Californian plants have responded to climate change in a surprising way, data collected under the direction of a forester born almost exactly 121 years ago have helped show. Albert Everett Wieslander headed surveys in the 1920s and 1930s covering 28 million hectares, over most of the state’s natural environment outside of deserts and larger agricultural areas. The data was originally intended to provide 220 detailed vegetation maps, but publication was halted by the Second World War after just 23 maps were released. Then in 2005 a digitized version of the raw data, now known as the Wieslander Vegetation Type Mapping (VTM) collection, was made available online.
Now, Solomon Dobrowski, from the University of Montana’s Department of Forest Management and his colleagues have compared this record with modern studies of plant populations for Northern California. “We used their survey plots,” Dobrowski told Simple Climate. “Basically they’d delineate a fixed area on the ground as roughly 800 square metres in size and they would document the types of trees and shrubs and other plants they found within that location.”
Prior studies of how plants are reacting to climate change have shown them moving to habitats in pursuit of their preferred temperatures as the planet warms. This typically means that they move towards the poles, or up mountainsides to higher altitudes. However, what Dobrowksi’s team found from their comparison, and published in top journal Science yesterday, at first glance seems to almost directly disagree with this. “I was mildly incredulous when my graduate student Shawn Crimmins first approached me and said that things are moving downhill,” Dobrowski admitted. “I asked him to go back and revisit his analysis and make certain that it was right. When he came back and said yes, we’ve dotted our i’s and crossed our t’s, I realised we had to revisit our assumptions about what was going on.”
Rainy Northern California
The explanation lies in the perhaps surprising observation that climate change has caused an increase in rain and snowfall in Northern California in the seven decades between surveys. The resulting moisture evaporates much less quickly at cooler, higher, altitudes than in lower locations. This means that the highest plant habitats see a much greater increase in moisture, Dobrowski explains. His team’s analysis focussed largely on conditions at plants’ optimum elevations, the height at which they’re most likely to be found. They found these optimums changed to locations with temperatures that maintained the same balance between water falling as rain and snow and evaporating.
“We found that they are at newer, warmer optimums for temperature because they went downslope,” Dobrowski said. “If they’re tracking water balance, then we would expect no change in the optimum water balance at which they’re located between the historic and the modern period, and that’s what we show. They’re trying to maintain the same balance of water that they previously were at in the 1930s. But because things have gotten wetter across the state they’re able to do that at lower elevations now.”
The fact that plant movement in response to climate change in Northern California appears to be in a different direction to those seen elsewhere doesn’t lead to an overall balancing. Instead, each situation sees them moving away from their historical homes Dobrowksi indicates. Plants moving uphill find themselves with less area to inhabit, while those moving downhill are likely to find themselves in unwelcoming urban environments. “The other thing is that ecological communities are coherent wholes,” he added. “We expect insects to move upslope and be more tightly linked to temperature changes. If you have a situation where the insects are moving upslope and the plants are moving downslope, now you can get a real interruption of a critical ecological process.”
Investment pays off
Finding that something other than temperature could be driving plant movements demonstrates the importance of the data from the 1930s, Dobrowski said. “There aren’t that many good historic data sets on species distributions,” he explained. “There’s lots of studies that take current data and then make a forecast into the future, where people have focussed more on temperature. We have more confidence in the forecasts for temperature data than we do in precipitation data. So, people have tended to shy away from a robust analysis.”
Wieslander’s rare collection of information was made possible by a “public works” investment made by the US Forest Service. This was one of the ways the US government tried to stimulate its economy after the financial crisis the world’s recent turmoil is often compared to – the Great Depression. That was lucky for Dobrowski – and arguably all of us – as no-one could have known at that time that the work would lead to a better idea of the likely outcomes of global warming. “People wonder about the value of science, and we don’t always know until some time later,” Solomon said. “Now we’re starting to really realise historical data sets are invaluable for understanding these effects.”