Species may not always move in the directions we predict under climate change

Meagan Oldfather, a plant ecologist from UC Berkeley, walked swiftly, scrambling over the rocks and scree of the White Mountain alpine, a mountain range northwest of Death Valley, at the border between California and Nevada. As climate changes, species are expected to move up mountains like these, marching higher in elevation to track historical conditions. With increasing temperatures and based on theory, we anticipate that populations that are higher up will fare well and those in lower elevations will decline. But these simplistic predictions can only take us so far. Even when ecologists can model species distributions under climate change behind their desks, to get the true scope of the situation and forecast future changes in species distributions, we need detailed field data like those that Oldfather has been collecting for the past five years.

How a plant species will react to climate change is much more complex than simply marching up a mountain, and will depend on the unique effects of temperature and water on various life stages. For example, warmer temperatures might be beneficial for producing seeds, but not for plant survival. New seedlings, leaf growth, and death will all influence the overall population growth, stability and decline. In places where a change in temperature is beneficial to one or more important life stages, the numbers of plants there will increase. Because of this, there may be places on the landscape where a species is not impacted by warmer temperatures, and other places where that same species will undergo a sharp decline. These nuances are critical and, in Oldfather’s view, quite intriguing. “I want more people to get excited about uncertainty,” she says. “We don’t always know what a species will do, but that mystery can be very exciting.”

Oldfather’s work entailed marking, counting and tracking thousands of plants for four years, in 16 populations across the entire elevational range of Ivesia lycopodioides in the White Mountains. She also set up treatments in nine of these populations to examine the effects of changes in temperature and precipitation on this species in the lowest, mid and highest elevation sites. To do this, she constructed small hexagonal greenhouses to warm the plants, and lugged five-gallon tanks of water to add into plots during snow-free months.

Ivesia lycopodioides or “clubmoss mousetail” is in the rose family, and is particularly suitable for population studies like this because they form basal rosettes, or grouped leaves at the base of the stem, making individuals easy to tell apart. The name “lycopodioides” literally means “lycopod-like,” and Oldfather delicately lifted the leaves to show me how truly similar they are to the bottlebrush of a lycopod. I bent down to take a closer look at the intricately dissected leaves. Alpine plants are the epitome of what some call “belly botany” — plants truly appreciated only when lying on your belly. But upon close inspection, they are often stunning. I. lycopodioides, with its lace-like leaves and elegant yellow flowers, was no exception.

Each year for four years Oldfather counted every leaf and flower of every individual in these plots, and documented all of the new seedlings. This information gave her the ability to estimate how the population is doing —  whether it is stable, growing, or shrinking. This type of research is called “population demography”, and is the equivalent of taking the heartbeat of a species in a specific location.

Although labor intensive, this study yields much needed information about the nuances of plant responses to climate change. 

Benjamin Blonder, a plant ecologist at UC Berkeley specializing in the impacts of climate change on plants, says “[e]xperimental studies like this one are time consuming but provide key insights into population ecology that are not readily obtained from observational studies.” Blonder also notes that “[t]his is a strong study that demonstrates the complexity of the demographic processes underlying plant responses to climate change.”

Oldfather and her colleagues found patterns of decline in the highest elevations, which is in direct contrast to predictions based on theory and distribution modeling. These high elevation sites were also the driest, and the observed contraction here suggests that water may be more important for these plants than temperature in determining how they might move across the landscape with climate change. However, her climate simulation treatments revealed that more water is not necessarily better in terms of plant persistence. Plants under those tiny greenhouses seem to decline the most when water was added, likely due to more competitive species moving in.

The findings from her study are key in establishing that there are many ways a species can be vulnerable across their range, and that predictions of shift may be more nuanced than we think. “With climate change work on range edges, it’s frustrating to tell people that a species will shift a certain way and then they don’t.” says Oldfather. “It’s much more complex than that — species are unique and complicated.”

“It is fascinating to me that one of the most basic questions of ecology — why species are where they are — is still relatively unknown,” Oldfather says. “So if we don’t study this complexity, how could we accurately predict the future?”