Have you ever headed to the mountains to escape the heat? On a warm summer day, it’s hard to deny the appeal.

While plants can’t move to higher ground on a whim, their geographic range may shift over time in response to changing environmental conditions. Climate change is one factor that can initiate this process, which is also known as range expansion. As climate conditions shift, optimal habitat for a particular species may also shift, pushing them into new territory.

In this new territory, they may interact with organisms that didn’t inhabit their previous home ranges. For example, warming temperatures might prompt a subalpine globeflower to creep into an alpine environment populated with plants it hadn’t previously encountered.

Range expansion isn’t inherently problematic, but the process has accelerated due to climatic changes, especially in high-altitude environments.

“The issue is when warming is so extreme that habitat at the top of the mountain is at risk of disappearing completely, or there’s so much encroachment that habitat disappears,” explains Megan Szojka, a PhD student in the UW Department of Botany. “When you have all these stratified habitats on a mountain, eventually the top one is going to have no more space to expand its range.”

This stratification of habitats is one of the reasons alpine ecosystems are ideal for studying range expansion. Moving up a mountainside, distinct habitat types exist in close proximity to one another.

Why study this flower?

From a conservation standpoint, it’s important to understand what happens as subalpine species spread to new environments and interact with lingering alpine plants.

Szojka’s research focuses on how the subalpine globeflower Trollius albiflorus responds to habitat changes that could be induced by climate change. Currently this species mainly occurs below the tree line, which provides a clear marker signaling the upper limit of its range. It also has distinctive flowers, so it’s easily recognizable.

While Szojka is gathering data at Niwot Ridge near Boulder, Colorado, results from her study may have implications for subalpine plants across a larger region. Trollius albiflorus is found in Wyoming as well, including in the Snowy Range.

“This story is about Trollius, but in general, species across the subalpine will all be experiencing this. Hopefully my research will elucidate patterns that are reflective of the whole community’s range expansion,” she explains.

Szojka’s research questions are twofold: First, what aspects of climate change help or hinder this plant’s range expansion into the alpine? Second, how do interactions with other species influence these outcomes?

To find out, she transferred a group of Trollius albiflorus plants from a subalpine habitat to a location above the treeline, then exposed subsets of the transplants to different treatments, including combinations of warming, shading by neighboring plants, increased snowpack, and added nitrogen.

The experiment

In 2021, Szojka’s team painstakingly transplanted nearly 300 individual globeflower plants from their subalpine habitat to a test site a few hundred feet higher. Data collection began in summer 2022 and will continue through summer 2024.

The globeflowers were transplanted into 48 plots, each about one square meter in size. The researchers used a snow fence to emulate the effects of higher snowpack at the beginning of the growing season, a change predicted to occur in this area due to later season snowstorms.

For plants exposed to warming treatments, plexiglass chambers were used to create miniature greenhouses. Lastly, some plots were treated with nitrogen fertilizer to mimic the effects of atmospheric nitrogen deposition from industrial activities.

Szojka’s experimental design also accounted for the interaction between the transplanted globeflowers and their new alpine neighbors, a variable that turned out to be key to the transplants’ responses to warming temperatures.

In each plot, half of the transplants’ roots were surrounded with PVC pipe to prevent them from interacting with the roots of other plants. In the other half of every plot, the community’s roots were permitted to grow without restriction.

Community over competition

Since the current alpine environment is colder than the globeflower’s preferred habitat, Szojka hypothesized that warming would improve the transplants’ performance. But that wasn’t the case. “It turns out that warming is super detrimental to our species,” she says. “They do extremely poorly in warming situations alone.”

However, when the transplanted globeflowers were permitted to interact with other plants in their new community, they tolerated warming conditions much more readily. Szojka’s results suggest that the negative effects of warming temperatures were mitigated by the presence of alpine species.

“We expected competition, but to the contrary, interactions between species really help,” she notes. In general, the globeflowers fared best in situations where they could interact with alpine species and received heavier snowpack.

Hungry herbivores

Szojka didn’t set out to study how elk affected the transplanted globeflowers, but it turned out these large herbivores were determined to munch their way into her study.

She noticed a significantly higher proportion of herbivory events in locations where the transplanted globeflowers performed best. Unfortunately for the globeflower, the elk consistently targeted the biggest, healthiest plants.

“You’d expect that the healthiest plants would produce the most seeds and carry the population forward to the next year, but herbivory events are stopping the ability of any really healthy plants to contribute,” Szojka says. “For the Trollius population to expand its range, it may be more reliant on the plants just squeaking by versus the ones doing super well.”

What’s next?

Szojka’s research suggests that interactions with alpine plants positively affect the globeflower’s ability to weather warming temperatures. Her next objective is to investigate the mechanism behind these positive interactions. First of all, is that facilitation happening aboveground or belowground?

A community of roots can work together, collectively bringing more water and nutrients to the surface than a single plant—essentially “getting more for everybody.” But facilitation can also happen aboveground, where plants can shade one another, decreasing evaporation and keeping the soil moist.

This summer, Szojka plans to quantify shading effects as well as monitoring soil moisture and temperature. The goal is to parse whether community interactions primarily benefit globeflowers underground, aboveground, or a combination.

Either way, Szojka’s results suggest that a subalpine species like Trollius albiflorus may benefit from interactions with alpine plants as its range shifts in response to changing climate conditions.

“Overall, there’s a lot of hope and it’s great news that the alpine community helps rather than hinders range expansion,” Szojka concludes. “Given enough time, even with the elk, I think they should be able to expand their range.”

To learn more, contact faculty advisor Lauren Shoemaker at lshoema1@uwyo.edu.

This article was originally published in the 2024 issue of Reflections, the annual research magazine published by the UW College of Agriculture, Life Sciences and Natural Resources.