Climate change is projected to significantly alter species distributions, leading to cascading risks for global biodiversity and ecosystem functioning. While numerous studies have demonstrated slow responses of terrestrial plant species, particularly in forest ecosystems, to rapid warming, grasslands, with their prevalence of short-lived species and direct exposure to harsh climatic conditions, may exhibit a faster response. This study focuses on the California Floristic Province (CFP), a global biodiversity hotspot encompassing diverse geographic, climatic, and edaphic gradients, to investigate whether grassland communities closely track climate change in terms of compositional shifts. The CFP harbors a rich diversity of plant species, including a high proportion of endemics, but is also heavily invaded by non-native species. Understanding the dynamics of CFP grasslands under climate change is crucial for early detection and accurate projection of impacts on biodiversity and ecosystem services. The researchers aimed to determine if grassland community composition in the CFP responds rapidly and consistently to climate change by shifting towards species associated with warmer and drier locations at a pace similar to the rate of climate change itself.

Previous research on the pace of compositional shifts in grasslands relative to climate change has yielded inconsistent findings, with some studies showing increases in grasses at the expense of forbs under warming and drying conditions, while others from manipulative experiments reported seemingly contradictory shifts. This variability highlights the challenge of assessing how grassland communities track climate change, underscoring the need for robust methods and extensive, long-term datasets. The existing literature predominantly focuses on forest ecosystems, leaving the response of other ecosystems, especially grasslands, relatively understudied. Therefore, a comprehensive analysis integrating long-term observational and experimental data is necessary to resolve the discrepancies and fully understand grassland responses to climate change.

To address the research question, the researchers combined long-term quantitative observations with manipulative experiments within the CFP. They first compiled twelve long-term observational records spanning a period of climate warming and drying, covering a broad geographical area and diverse environmental conditions (e.g., soil type, dominant vegetation). Leveraging 829,337 occurrence records from a large-scale community science program, along with 30-year temperature and precipitation climatologies, they estimated the realized climatic niches of 349 vascular plant species in the CFP. Community compositional shifts were then quantified using community temperature index (CTI) and community precipitation index (CPI), calculated from species' realized climatic niches and their relative abundances. This approach is sensitive to nuanced changes in species relative abundance, not requiring complete species turnover. The same approach was applied to three long-term global change experiments within the CFP, allowing comparison of observational and experimental results. Finally, species abundance changes were analyzed, and observational and experimental shifts were synthesized in the climate niche space to assess consistency and magnitude of shifts.

The study revealed that grassland communities in the coastal CFP shifted towards species associated with warmer and drier conditions at rates comparable to the observed climate change. Specifically, the communities experienced significant thermophilization (0.0216 ± 0.00592 °C yr⁻¹) and xerophilization (−3.04 ± 0.742 mm yr⁻¹). These trends were consistent across both the observational and experimental datasets. Eight out of twelve observational sites showed significant increases in CTI and significant decreases in CPI. The Jasper Ridge Global Change Experiment showed a significant increase in CTI (0.148 ± 0.0249 °C) and a decrease in CPI (−17.3 ± 2.76 mm) in the third phase of warming treatment, demonstrating a rapid response to experimental warming and drying. Additional watering and drought experiments further supported the coupled effects of thermophilization and xerophilization driven by climate change. Analysis of species abundance changes revealed that species becoming more abundant were generally associated with warmer and drier locations. Synthesis of observational and experimental data in the climate niche space showed consistent community compositional shifts, indicating a coupled thermophilization and xerophilization response to climate change.

The findings of this study contrast with the documented lagged responses in forest communities, highlighting the greater sensitivity of grassland ecosystems to climate change. The rapid rate of grassland community thermophilization observed in this study exceeds that reported for many forest communities. Several factors might contribute to the rapid response, including faster population turnover of common species, greater exposure to macroclimatic changes, and the dominance of non-native species. The study also reveals a region-wide xerophilization, often overlooked in temperate studies. The observed coupling between thermophilization and xerophilization, driven by the negative correlation between temperature and precipitation in the Mediterranean climate of the CFP, suggests that future combinations of climatic conditions exceeding the existing species' climatic niches may limit the capacity of communities to track changes in multiple climatic variables.

This study provides strong evidence for rapid and consistent directional shifts in grassland communities in the CFP in response to climate change. The findings highlight the vulnerability of grassland ecosystems to climate change and suggest potential implications for biodiversity and ecosystem functioning. Future research could focus on expanding the spatial scope to include a broader range of grassland types, incorporating intraspecific variation and adaptation in climatic niche estimations, and investigating the consequences of these compositional shifts on ecosystem services.

The study primarily focused on coastal CFP grasslands, with limited data from interior grassland communities. The estimation of climatic niches did not account for intraspecific variation or the capacity for species acclimation or adaptation. The warming treatment in the experiment also resulted in some drying, making it challenging to fully disentangle the individual effects of warming and drying. Despite these limitations, the consistent results across multiple observational and experimental datasets provide strong support for the rapid response of grassland communities to climate change.