Ol’ Green Gills.
Green about the gills – an 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, reflecting both the potential sickening effects of eating this mushroom as well as its most distinctive identifying feature: The greenish-gray color of the mature gills. No other mushroom in California shows this character! Although not seriously toxic (causing only short-term gastrointestinal upset), 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. The range of this species is expanding! With every passing year, there are more interfaces for humans to encounter them.
Beyond the perspective of public health, this is also an interesting species 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. In California, the earliest record I can find was collected by one F.G. Floyd near Pasadena in 1930 (curiously, around the same time Amanita phalloides was first recorded here). It is now widespread throughout the state, but there are some puzzling gaps in this species’ distribution.
One striking example is along the central coast (see map below). Note the large gap in west-of-Highway-99 occurrences, bounded southwards at Santa Barbara and the spine of the Santa Cruz Mountains in the north. A single dot in San Luis Obispo represents a recent record documented by the ever-vigilant Michelle Torres-Grant.
Distribution of Chlorophyllum molybdites in California (detail).
Some of this pattern can be explained by lack of coverage. Monterey is not (yet) a major nucleus 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).
Notice that swarm of documented Green Gill occurrences comes right up to (but not over) the Santa Cruz County line!
It’s also clear from looking at the distribution map that the gaps in C. molybdites distribution on the California coast cannot be explained by a lack of available spores to start new populations – this species is definitely not dispersal-limited. Whether their spores are being dispersed by wind, adhering to tires, or otherwise moving around with human activity, C. molybdites is a good traveler, having thoroughly colonized not only mainland North America but also remote islands like Hawaii, 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 establishing populations in places like southern Santa Cruz County?
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 hot and humid months.
Temperature may still be a primary factor shaping this species’ distribution – 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.
The Muggy East has lots and lots of Green Gills. Southern California and the Central Valley have plenty as well. Most areas of the Northwestern quadrant of the USA are too dry, too cold, or both.
Support for this nighttime-temperature 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 are surrounded by potential source populations), but which lack prolonged periods of warm nighttime temperatures.
By sheer happenstance, a sort of natural experiment to test this hypothesis can be found on the UC Berkeley campus: A steam vent lets out from one of the buildings at ground level, continuously 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 present 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 California, 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 creatures can be found within them. During last year’s heat waves and warmth that lingered 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 2021 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*.
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.
References:
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
doi: 10.3897/mycokeys.32.23831Obradovich N, Migliorini R, Mednick SC, Fowler JH. 2017. Nighttime temperature and human sleep loss in a changing climate. Science Advances
doi: 10.1126/sciadv.1601555Pierre-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.htmlVellinga, Else C. 2003. Chlorophyllum and Macrolepiota (Agaricaceae) in Australia. Australian Systematic Botany 16, 361–370