Pacific Poison Oak in Many Forms

Pacific Poison Oak. Just the mere mention or thought can make many of us itch and twitch. Some of us dare to breathe as we carefully avoid it's reaching branches on trails. Prior to the 2017 Nun's Fire, a fellow park employee and I joked that if a wildfire ever roared through Annadel State Park, everyone in Sonoma County would break out in poison oak from the oily smoke. We were rather surprised to hear nothing of the sort after all the fires of 2017, 2018, 2019 and 2020 around our richly carpeted poison oaky woodlands. Especially after 63% of Annadel burned in the Nun's Fire. My grandmother got poison oak rash from burn piles in the Sacramento Valley after they moved there.

For years, I had assumed that poison oak foliage all looked the same. I knew that the plant grows in a multitude of different forms: small bush, large bush, tree-like, groundcover-like, huge thick vines curling up tall trees to reach sunlight, from rootstock, from seeds spread by birds and critters, branches arching out over trails, branches reaching like spears up out of blackberry brambles. It loves to coexist with blackberry brambles. But the leaves all seemed to look similar: generally like oak leaves, mediumly lobed, smooth edged in between the lobes, of course in the pattern of "leaves-of-three," medium green till summer when they begin to turn to typical Autumn colors, flowing in the breeze so soft-looking (though I never meant to touch them to feel that).

Till I started noticing differences, about 10 years ago. While doing volunteer trail maintenance with Sonoma County Trails Council (SCTC), along North Burma Trail in Annadel State Park, I noticed oblong leaves with very shallow lobes, some without lobes at all. The individual leaves were very large, almost the size of my entire hand. "Huh," I thought. "The plant is evolving right before my eyes." For a few years, members of SCTC had been discussing how it seems as if poison oak likes the gradual climate change, increasing temperatures, is becoming more widely spread in more areas in thicker concentrations. It was not just our imagination.

So I started to notice the shapes of leaves. As I began to participate more and more on iNaturalist, I began to purposefully take observation photos of differently shaped poison oak leaves. Now, I recall having seen a lot of variety. Last Monday at Paulin Creek Open Space Preserve as I was doing the dodge-and-swerve around low-reaching branches of PO, I suddenly saw short plants along the ground with very large, very smooth-edged oval leaves -- each leaf as big as my foot! I actually got a perspective photo of my foot near one of these over-size leaves, without touching it of course. These plants are in full shade, up the embankment from a spring fed creek, each plant up to 2-feet tall and richly foliaged.

Yesterday was extraordinary: May 21, 2023 at Calabazas Creek RP & OSP, I noticed the most deeply lobed poison oak leaves ever. The lobes extended into the leaves by a full inch or so, on very large leaves (after 12 atmospheric rivers this past winter, everything is lush). These leaves were each about 7" long and about 4" wide. And oddly shaped. Then, another extraordinary observation a little farther down the trail: Pacific poison oak with deep lobes AND serrated edges. Even more oddly shaped overall. Whaaat?!

I am just a casual observer Nature-nerd. Who avoids getting poison oak rash very carefully. I am not seriously studying PO specifically, but noticing it as I nerd-out on all things nature around me. This spring, I feel like it's too bad the plant is so ... well, evil for most of us. Because especially after 12 atmospheric rivers, it sure does make a lovely lush green flowy groundcover plant.

Quite a few people I know who virtually live outdoors and have participated in trailwork for years never got poison oak, and rather bragged about it. Till they did. Once they got it the first time, there seemed to be no going back to "before." It seems that our bodies might even react worse with each exposure. Indeed, my husband never used to get it. Then once after a trailwork event, he broke out in PO rash on day 7 after exposure. The next time it was day 6, then the next day 5, and so on. Now, he begins rashing within hours of exposure, even after having carefully avoided the plants, washing with Technu before getting in the car, washing with Technu again in the shower at home. We always have a variety of treatment strategies on hand: Benedryl stick, Fluocinonide prescription steroid cream, lavender essential oil, 4X4 gauze pads for the really weepy spots if it gets that bad. Then there is always the hot-HOT water shower treatment for itch-gasms. Fortunately, neither of us has had to go so far as Prednisone, though some of our friends just don't even pause to think about that option, but just call their doctor before their eyes swell shut.

Publicado el 22 de mayo de 2023 16:00 por wildmare64 wildmare64


Many thanks for this account of a fascinating plant, Toxicodendron diversilobum.

There are so many remarkable and puzzling aspects to the biology of this species that it is hard to know where to start.

How can it have evolved to be so much more toxic to humans than to other animals?

How does the toxin persist despite combustion?

What are the adaptive advantages of such extreme plasticity in growth-form, which would be more understandable on remote oceanic islands with limited floras, where a single recruited species fills various niches?

What are the ecological relationships among a) plasticity in growth form, b) wide habitat-tolerance within the Californian region, and c) toxicity?

Can the ripe fruits be safely eaten by humans?

Southern Africa shares the mediterranean-type climate with California, and has a flora unusually rich in Anacardiaceae. Furthermore, several shrubby spp. of Searsia ( have trifoliate leaves similar to those of T. diversilobum. However, no searsia or other anacard in southern Africa emulates T. diversilobum in its plasticity of growth form, and none forms lianes.

Publicado por milewski hace 7 meses

By the way, you have a talent for writing, and it would be good to see more Posts from you, on various topics.

Publicado por milewski hace 7 meses

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