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Climate change outpaces the ability for trees to adapt

A new study has found that the prevailing methods used to predict how tree species will respond to climate change are inaccurate and unreliable. Led by the University of Arizona in collaboration with the U.S. Forest Service, the research was focused on the ponderosa pine — a widespread tree in the Western United States.

Space-for-time substitution method

The results of the study challenge the reliability of the commonly used “space-for-time substitution” method in forecasting species adaptation to global warming. This technique has been a cornerstone in ecological research for predicting how species might adapt to changing climate conditions. 

The space-for-time substitution method assumes that individuals of a species thriving in hotter climates can be proxies for understanding how those in cooler areas might respond to global warming. The idea is that, as temperatures rise, species at the cooler edge of their distribution would adapt similarly to their counterparts in warmer regions.

However, the new study reveals that this approach might be fundamentally flawed. When applied to the ponderosa pine, the method inaccurately projected a positive response to warming, whereas the actual response was detrimental. 

“We found that space-for-time substitution generates predictions that are wrong in terms of whether the response to warming is a positive or negative one,” said study co-author Margaret Evans, an associate professor in the UArizona Laboratory of Tree-Ring Research. “This method says that ponderosa pines should benefit from warming, but they actually suffer with warming. This is dangerously misleading.”

Critical insights on trees and climate

The experts meticulously analyzed ponderosa pine tree rings dating back to 1900 across the Western United States. They compared the trees’ actual growth with the predictions made by the space-for-time model. 

The findings were alarming: the model had failed to accurately predict the trees’ response to recent warming trends. The tree ring data painted a starkly different picture from the model’s predictions. 

In reality, the trees consistently exhibited negative impacts due to temperature variability. Warmer years resulted in smaller-than-average growth rings, contradicting the model’s suggestion of a beneficial effect of warming. 

“If it’s a warmer-than-average year, they put on a smaller-than-average ring, so warming is actually bad for them, and that’s true everywhere,” explained Professor Evan. 

Trees and rapid climate change

A key insight from the study is that the pace of climate change might be outstripping the trees’ ability to adapt. The research underscores a fundamental ecological principle: adaptation and migration are gradual processes, often lagging behind rapid environmental changes. 

While evolutionary adaptation and migration could theoretically enable species to cope with shifting temperatures, the current rate of climate change appears too swift for these natural processes to keep pace. 

The team also explored how variations in rainfall affect tree growth, confirming that increased rainfall universally benefits the trees, regardless of temporal or spatial context.

“These spatially based predictions are really dangerous, because the spatial patterns reflect an end point after a long period of time when species have had a chance to evolve and disperse and, ultimately, sort themselves out on the landscape,” said Professor Evans. 

“But that’s just not how climate change works. Unfortunately, the trees find themselves in a situation where change is happening faster than the trees can adapt, which is really putting them at risk of going extinct. It’s a word of caution for ecologists.”

The study is published in the journal Proceedings of the National Academy of Sciences.


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