Nitrogen exposure is changing the way plants defend themselves

Goldenrod plants swaying in the breeze may not strike us as champions of innovation. Yet, hidden in their stems lies an evolutionary story shaped by decades of human agricultural influence. What if simply adding fertilizer to soil could rewire the way plants defend themselves?

A study led by researchers at the University of Michigan shows that nutrient-rich environments can push goldenrod to adopt a fascinating plant defense: stem nodding. Professor Mia Howard and her team explored how nitrogen fertilization influences goldenrod defense traits.

“One of the exciting things about this finding is that there are so many hypotheses and theories predicting that resources would affect the evolution of plant defenses, and here we have the first experimental evidence to show that it does,” she said.

What is stem nodding?

Goldenrod plants face an annual threat from insects like gall midges. These insects inject eggs into the plant’s tip, triggering growth of protective galls where larvae mature. Goldenrod counters with nodding: a bend in the stem that hides the vulnerable tip during peak insect activity.

This trait is genetically encoded, not influenced by soil nutrients. Plants either nod or they don’t. That consistency makes goldenrod ideal for studying genetic responses to environmental changes.

“If the plant is a nodder, it’s going to nod. It’s also really easy to phenotype because you can just look at it,” Howard explained.

Nitrogen and goldenrod nodding

The team surveyed goldenrod in six paired plots (fertilized and unfertilized) set up in 1989.

Fertilized plots received nitrogen equivalent to what’s used in agriculture. In 2016, 2021, and 2022, they recorded significantly more nodding goldenrod in treated plots. In fact, nodding morphs were 3 to 6 times more common in fertilized areas.

The researchers observed a clear trend: as nitrogen exposure increased over the decades, the frequency of nodding goldenrod also rose.

This wasn’t just a short-term shift. It indicated a genetic change, likely caused by the spread of nodding clones in the high-nitrogen plots.

Bigger plants get attacked more

Fertilized goldenrod grew 30 to 40% taller than unfertilized ones. In 2022, they also produced about 50% more biomass. This growth made them more noticeable to insects.

As expected, galling increased – twice as many plants were galled in fertilized plots that year.

Interestingly, stem nodding didn’t always help. In 2021 and 2022, galling rates were similar in nodders and non-nodders. This suggests insects may have adapted, or other factors (like antibiotic defenses) could be at play.

Still, in 2016, nodding did reduce gall presence. Defense effectiveness seems to vary year by year.

Plants bend to defend themselves

Even if nodding fails sometimes, it persists. One reason is its low cost. Unlike chemical defenses that require energy, nodding is a simple structural adjustment. It does not reduce plant size, and in some studies, nodders grew taller and produced more seeds.

In nutrient-rich settings, plants must grow fast and outcompete neighbors for light. Howard’s team suggests nodding may be a defense that supports rapid growth. That makes it especially useful in fertilized environments where tall plants are more visible to herbivores.

How plants evolve to defend

This study highlights why long-term experiments matter. Evolution unfolds over decades. Without this three-decade setup, scientists wouldn’t have spotted the subtle shifts in goldenrod genetics.

“We were really fortunate that Kellogg biological station had started this experiment and continued to maintain it for three decades. Having these long term experiments is really valuable,” Howard said.

“You might not see a lot of differences in plant populations in the early years, but I think they are becoming a really good resource for studying evolution.”

Goldenrod reminds us that even small changes – like bending a stem – can be part of nature’s slow but powerful evolutionary story. Nitrogen, a simple nutrient, may shape how plants fight back for generations to come.

The research was made possible by three decades of continuous fieldwork at the Kellogg Biological Station, with support from multiple institutions, including NSF and USDA.

The study is published in the journal Oikos.

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