Leafing in temperate trees is crucial for climate models, as small shifts can significantly impact warming predictions. Researchers often study urban heat islands, which are warmer than rural areas, to understand plant responses to higher temperatures.
However, an MIT study suggests that urban heat islands are unreliable for predicting the effects of global warming. This is partly due to the limited genetic diversity of urban tree species, leading to misunderstandings of warming impacts.
Des Marais’ group primarily conducts research in tightly controlled lab settings, regulating temperature, humidity, and carbon dioxide levels. While a few field experiments use heaters to modify conditions around individual trees, these setups are minimal in scale.
Des Marais said, “When you’re looking at these longer-term trends that are occurring over space that’s quite a bit larger than you could reasonably manipulate, an important question is, how do you control the variables?”
Des Marais and his students have explored temperature gradients to study tree responses to warming. However, they’ve also turned their attention to genetics, comparing urban tree species to the same species found in nearby natural forests. Interestingly, they discovered genetic differences even among trees that looked alike.
While the model assumes only the temperature difference between urban and rural areas is changing, an overlooked factor is the genetic diversity of the trees. This unaccounted genotypic variation adds complexity to the findings, revealing that temperature alone doesn’t fully explain the differences observed.
Genetic differences showed that the urban trees studied were not representative of natural ones, masking the true impact of warming. Urban trees were less affected by spring leaf-out compared to their natural counterparts.
Due to travel restrictions during the pandemic, MIT postdoc Meghan Blumstein focused her research on red oaks in Cambridge, Massachusetts. She collaborated with researchers at the Harvard Forest and gathered three years of data on temperature, leafing timing, and genetics. Although the study centered on red oaks, the findings likely apply to other tree species.
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Researchers had just sequenced the oak tree genome, which allowed Blumstein and her colleagues to look for subtle differences among the red oaks in the two locations. The differences they found showed that the urban trees were more resistant to the effects of warmer temperatures than those in the natural environment.
Des Marais says, “Initially, we saw these results and were sort of like, oh, this is a bad thing. Ecologists are getting this heat island effect wrong, which is true. Fortunately, this can be easily corrected by factoring in genomic data. It’s not much more work because sequencing genomes is so cheap and straightforward. Now, if someone wants to look at an urban-rural gradient and make these kinds of predwell, that’. You just have to add some information about the genomes.”
“It’s not surprising that this genetic variation exists since growers have learned by trial and error over the decades which varieties of trees tend to thrive in the difficult urban environment, with typically poor soil, poor drainage, and pollution. As a result, there’s not much genetic diversity in our trees within cities.”
Journal Reference
- Meghan Blumstein, Sophie Webster et al. Genomics highlights an underestimation of phenology sensitivity to the urban heat island effect. PNAS. 10.1073/pnas.2408564122
Source: Tech Explorist
