The changing of leaves from green to red, gold, and orange is a landmark event in fall. For trees, this is more than just a season of beauty, as it pauses photosynthesis and begins the storage of nutrients for the winter.
But climate change could alter the details of this seasonal phenomenon.
A team of researchers, led by Allison Gill, conducted a meta-analysis using data from deciduous forests across the northern hemisphere to determine how the timing of leaf coloration is changing, and what cascading effects this may have on forests.
The process of leaves changing color and falling off is referred to as senescence.
Leaf senescence is the end stage of the growing season for a deciduous tree, and this process can ultimately affect future plant competition, plant growth, and carbon storage.
Phenology, the study of seasonal natural phenomena, has become increasingly valuable to understanding how the effects of climate change could make an impact on forests. Warming temperatures in particular have been shown to delay the timing of leaf senescence in many arctic and boreal forests.
To examine these trends, the team led by Gill compiled research collected from 1993 to 2010 in the eastern United States, Europe, and Japan. They specifically looked for papers that gave exact days of senescence as well as temperature changes for the reported time period.
The analysis was conducted using published studies from the peer-reviewed literature that reported autumn senescence dates for deciduous trees in the northern hemisphere, encompassing 64 publications with observations ranging from 1931 to 2010.
Ultimately, October temperatures were the biggest driver of leaf fall. Other factors were also important, such as day length, latitude, precipitation, and cooling degree days, which is the measure of how hot the temperature is during a given time period. Temperature was a strong driver in lower-latitude locations but less so in high-latitude areas, which indicates that many site-specific factors drive leaf senescence.
Areas farther north are predicted to feel the effects of climate change more severely and more quickly than low-latitude areas. While temperatures will rise more rapidly, day length does not appear to shift under climate change. It’s possible that species could adapt to these changes, but the effects of climate change could be occurring too quickly for species to keep up.
As temperatures rise and the timing of leaf senescence shifts, cascading effects could be felt by the tourism industry. Many places, such as New England, rely on the autumn colors to bring in visitors.
Gill highlights the necessity of more studies to examine these trends and determine the impacts on forests, communities, and industry.
“Regional, national and global networks of on-the-ground and remotely sensed data provide important information about changes in autumn phenology, but there is an opportunity to augment these data through smaller scale, independent studies as well.”
While this study indicates that temperature is a strong predictor of delayed leaf senescence, each forest is unique, and time will tell how climate will continue to affect fall foliage across the Northern Hemisphere.
“Among the meteorological measurements examined, October temperatures were the strongest predictors of date of senescence, followed by cooling degree-days, latitude, photoperiod and, lastly, total monthly precipitation, although the strength of the relationships differed between high- and low-latitude sites,” said the study.
“Autumn leaf senescence has been significantly more delayed at low (25° to 49°N) than high (50° to 70°N) latitudes across the northern hemisphere, with senescence across high-latitude sites more sensitive to the effects of photoperiod and low-latitude sites more sensitive to the effects of temperature. Delays in leaf senescence over time were stronger in North America compared with Europe and Asia,” the study said.