Vegetation Communities and Climate Change

Researchers walk to a transect in Point Mugu State Park. Photo from National Park Service.

By Frank Landis, Chairperson, Conservation Committee

Probably I should be writing an article about the virtues of planting large numbers of plants from the plant sale—which you should—but at the Chapter Council last Saturday, Greg Suba handed me a copy of “A Climate Change Vulnerability Assessment of California's Terrestrial Vegetation,” a document written in 2016 by James Thorne et al. of UC Davis for CDFW.  And, unfortunately for you, I've been reading this, rather than thinking about how to persuade you that gardens are one way for plants to migrate to avoid climate change. So, my apologies, you're going to get a semi-review of a document you're likely to never read. But it will be interesting nonetheless. I hope.

One grumble is that the bureaucratic neologism "vegetation community" crept into this document. I detest that phrase, even more so in the context of a report on climate change.  Here's the problem: the phrase was proposed as a compromise between two camps of ecologists, the plant community crowd and the vegetation mob. This dispute goes back a century, and it's about the nature of plant communities, whether they're superorganisms or assemblages.

If you're in the superorganism crowd, to you plant communities are groups of organisms that have coevolved with each other, like the parts of a watch. When this superorganism is disturbed, different species come out, restore the damage, and ultimately the superorganism goes through ecological succession to its climax state. That climax state is theoretically determined by climate, which in the original theory was supposed to be constant over the hundreds to thousands of years that it took for the superorganism to climax into equilibrium with its environment. When researchers realized many decades ago that plant communities in the past had different dominant species than they do now, they posited that there were simply different climax species under different environmental regimes, but it's still all superorganismic, y'all.

Then there's the vegetation crew, which I proudly belong to. We're perfectly happy with species coevolving with each other (there's even now a well-tested theory for that, Thompson's geographic mosaic of coevolution), but we see no evidence for vegetative superorganisms. To us, vegetation is composed of individual species, each of which follows its own plan. Vegetation types happen because certain species tend to dominate large patches, while most plant species do not. It's common to see a forest dominated by oaks, but quite rare to find a forest dominated by elderberries. While there is succession after fire, it's a probabilistic combination of survivors resprouting plus seeds in the seed bank germinating, plus whatever blew or washed in. The climate changes all the time at all scales, and while there are certain species that tend to win out (oaks, for example, or now eucalyptus), change over time depends quite a lot on accidents.

But wait, you say, what about all those pollinators, mycorrhizae, nitrogen fixing bacteria, all that stuff? What about them? They're not gears in the machine either, and they've all got ways of moving around and surviving through time too. The fact that they exist and form such complex relationships doesn't mean a superorganism is controlling their fates, any more than the fact that you live in a web of complex social relationships means that something planned all those relationships for you. Chance was involved in forming both.

There's a selection bias here: we tend to look at the complexity, be awed, and assume that something made it.  If you look at how it actually happens, what you see are plants, fungi, insects, and bacteria making absolutely enormous numbers of seeds, eggs, and spores, and almost all of these die. The ones that grow to maturity are the few that, by chance, ended up in the right spot with the right partners at the right time. The environment's flooded with propagules simply to counteract the extremely low odds of them flourishing. Chance can cause really complex ecosystems to emerge in an environment flooded by propagules, if you give it long enough and provide enough different propagules.

To us veg heads, change over time and after disturbance is the norm and there is no climatic equilibrium. When we look at deep time, we look to people like Margaret Davis, who found, through careful sampling of pollen cores in lakes back east, that after the ice ages, trees migrated from refugia in the Carolinas and elsewhere to rebuild the eastern forests. Species like birch migrated quickly with their windblown seeds, while species like beeches moved very slowly. The result is that you see the same forest species across the east, but the forests get richer as you head towards the south. Beeches barely made it across Lake Michigan before Europeans settled the place and changed the rules, while birches blew past Minnesota and well into Canada. Out here in the west, we see similar patterns in ancient woodrat nests, patterns that strongly suggest that species migrate across the landscape by their own devices, not as galumphing superorganisms.

Still, just as there are a lot of climate deniers, there are a lot of plant community ecologists, and so bureaucrats in various places have decided that "vegetation community" is a neutral term.  Obviously, there isn't a community of vegetations, but since the planners don't know that there's 50 years of evidence supporting one side over the other, they decided this was the way to avoid the debate altogether.

So now we get to Thorne et al.'s work, which isn't really about the fight over vegetation community, but they used the term in their text. They also did kind of a vegetation community approach to figuring out how vegetation might change, and it gives you a flavor of the state of the research.

Their targets were high level vegetation classifications. Chaparral is one (macrogroup California chaparral, which corresponds to WHR code mixed chaparral and chamise-redshank chaparral—and the fact that they thought it was important to still use the Wildlife Habitat Relationships code, which was obsolete in the 1990s, tells you something about the state of bureaucratic science in certain agencies), coastal sage scrub is another vegetation macrogroup, while each regional take on things like riparian forest, oak woodlands, and montane meadows got their own separate treatment. But I'm not angry about the way things got lumped...

Anyway, they started with maps of these vegetation macrogroups (which the state has, thanks in large part to CNPS' vegetation mapping program) and mapped climate data for each of these polygons.  Then they did a principal component analysis to see which bits of data were correlated with each other (the resulting graphs are quite pretty). Then they took two climate models, one chosen because it breaks on the "more moist" end of the dozen models they could have chosen, while the other was on the "more dry" end, remapped California's climate under these two models (and under two different climate change scenarios), and mapped where the vegetation macrogroup might like to grow under each scenario, assuming it was only influenced by climate (which, they repeatedly acknowledge, it isn't. That's just one factor). If you squint at this sideways, it looks a bit like a plant community-style analysis, does it not?

They also looked at some of the dominant plant species for vegetation types within the macrogroup, scored these plants on things like seed dispersal and resistance to fire and disturbance, and combined all these scores into a vulnerability assessment for how threatened each vegetation macrogroup was under each scenario. This is where the vegetation ecology approach of individual plants came out.

Oh, and because bureaucrats want to pretend that climate change only goes out to 2100 (it's only practical to tell the politician bosses there's a limited time horizon, or something), they only mapped the vegetation from 2070-2099, although they produced some summary statistics (like the acreage for each vegetation macrogroup) for time periods leading up to that. 

The results are about what you'd expect, with San Diego County becoming much more desertified. One surprise was that the work suggests that the Modoc Plateau in northeast California could become a redoubt for some of the more characteristic vegetation of the California Floristic Province. If the plants can migrate there in time, which is a problem they acknowledged but didn't attempt to analyze. 

What actually will happen depends quite a lot on chance.  Can plants move their seeds to where their kids will grow to maturity? Can their pollinators and dispersers follow them, or will they find new partners? Tune in next week for the next thrilling episode in...but that's sarcastic. The truth is that we simply don't have enough information to model the futures of thousands of native plant species, so we're stuck using simplifications, like vegetation macrogroups, to get an idea of how much things need to change. As such, I'm very, very glad they went to the trouble of doing this, even though I found bits of it annoying.

Hopefully you're now questioning any notions you might have had that nature's all doomed, because all those poor little coevolved organisms can't make their relationships work with the changing climate torturing the superorganisms to which they belong. Hopefully you're also becoming a veg-head and starting to figure out what you can do to help thousands of species migrate. 

There is something you can do, actually. You can get a few more species growing and reproducing in your garden, to support wild mycorrhizae as well as the stuff you get in the mixes, and make more living spaces and smorgasbords for your local pollinators and seed dispersers.  CNPS-SD can help you do that, if you come to the October plant sale and buy lots of plants and seeds. See you there!

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