"Green about the gills" – idiom describing the appearance of queasiness; about to vomit; sickly and uneasy.
In the spotlight: the Green Gill Parasol (Chlorophyllum molybdites). The common name of this species serves double duty, signaling both the potential sickening effects of eating this mushroom while simultaneously describing its most distinctive identifying feature: The greenish-gray color of the mature gills (a correlate of the green-gray spore deposit). In fact, no other mushroom in California shows this character! Although not seriously toxic, it certainly has the potential to be enqueasening. By the available numbers, this appears to be one of the species most frequently involved in mushroom poisonings in the United States.
There some obvious factors contributing to this mushroom's nauseating track record:
1. The species is widespread and abundant – it’s common for multiple tens of fruitbodies to appear in small areas simultaneously, often in striking fairy rings.
2. The fruitbodies are enticingly large and attractive.
3. They look similar to a few edible species.
4. This species’ preferred habitats are almost always in close proximity to humans (think irrigated lawns). So folks encounter it more frequently than the average mushroom.
5. It is expanding its range! With every passing year, there are more interfaces for humans to encounter them.
Besides the perspective of public health, this is an interesting species for biogeographers. This post is focused on the latter point in the list above. What factors are driving the expansion of this species? The geographic area occupied by C. molybdites has expanded in the past few decades – generally speaking, it has radiated northward as well as westwards from more tropical climes.
But there are some puzzling gaps in this species’ distribution…
One striking example is the central coast of California (see map below). Note the large gap from Santa Barbara to the spine of the Santa Cruz Mountains, with a single dot in San Luis Obispo (documented by the ever-vigilant Michelle Torres-Grant).
Some of this pattern can be ascribed to lack of coverage. Monterey and San Luis Obispo Counties are not (yet) major nuclei for folks posting mushrooms to iNat. But given that this is a big, showy mushroom that grows on people’s lawns, we can be confident that the general pattern here is real: Chlorophyllum molybdites is (apparently) either absent or very scarce west of the central Coast Ranges in places like Monterey, Salinas, Castroville, and Watsonville – places which definitely contain patches of appropriate substrate (watered lawns, soccer fields, urban parks, manicured cemeteries, golf courses).
It’s also clear from looking at the distribution map that the reason C. molybdites is absent from some places on the California coast is not due to a lack of available spores to start new populations.
This species is emphatically not dispersal-limited. Whether their spores are being dispersed by wind, adhering to tires, or otherwise moving with people, C. molybdites appears to be a proficient traveler, having thoroughly colonized not only North America but also Hawaii and apparently even some further-flung outposts such as Fiji and Tahiti!
So if not the lack of lawns or the lack of spores, what factors have so far prevented the Green Gills from invading Watsonville?
As evidenced by the populations of this species in Minnesota, Wisconsin, and Michigan we know that it’s not cold winter temperatures limiting the distribution of C. molybdites – this species is apparently able to go dormant and tolerate very cold winters, rebounding to fruit again in the more saunoid months.
Temperature may still be a primary factor shaping this species’ distribution though, albeit in a more nuanced way. Else Vellinga has hypothesized that this species is tightly constrained by nighttime temperatures exceeding some minimum temperature for extended stretches of consecutive dates. Think of the warm, muggy summer nights that are so characteristic of much of eastern USA, where this species is most common.
Support for this model can be found by studying the geographic pattern of occurrence of C. molybdites in California. It seems to be absent from areas with otherwise appropriate habitat, and surrounded by potential source populations, but lacking prolonged periods of warm nighttime temperatures.
By sheer happenstance, a sort of natural experiment can be found at UC Berkeley: A steam vent lets out from one of the buildings at ground level, significantly warming a small patch of lawn. UC Berkeley happens to be the academic home of Else C. Vellinga; global expert on Lepiotaceous fungi. A few years ago, she walked by the vent, and found Green Gill Parasols fruiting in the steamed lawn. She noted that it was a significant record for Alameda County (one of the first), and so history was made! Even micro-local warming is enough to make an appropriate substrate suitable habitat for C. molybdites. The spores are probably available almost everywhere in coastal California, they just need the right conditions to gain a foothold.
So what does the future hold?
The asymmetrical effect of global warming on nighttime vs. daytime temperatures has been investigated in a number of studies, and although the signal is complicated, it seems that in many places, global warming is driving a faster increase in nighttime temperatures than of daytime temperatures. From the perspective of humans, this is uncomfortable and dangerous. But from the perspective of C. molybdites this is just peachy. More warm nights in more places = more places where Green Gills can establish populations.
If a place has appropriate substrate (big grass lawns), is at least somewhat humid, and experiences spells of consecutive warm nights, sooner or later it’ll be colonized by Chlorophyllum molybdites. As climate change continues to warm the summer nights of central Calfiornia, it seems only a matter of time until Green Gills start to live here.
After more than a decade of of cultivating a useless preoccupation with the arbitrary geopolitical boundaries of Santa Cruz County, I like to think I’ve developed a good sense for what critters can be found within them. During last year’s heat waves and warmth lingering well into November, I could practically feel the mycelium growing somewhere within county lines… But by December rains and cooler temperatures finally arrived, and the window for the appearance of our inaugural Green Gills seemed to have closed.
Could the summer of 2019 be the year we finally encounter Green Gills in Santa Cruz?
Let’s get some skin in the game. I hereby offer a $20 bounty for Santa Cruz County for a first record of C. molybdites. I bet an additional $20 that it will come within the next five years, in case anyone wants to bet against. *Those seeking to collect on this bounty must provide documentation, preferably supported by a voucher specimen*.
Go make it happen… Judging by this phenology chart from iNaturalist data, NOW IS THE TIME.
I’ll be doing some investigating by bike, by car, by foot; prowling the hotter, lawn-ier areas of the county looking to prove myself right. I could use the money.
Ge ZW, Jacobs A, Vellinga EC, Sysouphanthong P, van der Walt R, Lavorato C, An YF, Yang ZL. 2018. A multi-gene phylogeny of Chlorophyllum (Agaricaceae, Basidiomycota): new species, new combination and infrageneric classification. MycoKeys 32: 65–90
Obradovich N, Migliorini R, Mednick SC, Fowler JH. 2017. Nighttime temperature and human sleep loss in a changing climate. Science Advances
Pierre-Louis K, Popovich N. 2018. Nights Are Warming Faster Than Days. Here’s Why That’s Dangerous. The New York TImes
Accessed on 30 Jul 2019: https://www.nytimes.com/interactive/2018/07/11/climate/summer-nights-warming-faster-than-days-dangerous.html
Vellinga, Else C. 2003. Chlorophyllum and Macrolepiota (Agaricaceae) in Australia. Australian Systematic Botany 16, 361–370