Gábor Ónodi

Revisiting and resurveying historical vegetation plots has become a central tool for documenting changes in plant communities. However, theoretical studies suggest that reliable detection of long‐term trends from a single resurvey is nearly impossible for species with high stochastic population fluctuations, though it may be relatively informative in more stable plant communities dominated by perennial species. To empirically test how well data from a single resurvey align with estimated long‐term trends in species richness and community turnover in European grasslands, we compiled permanent‐plot time series from dry to wet grasslands that were sampled multiple times (most often annually) during the monitoring period. We then extracted pairs of surveys (baseline survey and one resurvey) from the individual time series and compared changes inferred from these pairs with trends inferred from the full time series using Bayesian multilevel regressions. We found that in studied grasslands, data from a single resurvey can reliably reflect long‐term trends in community turnover but deliver less reliable inferences of trends in species richness. Synthesis . Despite limitations, single resurveys remain a useful tool for describing and understanding changes in plant diversity, at least in perennial grasslands. However, they should be used cautiously, especially regarding changes in species richness. In general, trend inference becomes more accurate as the number of resurveys increases and the monitoring period lengthens. Unfortunately, such long‐term, multiple‐resurvey schemes still remain scarce.

Monitoring the condition of grasslands is essential given their vital role in food security, carbon sequestration and other ecosystem services. Harvested aboveground biomass (HAB) and aboveground net primary production (ANPP) are among the most important grassland state indicators. However, spatially explicit production estimates are largely lacking, and grassland area estimations also remain uncertain. This study addresses these gaps for drought-prone Central European grasslands over 2017–2024. We synthesized grassland extent data, collected extensive field measurements on biomass (BM), and used remote sensing-based biophysical proxies to build an ensemble of six linear models for spatial extrapolation at 10 m resolution. Bayesian framework was used for the linear model fitting that also considers uncertainty of the observations. The ensemble mean ANPP was 310.7 ± 19 gBM m⁻², with modest interannual variability. Upscaled country-wide mean ANPP was 34.3 ± 13.3 Mt year⁻¹. The results indicate that, within the frame of the present study, the remote sensing-based linear model selection has a larger influence on the country totals than the grassland area database selection. The results highlight that both grassland area uncertainty and model construction are major sources of uncertainty in biomass estimation that have to be addressed in future studies.

Climate change is expected to facilitate the spread of many alien C ₄ grasses, which perform better in warmer climates than C ₃ species. Furthermore, extreme climatic events may promote plant invasions by suppressing native species. Despite several well-documented cases of C ₄ perennial grass invasions, the role of extreme events in facilitating plant invasions is rarely documented. Sporobolus cryptandrus , a North American C ₄ perennial bunchgrass, has recently been established in several European locations, including mass invasions in natural grasslands. Given the early stage of S. cryptandrus invasion and its potential for future spread, a better understanding of the dynamics and mechanisms of its invasion is urgently needed. In a natural sand steppe in Hungary, an EU priority habitat type with one of the largest S. cryptandrus populations in Europe, we monitored S. cryptandrus invasion and changes in species dominance in permanent plots annually from 2022 to 2024, which included an extreme drought in 2022. In the post-drought years, we found a gradual increase in both the frequency of S. cryptandrus and the number of plots where it was the dominant species. Following the drought-induced dieback of the dominant native grass, Festuca vaginata , the cover of S. cryptandrus almost doubled from 2023 to 2024 and approached that of F. vaginata . This expansion was positively related to F. vaginata dieback. Our study demonstrates that S. cryptandrus can spread and become dominant rapidly in natural grasslands. Furthermore, our results provide empirical evidence on how extreme events can accelerate an ongoing plant invasion process.

Grassland state assessment is essential given their vital role in food security, carbon sequestration and other ecosystem services. Harvested aboveground biomass (HAB), aboveground net primary production (ANPP) and net primary production (NPP) are among the most important grassland state indicators. However, spatially explicit production estimates are largely lacking, and grassland area estimations also remain uncertain. This study addresses these gaps for drought-prone Central European grasslands over 2017-2024. We synthesized grassland extent data, collected extensive field measurements, and used remote sensing-based biophysical proxies to build an ensemble of eight linear models for spatial extrapolation at 10 m resolution. The proxies explained 11-41% of observed biomass (BM) variability. The ensemble mean ANPP was 325.7±56.6 gBM m2 year1 (median: 316 gBM m2 year1), with modest overall interannual variability. Upscaled country-wide ANPP averaged 35.1±9.2 Mt BM year-1 annually (range: 31.9-38.3; median: 35.9 Mt BM year-1). Uncertainty from grassland area estimation was roughly twice that from model choice. Using literature and local data, NPP was estimated at 421±110 gC m2 year-1 (median value), showing low interannual variability. Results highlight grassland area uncertainty as the dominant source of error in biomass estimation, rather than the remote sensing models themselves.

Extreme events, such as droughts, may severely affect ecosystem structure and functions in the short term. While in a stable climate, ecosystems are expected to recover in the long term, in an era of accelerating climate change, it is a critical question whether ecosystems recover to their pre‐drought state and what limits recovery. In a temperate grassland in Central Hungary, we monitored permanent vegetation plots and analyzed changes in species and functional group dominance on a yearly basis from 1999 to 2023, a period with increasing temperature but no trend in precipitation. Extreme droughts in 2003 and 2022 caused mass diebacks of the dominant perennial bunchgrasses, Festuca vaginata and Stipa borysthenica . Following the 2003 drought, annual species became temporarily dominant. Although perennial grasses regained dominance by 2006, this included a shift in dominant species from Festuca to Stipa for over a decade. In 2016, however, a mass establishment of Festuca occurred, which, in turn, enabled Festuca to recover and regain dominance by 2021. A detailed sub‐sampling of grass seedlings from 1999 to 2008 revealed that high seedling emergence and survival were much less frequent for Festuca than for Stipa , which likely explains the shift in species dominance to Stipa and the delayed post‐drought recovery of Festuca . The long‐term (1901–2023) weather records for the study area showed no trend in the frequency of drought types that caused mass mortality of perennial grasses in the last 25 years. Collectively, these results suggest that our study system still has the capacity to recover from extreme droughts, but the recovery time of dominant species is unpredictable, and more frequent droughts in the future may prevent recovery and induce persistent change. In general, our results underscore the importance of demographic processes related to seedling establishment in understanding and predicting the recovery from mass dieback events in a changing climate.

Despite the increased frequency and severity of droughts in many regions and declining biodiversity, existing experimental evidence for changes in plant species richness associated with altered water availability is limited. The growing extent of drylands highlights the need to predict precipitation‐related changes in species richness, which requires a better understanding of the mechanisms. We carried out a field experiment applying a single extreme drought event and combined it with subsequent chronic alteration of summer precipitation for 7 years in a water‐limited temperate grassland. We assessed how altered precipitation regimes and a previous extreme drought affect species richness. We compared a simple analysis assuming only the net effect of precipitation on species richness to a complex approach by structural equation modelling that included both the direct effects of precipitation and indirect effects through the biomass of dominant grass species. Using simple analysis, we found significant positive and nonsignificant precipitation–species richness relationships in the presence and absence of extreme drought, respectively. The complex analysis disentangled direct and indirect pathways between precipitation and species richness. The indirect pathway acted only in the absence of drought. In this case, increasing precipitation increased the biomass of dominant species, which, in turn, decreased species richness, acting as a mediator variable. The direct relationship was positive, independent of the presence of drought. Synthesis. Consistent with the global relationship between water availability and species richness, we experimentally showed that decreasing precipitation decreases species richness. Furthermore, we found that increasing precipitation may also decrease plant species richness via an indirect pathway acting through the biomass of the dominant species. Our results highlight that species richness can become more sensitive to changes in precipitation after extreme drought events that eliminate or set back dominant species.

Repeated surveys of vegetation plots offer a viable tool to detect fine‐scale responses of vegetation to environmental changes. In this study, our aim was to explore how the species composition and species richness of dry grasslands changed over a period of 17 years, how these changes relate to environmental changes and how the presence of spring ephemerals, which may react to short‐term weather fluctuations rather than long‐term climatic trends, may influence the results. A total of 95 plots was surveyed in 2005 and resurveyed in 2022 in dry grasslands of the Kiskunság Sand Ridge (Hungary, Eastern Central Europe), where there has been a significant increase in mean annual temperature during the last decades, while no trends in precipitation can be identified. Db‐RDA was performed to reveal compositional changes. The changes in environmental conditions and naturalness state were assessed using ecological and naturalness indicator values. We also compared per‐plot richness of all species, native species and non‐native species of the old and the new relevés. All analyses were repeated after removing all spring ephemerals. We found clear temporal changes in species composition. Mean temperature indicator values increased, while mean soil moisture indicator values decreased during the 17 years. Also, decreasing per‐plot richness was detected both for all species and for native species, while mean naturalness increased. After the removal of spring ephemerals, the compositional changes were less obvious although still significant. The increase in the temperature indicator values remained significant even without the spring ephemerals. However, the decrease in the moisture indicator values, the decrease in the number of all species and native species, as well as the increase in naturalness indicator values disappeared when spring ephemerals were excluded from the analyses. Our study demonstrates that between‐year weather differences and long‐term environmental trends both contribute to observed vegetation changes.

Aims: More intense and frequent droughts predicted for the future will heavily affect grasslands worldwide and, along with various other disturbances, possibly trigger major vegetation shifts. Therefore, documenting these changes and understanding the mechanisms behind them is essential. Our study aimed to investigate how dominant species in a semiarid grassland respond to a combination of small‐scale experimental disturbances and naturally occurring droughts and uncover the underlying mechanisms.Location: Central Hungary.Methods: We conducted a small‐scale disturbance experiment in a semiarid temperate grassland and followed recovery for 18 years, including severe drought events. In 1999, we established 16 sites, eight dominated by Festuca vaginata and another eight by Stipa borysthenica, the two dominant species of sand grasslands in the region. At each site, three permanent plots were marked that received either a cutting or digging treatment or remained as controls. We monitored the cover and density changes of Festuca and Stipa annually.Results: In the early years following the disturbance, Festuca recovered at a similar rate under both disturbance treatments, while Stipa recovered faster in cut than in dug plots. When natural drought events caused major diebacks of both species, Stipa recovered very quickly and regained dominance in initially Stipa‐dominated plots, and it also took over in initially Festuca‐dominated control and cut plots. However, digging at Festuca‐dominated sites delayed drought‐induced Stipa colonisation and thus favoured Festuca recovery. We found that the poor performance of Stipa in dug plots was related to sharply reduced seedling establishment, which resulted from the low number of seeds captured by the bare soil surface after digging.Conclusions: Our results demonstrate that recurring drought events may induce dominance shifts in temperate grasslands, but small‐scale disturbances can modulate vegetation responses. Our findings emphasise the importance of post‐disturbance regeneration patterns in drought‐induced vegetation shifts and show that seed dispersal strategy may have a major effect on vegetation dynamics.

Ecological theory posits that temporal stability patterns in plant populations are associated with differences in species' ecological strategies. However, empirical evidence is lacking about which traits, or trade-offs, underlie species stability, especially across different biomes. We compiled a worldwide collection of long-term permanent vegetation records (greater than 7000 plots from 78 datasets) from a large range of habitats which we combined with existing trait databases. We tested whether the observed inter-annual variability in species abundance (coefficient of variation) was related to multiple individual traits. We found that populations with greater leaf dry matter content and seed mass were more stable over time. Despite the variability explained by these traits being low, their effect was consistent across different datasets. Other traits played a significant, albeit weaker, role in species stability, and the inclusion of multi-variate axes or phylogeny did not substantially modify nor improve predictions. These results provide empirical evidence and highlight the relevance of specific ecological trade-offs, i.e. in different resource-use and dispersal strategies, for plant populations stability across multiple biomes. Further research is, however, necessary to integrate and evaluate the role of other specific traits, often not available in databases, and intraspecific trait variability in modulating species stability.