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A new study uses museum specimens to find out how the leaves of poison ivy in Pennsylvania have changed with the increased carbon dioxide in the atmosphere that is fueling climate change.

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Mason Heberling, associate curator of botany at the Carnegie Museum of Natural History in Pittsburgh, said the study used many of the museum’s herbarium collection of 133 dried poison ivy specimens from Pennsylvania, dating from 1838 to the 2010s.

The study was pioneered by then-Chatham University undergraduate Alyssa McCormick, a research intern. It’s based on a pair of influential studies from the early 2000s, which had been featured in one of the museum’s exhibits.

In one of those studies, poison ivy was grown indoors with high and regular carbon dioxide levels. The other study artificially enriched a forest plot with carbon dioxide. The studies showed poison ivy could be a winner when it comes to climate change, growing larger leaves with more CO2 and becoming more toxic to people. 

But those were experimental studies. Heberling and McCormick wondered about an observational one.

‘We kind of stared at the specimens for a really long time and thought, “What can we measure from them?”’ Heberling said. 

Their methods

They measured stomatal density, or the number of stomata, or pores, on the leaves, which are how plants take in carbon dioxide for photosynthesis and release oxygen and water.

“You put a little bit of nail polish on the dried leaf,” Heberling said, “let the nail polish dry on the leaf, and then you can pull off the nail polish, and it makes kind of a cast of the underside of the leaf without damaging the specimen.”

Heberling said the amount of stomata that some plant species make correlates with carbon dioxide. So, they wanted to know if poison ivy adjusted its stomatal density with increased carbon dioxide. 

But they wanted to answer a second question: Do all plants do this, or is this particular to poison ivy?

To help answer their question, they included another woody vine, Virginia creeper, which often grows alongside poison ivy, and staghorn sumac and poison sumac. They are both related to poison ivy but are trees, not vines.

They also looked at leaf size, and McCormick took tiny samples of the leaves, which were sent away for isotopic analysis to measure different forms of carbon in them. 

The earliest specimens in the study are from around 1840 when atmospheric carbon dioxide was about 280 parts per million. 

“And today we’re well over 400,” Heberling said. “So that’s a pretty big increase in carbon dioxide over the last 200 years.”

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What they found

What they found was that, unlike the two tree species, poison ivy and Virginia creeper decreased their stomatal density with increased carbon dioxide, which means that per unit area, fewer stomata were present. 

The leaves of the vines tended to be larger over time, as in the previous studies. Heberling said at face value, the study measures an interesting shift in traits, but if the leaf area is larger, without increasing the number of pores, the plants might benefit from increased light capture for photosynthesis without losing as much water. but, Heberling said, they don’t really know. 

The study does point to vines responding differently to carbon dioxide than trees.

“Instead of using that extra carbon dioxide towards structure making wood and tree stems,” he said, “they’re able to allocate that towards leaf production.”

A previous study suggested that water use efficiency is increasing in poison ivy.

“And we actually found the opposite,” Heberling said. “With the isotopic data, it appears that poison ivy, according to the metrics that we measured in Pennsylvania, is actually decreasing in its water use efficiency.”

Heberling said a big takeaway from the study is that poison ivy does seem to be doing better with climate change, and specifically more CO2 in the atmosphere, but so are other vines.

“I hear a lot of people talking about vines taking over their yard,” he said. “Not only poison ivy, but wild grapes and other things. And so this study kind of lends to that there is definitely a carbon dioxide enrichment story here in vines.” 

That could have implications for Pennsylvania forests. Heberling said ecologists are looking at how an increase in poison ivy plants and other woody vines due to higher CO2 levels could change the composition of forests. For example, vines could affect forest regeneration by suppressing saplings from reaching the tree canopy.

On the other hand, poison ivy is a native plant in Pennsylvania and a food source for birds.

But what about the itch?

The museum study didn’t measure poison ivy’s toxicity through the production of the oily urushiol that gives many of us an itchy rash, but Heberling said more studies using the herbarium collection are in the works. 

The new CO2 study is published in the September issue of the American Journal of Botany and is co-authored by Heberling, McCormick, Molly Ng, and Ryan Utz.