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Past variability may help forests cope with climate change

Global warming will not only bring warmer temperatures to ecosystems across the globe, but will also cause greater variation in temperatures. The extent to which plants can cope with this variability may be an important indicator of their potential to survive or thrive as the climate changes in future and brings with it more extremes in environmental parameters. 

In a new study, published in the first issue of Environmental Research: Ecology, researchers investigated whether forests that had been exposed to variable conditions in the past were more tolerant of the increased variability that is seen today as an effect of global warming. They did this by assessing the productivity of forests all over the world. 

“Global climate patterns are becoming increasingly variable. This means more extremes, which threaten forest health and productivity,” explained study lead author Winslow Hansen, a forest ecologist at Cary Institute of Ecosystem Studies. “They say adversity makes you stronger. Here, we were essentially testing that adage for trees. Are forested regions that experienced more variable conditions in the past better prepared to tolerate variable climate now and in the future?”

The researchers studied records of regional climate variability during two 20-year study periods, namely 1950–1969 and 2000–2019. The records gave data on monthly minimum, mean and maximum temperatures, total precipitation, and mean vapor pressure deficit (a measure of how dry the air is). This global data was gridded at a 0.5° spatial resolution. 

The team then related variability in these environmental parameters to vegetation productivity in forests. To assess modern forest productivity, the researchers used global vegetation data detected by NASA’s MODIS satellites. The ‘enhanced vegetation index’ (EVI) is a satellite-derived measure of ‘greenness’ which is a reliable proxy for leaf cover and forest productivity. The MODIS satellites ‘Aqua’ and ‘Terra’ produce a global map of vegetation cover every eight days. 

By pairing climate data with satellite records of forests, the researchers were able to assess how climate variability in the past and present shapes current forest productivity. They found that regions where temperature was more variable in the past continue to experience more temperature variability today. Forests in these regions tend to tolerate this increasing variability better.

“Our findings show that historic temperature variability casts legacy effects on current forest productivity,” said Hansen. “In places where historic temperature variability was 0.66°C greater than the global average, forests were 19x less sensitive to current temperature variability. This trend was true globally, with important distinctions among biomes.”

“We are seeing global temperature change in two distinct ways: average temperatures are rising and temperature is becoming more variable year to year. These indicators are changing with varying degrees of intensity in different regions. In some places, rising mean temperature is likely to have a greater impact on forests than increasing year to year temperature variability, and vice versa,” explained study co-author Naomi Schwartz of University of British Columbia.

“While climate models predict relatively modest overall warming in the tropics through the 21st century, year-to-year temperature variability is expected to substantially increase,” said Hansen. “Our analysis indicates that tropical forests could be harder hit by effects of increasing variability than rising mean temperatures.”

However, the story is not the same for boreal forests. As Hansen explains, in boreal forests “… year-to-year temperature variability is expected to increase moderately relative to past conditions, but average temperature is rising at least twice the global average. Decadal warming trends, and exacerbating effects of fire and insect outbreaks, may threaten boreal forests more than interannual temperature variability.”

The results of the study demonstrate that exposure to temperature variations in the past shapes how forests respond to temperature variability today. However, the same is not true for forest responses to variability in precipitation and vapor pressure deficit. This may be due to physiological tradeoffs inherent in how trees cope with dry conditions.

“We often think of climate change as a monolithic phenomenon. But in reality, climate is changing in many different ways all at once, and we expect this to cause really different impacts across ecosystems, including forests,” said Hansen. “Our study highlights how forest adaptation strategies need to be developed that account for the nuanced effects of climate change.”

The analysis also highlighted sensitive ‘hotspots’ across all biomes, and indicated regions in which forests may be most at risk as the planet warms and temperatures become more extreme. These include boreal forests in eastern North America, temperate forests of the south-central and southeast United States, temperate forests in Asia, and tropical forests in the southern Amazon. 

The researchers hope that the framework they have developed can help set conservation priorities, support forest adaptation efforts, and improve carbon accounting. They state that understanding how forests are responding to climate change is critical to planning effective forest management and climate policy in the decades ahead.

“As climate conditions become increasingly variable, there is a critical need to identify where and how forests are changing,” said Hansen. “Our analysis offers a framework to hone this understanding on a global scale – helping to improve targeted conservation policies that protect forests, their inhabitants, and the essential services they provide.” 

By Alison Bosman, Staff Writer

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