Where have all the flowers gone? Climate change is driving the loss of forbs and diversity in Californian grasslands
Reference: Harrison, S.P., E.S. Gornish, and S. Copeland. 2015. Climate-driven diversity loss in a grassland community. Proceedings of the National Academy of Sciences of the United States of America. 112: 8672-8677. https://doi.org/10.1073/pnas.1502074112
How is climate change impacting plant communities?
This vexing question is one that keeps plant ecologists like myself awake at night scouring the current literature and planning the next experiment. We know that arid states like California are already experiencing increasingly long droughts and extreme temperature increases. Climate projections predict even more drying in these systems in the future. Unfortunately, there hasn’t been a great consensus among studies as to exactly how the changing climate will impact plant communities, particularly when it comes to species diversity. Many researchers have chosen to focus on changes in the ranges and populations of individual species rather than whole communities. Those that have studied whole communities have found everything from increased overall diversity to significantly decreased diversity. It seems that the impacts of climate change on diversity tend to be very context dependent in these experimental studies.
Few researchers have directly observed changes in plant community diversity in response to natural (not experimentally manipulated) trends in climate over an extended period of time. Dr. Susan Harrison at the University of California-Davis and her colleagues took it upon themselves to do just that in a 15 year observational study of 80 California grassland sites published in 2015 in the journal Proceedings of the National Academy of Sciences.
Plant diversity matters- for a lot of reasons.
Plant communities with high numbers of species (species richness) have been shown to provide a greater number of ecosystem services such as high productivity and invasion resistance. They also tend to be more stable and retain functions like nutrient cycling under environmental fluctuation than low diversity communities. When forbs (otherwise known as wildflowers or flowering plants other than grasses) decline in a community, pollinators and grazing wildlife species such as elk ultimately suffer too.
Decreases in precipitation are killing drought sensitive annual forbs and diversity is declining in CA grasslands.
The sites observed in the study by Harrison et al. were located at the UC-Davis McLaughlin Natural Reserve. These grasslands were of varied dominant community types: native annual, native perennial, invasive forb, and invasive grass. Overall, native annual forbs were generally the most common plant type across sites as is typical for California grasslands. The sites also had soils of different fertility levels (which corresponded with different numbers of species) and had been exposed to different grazing treatments and fire histories. In order to rule out possible influences of the size of the area examined, the researchers sampled plant abundances both at a local level (5m2 at each site) and a landscape level (all 80 sites together).
Seasonal precipitation, rainfall, solar radiation and humidity were all considered as potential climatic factors driving changes in diversity. Over the 15 year study period, there was a consistent decrease in mid-winter precipitation, typical of climate change effects seen in the arid American west, which are anticipated to negatively affect diversity since many grassland plants are newly emerging seedlings during this time.
The researchers also decided to take a physiological approach to understanding the climate-diversity relationship by measuring the specific leaf area (SLA) or leaf area to mass ratio of each species. SLA is related to drought sensitivity because there is a high water demand for plants that invest in a large, thin leaf (high SLA, ie more surface area for evaporation or water loss).
Overall, diversity declined in all of the plant communities regardless of the type, scale of observation, soil fertility, grazing treatment, or fire history. The native annual forb species were the most likely to decline over the 15 year period, which is exceptionally important to overall diversity considering that was the dominant plant type at all of the sites. Declines in mid-winter precipitation were best able to explain the losses of diversity compared to the other examined climatic factors. Species which had a high SLA value were more likely to decline in abundance when observing the community at the local scale. This agrees with the researcher’s predictions that this physical trait is a good proxy for drought sensitivity and further enforces the role that precipitation decline played in diversity loss. Although these results do not indicate an optimistic future for the diversity of plant communities in California’s grasslands, they do provide an important insight into what we can expect for the future. Hopefully ecologists and land managers alike can use this information to develop a mitigation plan for future climate scenarios.