Climate change drives California’s stunning wildflowers off

One of the great joys in life is hopping in the car every spring to go see the wildflowers popping up all over California. Hillsides that spent the winter a dull brown transition briefly to green, and then explode in a dazzling array of colors – orange, yellow, purple, red, blue – as if someone tripped and spilled a Crayola box of primary and secondary colors on the landscape. But it might not always be like that.

It is by now well known that climate change has a handful of predictable consequences. Some species move up mountains, chasing their desired temperature to ever higher elevations. Others change their latitude, following their optimal habitats away from the equator and towards the poles. Still other species stay where they are, but change the timing of their development along with the changing seasons. The least fortunate of species can’t adapt fast enough to changing environmental conditions, and while defiantly attempting to hang onto their existence, will ultimately perish.

But what our changing world means for ecological diversity is a tougher question to answer. Over the last 20 years, European mountains in the boreal and temperate regions have seen an increase in species richness. Warming there has allowed for longer growing seasons and higher productivity. This makes sense: the species that have always lived on mountain peaks are now being joined by those escaping lower elevations. Elsewhere in Europe, the story is playing out differently. In more southerly latitudes where the climate skews more Mediterranean, declines in species richness have been reported for mountain summits. Globally, there are no clear trends. Instead, species diversity is probably somewhat more predictable at smaller scales. A 15-year observational study of a Northern California grassland parallels the findings from southern Europe, suggesting that the proper scale to consider is the biome – areas dominated by particular communities of vegetation with similar climates.

Susan P. Harrison and Stella Copeland, researchers from the Department of Environmental Science and Policy at the University of California, Davis, together with Elise S. Gornish of the UC Davis Department of Plant Sciences monitored species richness at 80 sites within the University of California’s McLaughlin Natural Reserve, a 2,776 hectare site (100 hectares equals one square kilometer) in Northern California, each April and June between 2000 and 2014. The climate there is classified as Mediterranean; mean annual temperature is 46.4 degrees F in January and 77 degrees in June. On average, the area sees 62 centimeters of rain each year, mostly falling between October and April. Of the 80 sites, 38 were on relatively infertile serpentine soils, which are good hosts for native species, and 42 were on more fertile soils that were more heavily dominated by exotic grasses. Each of the 80 sites was divided into five plots, each measuring one square meter.

No matter whether Harrison looked at the local scale (5 square meters) or the broader landscape scale (80 sites = 27 square kilometers), or whether she considered just native species or included exotics as well, species richness declined over the 15 years. The biggest contributor to the decline was a loss in native annual forbs, otherwise known as wildflowers, though other types of plants declined as well. However, the decline could not be pinned to large decreases in a small number of common species. Instead, there were small changes at each site by a large number of species. The entire community, it seems, has been suffering.

Meanwhile, the non-native species, typically thought to be hardier and more adaptive, did not increase (though they didn’t really decline, either) over that course of the study.

Of all the possible causes for the wildflower decline, the only one that was supported by the data was precipitation. Diversity declined equally on sites that had been grazed or left ungrazed, and a wildfire that came through the area in 2009 did not seem to severely impact the landscape either. The lack of increase in exotic grasses means that there were no strong changes in soil nitrogen deposition either. Put most simply, the wildflowers declined because the area became progressively drier, less humid, and sunnier thanks to the retreat of rainclouds.

(The current California drought is only four years old. Even when the researchers only considered years up until the drought began in 2012, declines in plant diversity remained significant.)

The combined pattern of decreased rainfall, humidity, and cloud cover is known as aridification, and it’s a process that’s been predicted to intensify thanks to climate change in Mediterranean climates like in California. While Harrison’s observations aren’t entirely identical to climate-related predictions which also include increases in temperature, the data largely suggest that “the community changes we observed should be a reasonable model for those to be expected in coming decades.”

While many people implicitly assume that climate change is inextricably linked with a decrease in biodiversity, that is not necessarily the case, at least not across the globe. Indeed, some ecosystems seem to be becoming more diverse. Climate change is not having universal consequences now, and it will not in the future. But the researchers do conclude that declining diversity is very likely for water-limited climates. This study moves beyond the study of European mountaintops, because the researchers have spotted the trend even within a five square meter patch of grassland, a scale that is even “visible to the relatively casual observer.” This is climate change that you can see, at least if you’re paying attention.

California is becoming drier, wildflowers are becoming scarcer, and California may just become slightly less beautiful. – Jason G. Goldman | 24 June 2015

Source: Susan Harrison, Elise Gornish, and Stella Copeland (2015). Climate-driven diversity loss in a grassland community. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1502074112.

Header image: Native-dominated grasslands (foreground) and exotic grass-dominated grasslands (background); courtesy of Catherine E. Koehler/McLaughlin Natural Reserve/University of California.

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