It’s challenging to accurately foresee what the consequences of climate change will be, considering the numerous factors at play. To estimate the effects of a warmer climate on flora, certain scientists examine urban “heat islands,” where urban architecture causes temperatures to consistently be a few degrees higher than those in adjacent rural zones. This allows for direct comparisons of plant reactions.
However, a recent investigation by scholars from MIT and Harvard University has discovered that, at least concerning forests, urban heat islands are an inadequate substitute for global warming, which might have caused experts to downplay the effects of warming in various situations. The researchers found that this inconsistency is largely due to the reduced genetic variation of urban tree species.
The results are published in the journal PNAS, in a study authored by MIT postdoctoral researcher Meghan Blumstein, civil and environmental engineering professor David Des Marais, along with four others.
“The attraction of these urban temperature gradients is, well, they’re already established,” remarks Des Marais. “We can’t predict the future, so why not compare rural and urban settings across space?” Since such information is easily accessible, methodologies contrasting the growth of flora in urban environments with similar species outside those areas have been extensively employed, he notes, and have proven to be quite beneficial. Researchers acknowledged some limitations to this method, including notable differences in nutrient availability, such as nitrogen. Nonetheless, “many ecologists realized they weren’t ideal, but it was the best we had,” he states.
The majority of the investigations undertaken by Des Marais’ team are conducted in laboratories, under conditions strictly regulated for temperature, humidity, and carbon dioxide levels. While there are a few experimental sites where conditions are altered in the field, such as using heaters around one or a few trees, “those are exceedingly small-scale,” he explains. “When considering these long-term trends occurring over a space much larger than one can reasonably manipulate, a crucial inquiry is, how do you manage the variables?”
Temperature gradients have provided one strategy to tackle this issue, yet Des Marais and his students have also concentrated on the genetics of the tree species involved, comparing those found in urban areas to the same species found in a nearby natural forest. It became evident that differences existed, even among trees that seemed alike.
“So, lo and behold, you think you’re merely allowing one variable to change in your model, which is the temperature disparity from an urban to a rural context,” he states, “but in reality, it appears that there was also a genetic variability that hadn’t been considered.”
The genetic variances indicated that the plants under investigation were not indicative of those in their natural habitat, with the researchers concluding that this difference was actually obscuring the impacts of warming. They discovered that urban trees were less impacted regarding the timing of when the plants’ leaves emerged and expanded, or “leafed out,” in the spring.
The project initiated during the pandemic lockdown when Blumstein was a graduate student. She had secured funding to investigate red oak genotypes across New England but was unable to travel due to lockdown restrictions. Thus, she focused on trees easily accessible in Cambridge, Massachusetts. She then partnered with researchers at the Harvard Forest, a research-based forest in rural central Massachusetts. Together, they gathered three years of data from both sites, including temperature profiles, leafing-out timings, and genetic data of the trees. Although the study specifically examined red oaks, the researchers indicate that the findings are likely applicable to trees in general.
At that time, scientists had just sequenced the oak tree genome, enabling Blumstein and her colleagues to identify subtle variations among the red oaks in the two locations. The distinctions they uncovered indicated that urban trees exhibited greater resilience to the implications of warmer temperatures compared to those in the natural environment.
“Initially, we analyzed these findings and thought, oh, this is unfavorable,” Des Marais illustrates. “Ecologists are misinterpreting this heat island effect, which is accurate.” Fortunately, this can be easily rectified by incorporating genomic information. “It’s not excessively labor-intensive, as sequencing genomes has become quite economical and straightforward. Now, if someone wishes to examine an urban-rural gradient and generate these sorts of predictions, that’s acceptable. You just need to include some genomic information.”
It’s not unexpected that this genetic diversity exists, he notes, since cultivators have learned through experience over the years which tree varieties tend to flourish in the challenging urban landscape, characterized by typically poor soil, inadequate drainage, and pollution. “Consequently, there’s a marked lack of genetic variability in our trees within cities.”
The implications could be substantial, Des Marais suggests. When the Intergovernmental Panel on Climate Change (IPCC) releases its periodic assessments on climate conditions, “one of the tools the IPCC utilizes to project future responses to climate change concerning temperature involves these urban-to-rural gradients.” He hopes these new insights will be included in their upcoming report, which is presently being drafted. “If these findings hold true beyond red oaks, it implies that the urban heat island technique for examining plant responses to temperature is underestimating the strength of that response.”
The research group also included Sophie Webster, Robin Hopkins, and David Basler from Harvard University and Jie Yun from MIT. The project received support from the National Science Foundation, the Bullard Fellowship at the Harvard Forest, and MIT.