tope, spurdog) exhibited long‐term declines, and the largest (common skate complex) became locally extirpated (as did angelshark). Smaller species increased (spotted and starry ray, lesser‐spotted dogfish) as did smooth‐hound, likely benefiting from greater resilience to fishing and/or climate change. This indicates a fundamental shift from historical dominance of larger, commercially valuable species to current prevalence of smaller, more productive species often of low commercial value. In recent years, however, some trends have reversed, with the (cold‐water associated) starry ray now declining and thornback ray increasing. This shift may be attributed to (i) fishing, including mechanised beam trawling introduced in the 1960s–1970s, and historical target fisheries for elasmobranchs; (ii) climate change, currently favouring warm‐water above cold‐water species; and (iii) habitat loss, including potential degradation of coastal and outer estuarine nursery habitats. The same anthropogenic pressures, here documented to have impacted North Sea elasmobranchs over the past century, are likewise impacting shelf seas worldwide and may increase in the future; therefore, parallel changes in elasmobranch communities in other regions are to be expected.     

Trends in water quality and discharge confound long-term warming effects on river macroinvertebrates

1. Climate‐change effects on rivers and streams might interact with other pressures, such as pollution, but long‐term investigations are scarce. We assessed trends among macroinvertebrates in 50 southern English streams in relation to temperature, discharge and water quality over 18 years (1989–2007). 2. Long‐term records, coupled with estimates from inter‐site calibrations of 3–4 years, showed that mean stream temperatures in the study area had increased by 2.1–2.9 °C in winter and 1.1–1.5 °C in summer over the 26 year period from 1980 to 2006, with trends in winter strongest. 3. While invertebrate assemblages in surface‐fed streams were constant, those in chalk‐streams changed significantly during 1989–2007. Invertebrate trends correlated significantly with temperature, but effects were spurious because (i) assemblages gained taxa typical of faster flow or well‐oxygenated conditions, contrary to expectations from warming; (ii) more invertebrate families increased in abundance than declined and (iii) concomitant changes in water quality (e.g. declining orthophosphate, ammonia and biochemical oxygen demand), or at some sites changes in discharge, explained more variation in invertebrate abundance and composition than did temperature. 4. These patterns were reconfirmed in both group‐ and site‐specific analyses. 5. We conclude that recent winter‐biased warming in southern English chalk‐streams has been insufficient to affect invertebrates negatively over a period of improving water quality. This implies that positive management can minimize some climate‐change impacts on stream ecosystems. Chalk‐stream invertebrates are sensitive, nevertheless, to variations in discharge, and detectable changes could occur if climate change alters flow pattern. 6. Because climatic trends now characterize many inter‐annual time‐series, we caution other investigators to examine whether putative effects on ecological systems are real or linked spuriously to other causes of change.     

Local scale processes drive long‐term change in biodiversity of sandy beach ecosystems

Evaluating impacts to biodiversity requires ecologically informed comparisons over sufficient time spans. The vulnerability of coastal ecosystems to anthropogenic and climate change‐related impacts makes them potentially valuable indicators of biodiversity change. To evaluate multidecadal change in biodiversity, we compared results from intertidal surveys of 13 sandy beaches conducted in the 1970s and 2009–11 along 500 km of coast (California, USA). Using a novel extrapolation approach to adjust species richness for sampling effort allowed us to address data gaps and has promise for application to other data‐limited biodiversity comparisons. Long‐term changes in species richness varied in direction and magnitude among beaches and with human impacts but showed no regional patterns. Observed long‐term changes in richness differed markedly among functional groups of intertidal invertebrates. At the majority (77%) of beaches, changes in richness were most evident for wrack‐associated invertebrates suggesting they have disproportionate vulnerability to impacts. Reduced diversity of this group was consistent with long‐term habitat loss from erosion and sea level rise at one beach. Wrack‐associated species richness declined over time at impacted beaches (beach fill and grooming), despite observed increases in overall intertidal richness. In contrast richness of these taxa increased at more than half (53%) of the beaches including two beaches recovering from decades of off‐road vehicle impacts. Over more than three decades, our results suggest that local scale processes exerted a stronger influence on intertidal biodiversity on beaches than regional processes and highlight the role of human impacts for local spatial scales. Our results illustrate how comparisons of overall biodiversity may mask ecologically important changes and stress the value of evaluating biodiversity change in the context of functional groups. The long‐term loss of wrack‐associated species, a key component of sandy beach ecosystems, documented here represents a significant threat to the biodiversity and function of coastal ecosystems.     

Wild bee community change over a 26‐year chronosequence of restored tallgrass prairie

Restoration efforts often focus on plants, but additionally require the establishment and long‐term persistence of diverse groups of nontarget organisms, such as bees, for important ecosystem functions and meeting restoration goals. We investigated long‐term patterns in the response of bees to habitat restoration by sampling bee communities along a 26‐year chronosequence of restored tallgrass prairie in north‐central Illinois, U.S.A. Specifically, we examined how bee communities changed over time since restoration in terms of (1) abundance and richness, (2) community composition, and (3) the two components of beta diversity, one‐to‐one species replacement, and changes in species richness. Bee abundance and raw richness increased with restoration age from the low level of the pre‐restoration (agricultural) sites to the target level of the remnant prairie within the first 2–3 years after restoration, and these high levels were maintained throughout the entire restoration chronosequence. Bee community composition of the youngest restored sites differed from that of prairie remnants, but 5–7 years post‐restoration the community composition of restored prairie converged with that of remnants. Landscape context, particularly nearby wooded land, was found to affect abundance, rarefied richness, and community composition. Partitioning overall beta diversity between sites into species replacement and richness effects revealed that the main driver of community change over time was the gradual accumulation of species, rather than one‐to‐one species replacement. At the spatial and temporal scales we studied, we conclude that prairie restoration efforts targeting plants also successfully restore bee communities.     

Trends in water quality and subtidal benthic communities in a temperate estuary: Is the response to restoration efforts hidden by climate variability and the Estuarine Quality Paradox?

The analysis of temporal patterns in water quality and benthic assemblages in estuaries constitutes an important methodological issue for discriminating the effects of natural and anthropogenic pressures. Temporal trends in water quality and in the subtidal benthic community over a 5-year interval in the Mondego estuary (Portugal) were investigated with the aim of assessing changes in environmental quality as a response to restoration efforts and climate variability. Particularly, we addressed the following questions: (a) Would trends in water quality and benthos behave consistently over the whole study period for the different zones of the monitoring network and indicate improvement or degradation in ecological condition? (b) Could we distinguish the effects of climate variability and restoration efforts in water quality and benthos from trend analysis results? (c) Could the response of the benthic communities and water quality be useful to guide the planning of future management actions in this system?Clear cause–effect relationships regarding the ecological response to restoration efforts and climate variability were indeed challenging to identify and interpret. In fact, the response of water quality and benthic communities to restoration efforts seemed to have been masked by the effects of climatic variability. Furthermore, the present study illustrated clearly the high environmental variability inherent to estuarine systems and the difficulty of clearly distinguishing natural from anthropogenic stressors, in agreement with the “Estuarine Quality Paradox”. Implications for ecological quality assessment and management of the Mondego estuary and other poikilohaline systems are discussed, namely with regard to the “one-out, all-out” principle required by the European Water Framework Directive (WFD).     

Temporal dynamics of marginal steppic vegetation over a 26‐year period of substantial environmental change

Questions: (1) Is climate a strong driver of vegetation dynamics, including interannual variation, in a range margin steppic community? (2) Are there long‐term trends in cover and species richness in this community, and are these consistent across species groups and species within groups? (3) Can long‐term trends in plant community data be related to variation in local climate over the last three decades? Location: A range margin steppic grassland community in central Germany. Methods: Cover, number and size of all individuals of all plant species present in three permanent 1‐m² plots were recorded in spring for 26 years (1980–2005). Climatic data for the study area were used to determine the best climatic predictor for each plant community, functional group and species variable (annual data and interannual variation) using best subsets regression. Results: April and autumn temperature showed the highest correlation with total cover and species richness and with interannual variations of cover and richness. However, key climate drivers differed between the five most abundant species. Similarly, total cover and number and cover of perennials significantly decreased over time, while no trend was found for the cover and number of annuals. However, within functional groups there were also contrasting species‐specific responses. Long‐term temperature increases and high interannual variability in both temperature and precipitation were strongly related to long‐term trends and interannual variations in plant community data. Conclusions: Temporal trends in vegetation were strongly associated with temporal trends in climate at the study site, with key roles for autumn and spring temperature and precipitation. Dynamics of functional groups and species within groups and their relationships to changes in temperature and precipitation reveal complex long‐term and interannual patterns that cannot be inferred from short‐term studies with only one or a few individual species. Our results also highlight that responses detected at the functional group level may mask contrasting responses within functional groups. We discuss the implications of these findings for attempts to predict the future response of biodiversity to climate change.     

Woodland restoration on agricultural land: long‐term impacts on soil quality

Woodland restoration is underway globally to counter the negative soil quality and ecological impacts of agricultural expansion and woodland fragmentation, and restore or enhance biodiversity, ecosystem functions and services. However, we lack information about the long‐term effects of woodland restoration on agricultural soils, particularly at temporal scales meaningful to woodland and soil development. This study utilized soil and earthworm sampling across a chronosequence of sites transitioning from “agricultural land” to “secondary woodland” (50–110 years) and “ancient woodland” (>400 years), with the goal of quantifying the effects of woodland restoration on agricultural land, on key soil quality parameters (soil bulk density, pH, carbon and nitrogen stocks, and earthworm abundance, biomass, species richness and diversity). Broad‐leaved woodland restoration led to significantly greater soil organic carbon (SOC) stocks compared to arable land, and young (50–60 years) secondary woodland increased earthworm species and functional diversity compared to both arable and pasture agricultural land. SOC stocks in secondary broad‐leaved woodlands (50–110 years) were comparable to those found in long‐term ancient woodlands (>400 years). Our findings show that broad‐leaved woodland restoration of agricultural land can lead to meaningful soil ecological improvement and gains in SOC within 50–110 years, and provide intel on how restoration activities may be best targeted to maximize soil quality and functions.     

The use of long‐term monitoring data for studies of planktonic diversity: a cautionary tale from two Swiss lakes

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Fish species richness in spring‐fed ponds: effects of habitat size versus isolation in temporally variable environments

1. Species richness in a habitat patch is determined by immigration (regional) and extinction (local) processes, and understanding their relative importance is crucial for conservation of biodiversity. In this study, we applied the Island Biogeography concept to spring ponds connected to a river in southwestern Japan to examine how immigration and extinction processes interact to determine fish species richness in temporally variable environments. 2. Fish censuses were conducted 15 times in 13 study ponds at 1–4 month intervals from August 1998 through October 2000. Effects of habitat size (pond area), isolation (distance from the river) and temporal environmental variability (water level fluctuation) on (i) species richness, (ii) immigration and extinction rates and (iii) population size and persistence of each fish species were assessed. 3. The results revealed predominant effects of distance on species richness, immigration/extinction rates and population size and persistence. Species richness decreased with increasing distance but was not related to either pond area or water level fluctuation. A negative effect of distance on immigration rate was detected, while neither pond area nor water level fluctuation had significant effects on extinction rate. Further, population size and persistence of four species increased with decreasing distance, suggesting that, in ponds close to the river, immigrants from the river reduce the probability of extinction (i.e. provide a rescue effect), contributing to the maintenance of high species richness. 4. Overall results emphasise the importance of immigration processes, rather than extinction, in shaping patterns of species richness in our system. The predominant importance of immigration was probably because of (i) high temporal variability that negates habitat‐size effects and (ii) continuous immigration that easily compensates for local extinctions. Our results suggest that consideration of regional factors (e.g. connectivity, locations of source populations and barriers to colonisation) is crucial for conservation and restoration of local habitats.     

Temporal changes of biodiversity in urban running waters – Results of a twelve-year monitoring study

Freshwater biodiversity underlies severe threats, mainly suffering from habitat degradation by anthropogenic land use, in particular by urbanisation. However, recent long-term studies indicate recovery of stream macroinvertebrate diversity due to improved water quality at least in North America and Europe. We monitored macroinverbrates at 56 urban stream sites over a 12-year period (2009–2020) in Braunschweig, a German urban district. We utilised these data to investigate spatio-temporal changes in taxon richness and assemblage structure as well as factors potentially affecting the resulting patterns. Overall taxon richness was increasing over the study period, comprising both all taxa and taxa being indicators for healthy stream conditions. 53.6% of the sites had significant positive trends becoming most eminent since 2014, despite decelerating since 2018, the beginning of an extra-ordinary dry period. Only 10.7% of the study sites had negative trends. Assemblage structure was shaped by environmental factors like stream width and water quality. Over-average taxon richness including positive trends and higher numbers of indicator taxa of healthy stream conditions was found in streams with higher flow velocity, good saprobic conditions and more natural streambed structure. In contrast, low taxon richness and predominance of tolerant taxa were found in streams with more degraded conditions. Most of the environmental conditions having positive effects on taxon richness were improved by various programs set up by the environmental authorities. We therefore conclude, if urban stressors like organic pollution and structural degradation can be mitigated by revitalisation and water quality improvement, urban streams can have good potential for increasing biodiversity and improving ecological functioning.     

Biodiversity monitoring by community‐based restoration groups in New Zealand

In recent decades, community groups have transformed habitat restoration, pest control and species translocations in New Zealand. Large areas of wild New Zealand benefit hugely from ongoing management by community‐based restoration groups. Areas near cities and towns have especially good access to pools of keen volunteers. Community groups are involved in monitoring progress with their work, as well as monitoring biodiversity changes in general at their project sites. New tools powered by modern technologies are creating the opportunity for New Zealand's community volunteers to play a transformative role in biodiversity monitoring for either purpose. These tools are reducing the resources and expertise required for species detection and identification. Smartphones with cameras, GPS, audio recorders and data apps make it easier than ever to record species observations. Crowd‐sourced identification of species in photographs and sounds loaded onto NatureWatch NZ allow volunteers to make observations of a much wider range of taxa than just common birds and trees. Realising this potential requires community groups, scientists and their institutions to collaborate in building and maintaining simple, accessible monitoring systems that (i) require and promote standard monitoring methods, (ii) provide efficient data entry in standard formats, (iii) generate automated results of use to community groups and (iv) facilitate public sharing of data to contribute to regional, national and global biodiversity monitoring. Some New Zealand monitoring systems developed recently to assist community‐based restoration groups with monitoring mammalian predator control are good examples of this approach. Making this happen at a large scale across many community groups and taxa requires increased and coordinated long‐term institutional support for monitoring systems and training.     

Patterns of temporal community turnover are spatially synchronous across boreal lakes

1. Ecosystems are often exposed to broad‐scale environmental change, which can potentially synchronise community dynamics and biodiversity trends. Detection of temporal coherence may, however, depend on the metrics used and their sensitivity to detect change, requiring several lines of evidence to elucidate the full range of temporal responses to environmental change. 2. Here, we tested whether the patterns of synchrony among littoral invertebrate communities of Swedish lakes over 20 years (1988–2007) differed when analysed using univariate (taxon richness, evenness, Shannon diversity and total abundance) or multivariate (temporal turnover in community composition) metrics. We included both culturally acidified and circumneutral lakes to examine whether anthropogenic stress influenced the patterns of synchrony. 3. Average total abundance, taxon richness and temporal turnover in community composition changed monotonically with time, while evenness and Shannon diversity fluctuated around a long‐term mean. However, among‐lake variability was high, resulting in a weak temporal coherence. Only trends of temporal turnover changed synchronously across lakes, irrespective of their acidification history. 4. Spatially synchronous trends in turnover across lakes were correlated with increasing water colour and decreasing sulphate concentrations, showing the importance of regional drivers of spatiotemporal coherence. 5. Our results underpin an increasing body of evidence that the detection of diversity patterns varies among metrics that ignore (taxon richness, evenness, Shannon diversity) or consider (turnover) species identities. More generally, our results suggest that community‐level studies of synchrony are suitable for elucidating the role of intrinsic versus extrinsic factors in mediating complex community assembly processes in the long term. This, in turn, contributes to our understanding of temporal patterns of biodiversity.     

Abundant and rare picoeukaryotic sub‐communities present contrasting patterns in the epipelagic waters of marginal seas in the northwestern Pacific Ocean

In this work, they compared patterns of abundant and rare picoeukaryotic sub‐communities in the epipelagic waters (surface and 40–75 m depth subsurface layers) of the East and South China Seas across seasons via 454 pyrosequencing of the V4 region of 18S rDNA. They also examined the relative effects of environmental filtering, dispersal limitations and seasonality on community assembly. Their results indicated that (i) in the surface layer, abundant taxa are primarily influenced by dispersal limitations and rare taxa are primarily influenced by environmental filtering, whereas (ii) in the subsurface layer, both abundant and rare sub‐communities are only weakly influenced by environmental filtering but are strongly influenced by dispersal limitations. Moreover, (iii) abundant taxa exhibit stronger temporal variability than rare taxa. They also found that abundant and rare sub‐communities display similar spatial richness patterns that are negatively correlated with latitude and chlorophyll a and positively correlated with temperature. In summary, environmental filtering and dispersal limitations have different effects on abundant and rare picoeukaryotic sub‐communities in different layers. Thus, depth appears as an essential variable that governs the structuring patterns of picoeukaryotic communities in the oceans and should be thoroughly considered to develop a more comprehensive understanding of oceanic microbial assemblages.     

Algal richness and life‐history strategies are influenced by hydrology and phosphorus in two major subtropical wetlands

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Explaining ecological shifts: the roles of temperature and primary production in the long‐term dynamics of benthic faunal composition

Predicting the ecological consequences of environmental change requires that we can identify the drivers of long‐term ecological variation. Biological assemblages can exhibit abrupt deviations from temporal trends, potentially resulting in irreversible shifts in species composition over short periods of time. Such dynamics are hypothesised to occur as gradual forcing eventually causes biological thresholds to be crossed, but could also be explained by biota simply tracking abrupt changes to their environment. Here, we modelled temporal variation in a North Sea benthic faunal assemblage over a 40‐year period (1972–2012) to test for changes to temporal trends of biota and determine whether they could be explained by underlying patterns in sea temperature and primary production. These extrinsic factors were postulated to influence community dynamics through their roles in determining and sustaining the metabolic demands of organisms, respectively. A subset of mainly large and long‐lived taxa (those loaded on the first principal component of taxa densities) exhibited two significant changes to their temporal trends, which culminated in a shift in assemblage composition. These changes were explained by an increase in pelagic primary production, and hence detrital food input to the seabed, but were unrelated to variation in sea temperature. A second subset of mainly small and short‐lived taxa (those loaded on the second principal component) did not experience any significant changes to their temporal trends, as enhanced pelagic primary production appeared to mitigate the impact of warming on these organisms. Our results suggest that abrupt ecological shifts can occur as biota track underlying variation in extrinsic factors, in this case primary production. Changes to the structure of ecosystems may therefore be predictable based on environmental change projections.     

Consistent patterns in diatom assemblages and diversity measures across water‐depth gradients from eight Boreal lakes from north‐western Ontario (Canada)

1. Until recently, the distribution of diatom species assemblages and their attributes (e.g. species richness and evenness) in relation to water depth have been identified but not quantified, especially across several lakes in a region. Here, we examined diatom assemblages in the surface sediment across a water‐depth gradient in eight small, boreal lakes in north‐western Ontario, minimally disturbed by human activities. 2. Surface‐sediment diatom assemblages were collected within each lake along a gentle slope from near‐shore to the centre deep basin of the lake, at a resolution of ～1 m water depth. Analysis of sedimentary samples provided an integrated view of assemblages that were living in the lake over several years and enabled a high‐resolution analysis of many lakes. The study lakes ranged in water chemistry, morphology and size and are located along an east–west transect approximately 250 km long in north‐western Ontario (Canada). 3. The majority of diatom species were distributed along a continuum of depth, with those taxa having similar habitat requirements forming distinct, though overlapping, assemblages. Three major zones of diatom assemblages in each lake were consistently identified: (i) a near‐shore assemblage of Achnanthes (sensu lato), Nitzschia, Cymbella (sensu lato) and other benthic species; (ii) a mid‐depth assemblage of small Fragilaria (sensu lato)/small Aulacoseira and various Navicula taxa; and (iii) a deep‐water assemblage of planktonic origin (mainly Discotella spp.). 4. The depth of the transition between assemblage zones varied between the eight lakes. The boundary between the deep‐water planktonic zone and the mid‐depth benthic zone varied according to water chemistry and was probably related to light attenuation. The boundary was deeper in lakes with the lower dissolved organic carbon and total phosphorus (TP) (i.e. less light attenuation) and vice versa. 5. Generally, species richness, species evenness and turnover rate of species as a function of depth were significantly lower in the planktonic assemblage zone in comparison with the two zones nearer the shore. Reproducibility of species and assemblage distributions across the depth gradient of the lakes illustrated that, despite potential for sediment transport, detailed ecological characterisation of diatom species can be gleaned from sedimentary data. Such data are often lacking, particularly for near‐shore benthic species.     

Long‐term trends in restoration and associated land treatments in the southwestern United States

Restoration treatments, such as revegetation with seeding or invasive species removal, have been applied on U.S. public lands for decades. Temporal trends in these management actions have not been extensively summarized previously, particularly in the southwestern United States where invasive plant species, drought, and fire have altered dryland ecosystems. We assessed long‐term (1940–2010) trends in restoration using approximately 4,000 vegetation treatments conducted on Bureau of Land Management lands across the southwestern United States. We found that since 1940, the proportions of seeding and vegetation/soil manipulation (e.g. vegetation removal or plowing) treatments have declined, while the proportions of prescribed burn and invasive species treatments have increased. Treatments in pinyon‐juniper and big sagebrush communities declined in comparison to treatments in desert scrub, creosote bush, and riparian woodland communities. Restoration‐focused treatment objectives increased relative to resource extraction objectives. Species richness and proportion of native species used in seeding treatments also increased. Inflation‐adjusted costs per area rose 750% for vegetation/soil manipulation, 600% for seeding, and 400% for prescribed burn treatments in the decades from 1981 to 2010. Seeding treatments were implemented in warmer and drier years when compared to the climate conditions of the entire study period and warmer and wetter years relative to several years before and after the treatment. These results suggest that treatments over a 70‐year period on public lands in the southwestern United States are shifting toward restoration practices that are increasingly large, expensive, and related to fire and invasive species control.     

Long‐term effects of plant diversity and composition on plant stoichiometry

Plant elemental composition can indicate resource limitation, and changes in key elemental ratios (e.g. plant C:N ratios) can influence rates including herbivory, nutrient recycling, and pathogen infection. Although plant stoichiometry can influence ecosystem‐level processes, very few studies have addressed whether and how plant C:N stoichiometry changes with plant diversity and composition. Here, using two long‐term experimental manipulations of plant diversity (Jena and Cedar Creek), we test whether plant richness (species and functional groups) or composition (functional group proportions) affects temporal trends and variability of community‐wide C:N stoichiometry. Site fertility determined the initial community‐scale C:N ratio. Communities growing on N‐poor soil (Cedar Creek) began with higher C:N ratios than communities growing on N‐rich soil (Jena). However, site‐level plant C:N ratios converged through time, most rapidly in high diversity plots. In Jena, plant community C:N ratios increased. This temporal trend was stronger with increasing richness. However, temporal variability of C:N decreased as plant richness increased. In contrast, C:N decreased over time at Cedar Creek, most strongly at high species and functional richness, whereas the temporal variability of C:N increased with both measures of diversity at this site. Thus, temporal trends in the mean and variability of C:N were underlain by concordant changes among sites in functional group proportions. In particular, the convergence of community‐scale C:N over time at these very different sites was mainly due to increasing proportions of forbs at both sites, replacing high mean C:N (C4 grasses, Cedar Creek) or low C:N (legumes, Jena) species. Diversity amplified this convergence; although temporal trends differed in sign between the sites, these trends increased in magnitude with increasing species richness. Our results suggest a predictive mechanistic link between trends in plant diversity and functional group composition and trends in the many ecosystem rates that depend on aboveground community C:N. Synthesis We compared the effect of plant diversity on the temporal dynamics of community stoichiometry in two long‐term grassland diversity experiments: the Cedar Creek and Jena Experiments. Changes in community C:N ratios were accelerated by increasing diversity at both sites, but in opposite directions depending on soil fertility. Stoichiometry changes were driven by shifts of functional group composition differing in their elemental compositions, the identity of the functional groups depending on the site. Thus, we highlighted that community turnover constrained the effect of diversity on plant stoichiometry at both sites