Poison ivy leaves & Sexual dimorphism

I’ve spent quite a bit of time hand pulling poison ivy along trails where we run Crow’s Path programs. The plants are woody, and their extensive perennial root system readily sends up shoots every few inches along their length. They are particularly prone to this when the soil is disturbed or the roots damaged, so a well-weeded poison ivy patch will look much denser above ground the following year as new growth sprouts up from portions of the root left intact.

Poison ivy is dioecious, meaning that the small, white flowers tucked beneath the upper leaves are either male or female and a plant will contain only either male or female sexed flowers. While poison ivy relies heavily on vegetative reproduction to form these thick “stands,” female plants produce small pumpkin-like fruits (botanically, these fruits are drupes, similar to peaches, with a fleshy layer surrounding a pit that contains a single seed) in late summer dispersed by a range of birds and mammals. 

I’ve personally watched red squirrels, gray squirrels, catbirds, cardinals, and deer all eat the berries, and in a study where researchers left poison ivy fruits out at a feeder (this was done after migratory birds had left the study area), ruffed grouse were a major consumer of the fruits (source). But the good of the scorpion is not the good of the frog, and what’s edible to the grouse is most definitely not edible to us, as all parts of the poison ivy plant contain the itch-inducing oil urushiol (check out our quiz on poison ivy facts and fiction).

Sexual dimorphism in plants, as with animals, is a strategy that decreases competition between sexes. Because pollen production requires greater stores of nitrogen while seed and fruit production require more carbon (largely in the form of simple sugars in fruit and starch in seeds), habitat needs are different for the two sexes. In dioecious plants like poison ivy, this can play out as sex bias at different habitat types since the different energetic and nutrient costs associated with plants being either male or female may be favored by different ecological conditions at a site (a female plant will perform stronger in more favorable growing conditions where photosynthetic rates will be higher: source).

In boxelder, individual trees are either mostly male or mostly female. Significant sex bias appears in boxelder populations based on habitat characteristics, with a male bias in drought-prone habitats and a female bias in moist, streamside habitats. It is easier for male plants whose leaves have more prominently toothed margins (increased photosynthetic rates), thicker cuticles, (which helps shed heat), and decreased stomatal density (stomata are the holes found mostly on the underside of leaves that allow for gas exchange; they can be a big source of water loss; source) to outcompete female plants on drier sites. Female plants are able to further save on energy costs associated with reproduction by producing shorter reproductive branches than their counter parts in male plants (source).

It’s hard to map this directly onto poison ivy, particularly since it is quite difficult to differentiate between male and female poison ivy plants out in the field (there’s a short flowering period late-May into June and many younger/shorter stemmed poison ivy plants don’t produce any flowers). But there does appear to be a similar pattern with poison ivy, which is found in a wide range of ecological conditions. Female clonal stands tend to be in shadier areas (personal observation) and have leaves with wavy margins or that lack teeth entirely, while male leaves are more coarsely toothed (source). 

I’ve also noticed that sexually mature female plants are taller and have more branches than male plants (here in Vermont we have few vining poison ivy plants, most are upright stems between 6” and 24” tall). This may be connected to female plants reaching sexual maturity later than males as height corresponds to age, and a larger plant has greater energy reserves to support the greater energetic costs of fruit production. 

This mirrors the hermaphroditic patterns of jack in the pulpit. In its first year or two, jack in the pulpit will produce male flowers. But after that it will alternate between male and female flowers based on size, sex, and possibly other factors like nutrient availability in the prior growing season (source, source). Jack in the pulpit female plants also tend to have two leaves rather than one as is common with males.