(Poster image credit: Dr. Joan Dudney)

Our research on grassland ecosystems in the California Floristic Province (CFP) sheds light on how community composition is rapidly shifting to species associated with hotter and drier locations in response to climate change.

This study began by assembling long-term datasets from 12 observational sites (Fig. 1) by Dr. Josephine Lesage during her dissertation research. Drs. Kai Zhu and Yiluan Song then added data from three global change experiments. With datasets contributed by researchers across institutions, we were able to paint a comprehensive picture of how Californian grassland communities are responding to climate change. The question now is, how?

Figure 1. A researcher conducting sampling at Jasper Ridge to study long-term changes in grassland communities. Credit: Dr. Richard Hobbs.

Our work was built on a line of research on community thermophilization  (i.e., a shift toward species that thrive in warmer conditions) measured by the Community Temperature Index, including our own work (Feeley et al., 2020; Freeman et al., 2021). We placed plant species and communities in what we call the "climate space," specifically the temperature and precipitation conditions they are associated with (Fig. 2A), instead of grouping plants based solely on their taxonomy or guilds. This method allowed us to uncover consistent patterns directly in response to climate change, which might otherwise be hidden.

One of our key findings was that grassland communities in California are rapidly shifting at rates comparable to those of rising temperatures and changing precipitation patterns. This surprising result changed the popular belief that all terrestrial ecosystems are “lagging” behind, based on evidence from forest communities (Lenoir et al., 2020). Compared to forests that accumulate climatic debts that will manifest later (Bertrand et al., 2016), grasslands are responding to climate change continuously. In addition, we found clear evidence of xerophilization (i.e., a shift toward species that thrive in drier conditions), particularly in the Mediterranean climate of the CFP—a trend that’s been inconsistent in temperate regions.

To unpack the observed community-level shifts on a lower level of organization, we identified which species have increased or decreased in relative abundance. This added an important layer to our understanding of how individual species are faring under climate change. While communities as a whole may appear to be “keeping pace” with climate change, this doesn’t necessarily mean that all species maintain stable populations. In some cases, these shifts may be driven by the increased dominance of non-native species.

Notably, we found a consistent pattern in both our observational and experimental data: shifts toward hotter and drier species were closely coupled, maintaining a stable ratio (Fig. 2B). This coupling could stem from the distinct species niches shaped by the Mediterranean climate or the simultaneous warming and drying trends. We think that the former plays a more important role, suggesting that there may be limits to how much communities can track unprecedented climate conditions.

Figure 2. Grassland community shifts synthesized from both the observations and experiment. (A) Community compositions at the 12 observational sites (blue) and the experimental site (orange) are described by the median Community Temperature Index (CTI, °C) and Community Precipitation Index (CPI, mm), positioned in estimated species’ climate niche centroids (median, gray). (B) Communities shift in a consistent direction in the climate space in the observations (blue) and experiment (orange). For the observational sites, arrows point from the start to the end of the sampling period; for the experiment site, arrows point from ambient to warming treatments. Arrows are set to be semi-transparent for non-significant relationships in either CTI or CPI (two-sided t-test, p > 0.05).

These findings have immediate implications for biodiversity conservation in the CFP, one of the most diverse and threatened ecosystems in the world (Harrison et al., 2024). As grasslands shift toward a certain set of species, it becomes urgent to protect those at risk of being left behind. Restoration efforts will also need to account for these shifts by planting species adapted to the anticipated climate conditions.

This research has also contributed to broader discussions on ecological acclimation (Felton et al., 2022), which explored how different ecosystems keep pace with climate change with fast and slow processes. We hypothesize that grasslands and forests might exist on a spectrum, with grasslands showing faster acclimation and forests vice versa. Our curiosity now lies in predicting acclimation based on ecosystem properties and delving deeper into how both the structure and function of these systems adjust to ongoing climate change.

Article (Open Access)

Zhu, K., Song, Y., Lesage, J.C., Luong, J.C., Bartolome, J.W., Chiariello, N.R., Dudney, J., Field, C.B., Hallett, L.M., Hammond, M., Harrison, S.P., Hayes, G.F., Hobbs, R.J., Holl, K.D., Hopkinson P., Larios, L., Loik, M.E., Prugh, L.R. (2024). Rapid shifts in grassland communities driven by climate change. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-024-02552-z

References

Bertrand, R., Riofrío-Dillon, G., Lenoir, J., Drapier, J., de Ruffray, P., Gégout, J.-C., & Loreau, M. (2016). Ecological constraints increase the climatic debt in forests. Nature Communications, 7, 12643–12643.

Feeley, K. J., Bravo-Avila, C., Fadrique, B., Perez, T. M., & Zuleta, D. (2020). Climate-driven changes in the composition of New World plant communities. Nature Climate Change, 10(10), 965–970. https://doi.org/10.1038/s41558-020-0873-2

Felton, A. J., Shriver, R. K., Stemkovski, M., Bradford, J. B., Suding, K. N., & Adler, P. B. (2022). Climate disequilibrium dominates uncertainty in long‐term projections of primary productivity. Ecology Letters, 25(12), 2688–2698. https://doi.org/10.1111/ele.14132

Freeman, B. G., Song, Y., Feeley, K. J., & Zhu, K. (2021). Montane species track rising temperatures better in the tropics than in the temperate zone. Ecology Letters, 24(8), 1697–1708. https://doi.org/10.1111/ele.13762

Harrison, S., Franklin, J., Hernandez, R. R., Ikegami, M., Safford, H. D., & Thorne, J. H. (2024). Climate change and California’s terrestrial biodiversity. Proceedings of the National Academy of Sciences, 121(32), e2310074121. https://doi.org/10.1073/pnas.2310074121

Lenoir, J., Bertrand, R., Comte, L., Bourgeaud, L., Hattab, T., Murienne, J., & Grenouillet, G. (2020). Species better track climate warming in the oceans than on land. Nature Ecology & Evolution, 4(8), Article 8. https://doi.org/10.1038/s41559-020-1198-2