Climate change has caused substantial alterations in forest ecosystems worldwide through changes in temperature and precipitation patterns. A new study from Osaka Metropolitan University (OMU) suggests that these effects are also evident in Japan, where forests are undergoing changes in structure and shifts towards tree types adapted to warmer climates.

A research group led by Associate Professor Tetsuro Yoshikawa and Professor Yayoi Takeuchi of OMU investigated forest changes by compiling long-term monitoring data from 191 forest plots across Japan from the 1950s to 2020.

"Japan spans a wide range of climatic zones and contains diverse forest biomes, making it an ideal setting for investigating forest change," Associate Professor Yoshikawa, the first author of the study, explained.

The researchers analyzed annual rates of change in Total Basal Area (TBA), the sum of tree trunk cross-sectional areas measured at chest height within a plot. TBA reflects both tree size and density and provides a measure of the forest’s structure. They also investigated the changes in the relative dominance of four major tree groups: boreal conifers, deciduous broadleaved trees, temperate conifers, and evergreen broadleaved trees.

They found that changes in forest structure were closely associated with climate change, as greater increases in mean annual temperature were associated with increases in TBA. On the other hand, more rapid increases in annual precipitation were associated with decreases in TBA.

As increased precipitation can be associated with more severe typhoon disturbances, the findings support the hypothesis that such disturbances have contributed to the decline in TBA in recent years.

The researchers also found that the relative dominance of the cold-adapted boreal conifers declined, while that of warm-adapted deciduous broadleaved trees increased in some regions. This pattern is consistent with a phenomenon known as thermophilization: an increase in dominance of species adapted to warmer climates in communities.

"These changes suggest that warming is affecting not just the growth of individual trees but also the composition of Japan's forests," Professor Takeuchi said. "Over time, such changes could contribute to shifts in the distribution of forest biomes."

Overall, the findings highlight the complex effects of climate change on forests in Japan.

“Although our findings capture changes that occurred in Japanese forests up to 2020, the impacts of climate change are expected to become even more pronounced in the future,” Associate Professor Yoshikawa concluded. “Continued long-term monitoring of forest ecosystems will be increasingly important for detecting ecological changes and improving our ability to predict their consequences for forest environments and people's lives.”

The findings were published in Forest Ecosystems.

Conflict of interest
The authors declare no conflict of interest.
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