Spring comes earlier, autumn foliage later as climate change alters forests

If you are planning a fall foliage trip this year, you may need to set off a little later than expected. The peak autumn foliage season is forecast to arrive later than usual.

According to the Korea Forest Service, fall colors will spread gradually from the central to southern regions, starting with Mount Seorak on Oct. 20. Foliage is expected to reach its peak nationwide only in late October, with peak dates forecast for Oct. 28 at Mount Songni, Nov. 4 at Mount Naejang and Nov. 6 at Mount Halla.

The average peak date for maples and oaks nationwide is expected to be Oct. 31, about 0.8 days later than the average peak date over the past five years.

The delayed foliage season is not an isolated development this year. As temperatures rise, spring is arriving increasingly early while autumn is shifting later, a pattern that researchers have consistently observed in Korea’s forests. The seasonal clock that we have long taken for granted is changing.

A 2021 study by researchers at the Korea National Arboretum and Seoul National University’s Graduate School of Environmental Studies tracked 25 native plant species in Korea over a decade. It found that the timing of leaf emergence in broadleaf trees had advanced by 13 days over the 10-year period, while the onset of autumn foliage had been delayed by 3.7 days.

The researchers estimated that a 1-degree Celsius rise in average spring temperatures advanced leaf emergence by 3.6 days, while a 1-degree increase in average autumn temperatures delayed fall foliage by 1.5 days.

Extending the observation period has made the changes even more apparent.

A study titled “Long-Term Changes in Plant Phenology in Korea (2009-2024),” published last year in the Korean Meteorological Society’s journal Atmosphere, analyzed seasonal changes in 20 deciduous broadleaf tree species observed at 10 arboretums nationwide.

From 2009 to 2024, leaf buds burst an average of 0.94 days earlier each year, flowering began 0.83 days earlier and leaves unfolded 0.79 days earlier. In other words, spring plant development advanced by roughly 13 to 15 days over the 16-year period.

By contrast, the peak autumn foliage period — when 90 percent to 100 percent of leaves have changed color — was delayed by an average of 0.33 days each year, shifting approximately five days later over the same period.

As a result, the growing season for trees — the period from when leaves emerge in spring until they change color in autumn — lengthened by more than 17 days on average.

“This shows that the seasonal clock of forests is changing, with spring arriving earlier and autumn coming later,” said Jung Soo-jong, a professor at Seoul National University’s Graduate School of Environmental Studies.

Longer growing season: Good for carbon absorption?

Is a longer growing season good for forests? With leaves remaining active for longer, trees have more time to photosynthesize and potentially absorb carbon.

However, a forest’s capacity to absorb and store carbon cannot be assessed solely by how long trees photosynthesize. Soil also stores substantial amounts of carbon, and rising temperatures can stimulate microbial activity in the soil, potentially increasing carbon dioxide emissions. The amount of carbon absorbed through photosynthesis also depends on temperature and water availability, not just the length of the growing season.

“Carbon absorption capacity is determined by the balance between plant photosynthesis and carbon emissions from the soil, as well as factors such as drought and extreme weather events,” said Noh Nam-jin, a professor in the Department of Forest Resources at Kangwon National University.

More complex changes are also taking place within forests.

“When leaves emerge earlier in spring, the canopy of taller trees may become dense before understory plants — small plants growing on the forest floor — that bloom briefly in early spring have had enough time to complete their life cycles,” said Park Pil-sun, a professor in the Department of Forest Sciences at Seoul National University.

The leaves of taller trees block sunlight, potentially preventing understory plants from producing fruit properly, she explained.

A later autumn is not necessarily beneficial, either. After completing their growth, trees begin preparing for winter. If they do not have enough time for this process, they may become more vulnerable to severe cold.

“In a country like Korea, where the four seasons are distinct, a shorter period of preparation for winter weakens trees’ tolerance to low temperatures,” said Kim Hyun-jun, a professor in the Department of Forest Resources at Chonnam National University.

This is one reason why a simplistic response, such as indiscriminately introducing subtropical tree species, is not viable. Subtropical climates are generally defined by having an average temperature of 18 degrees Celsius or below in the coldest month and at least eight months with average monthly temperatures of 10 degrees or higher.

The problem is that researchers still do not fully understand how these changes will affect forests. A delay of several days in peak foliage is not simply a matter of adjusting the dates of autumn trips. The growth cycles of plants and their preparations for winter are changing as well.

The effects of climate change are also not driven by rising temperatures alone. Multiple factors, including droughts, torrential rain and strong winds, interact in complex ways, making it difficult to assess the overall impact through short-term observations.

Researchers have recently warned that “the climate is changing faster than we can monitor it,” Kim said.

That makes it all the more important to collect and maintain long-term data across multiple indicators. Noh emphasized the need to monitor soil carbon emissions, air temperature, precipitation, soil temperature and soil moisture together over extended periods.

The accumulated data should also be used to guide forest management.

“Rather than applying a uniform management approach nationwide, we need to closely monitor seasonal changes by tree species and region,” Jung said, adding that stable, long-term observations are essential to strengthening forests’ resilience to climate change.

This article from the Hankook Ilbo, the sister publication of The Korea Times, is translated by a generative AI system and edited by The Korea Times.