Submitted by Jane Durkin on Mon, 23/12/2024 - 11:05
Research into forest dynamics under the influence of typhoons in Hong Kong shows that natural rainforests in East Asia are surprisingly resilient to the storms.
Currently, nearly half of the forest restoration commitments globally are achieved by monoculture plantations. Previous research has shown that monocultures fail to provide the biodiversity benefits compared to naturally regenerated forests. The findings of the new study indicate that planted monocultures are also significantly more susceptible to tropical cyclones than natural forests.
The research team used computational fluid modelling and repeated LiDAR technology to track the wind flows over landscapes and measure structural changes across millions of trees that had just experienced an extreme typhoon event.
They found that natural forests suffered much less damage compared to plantations. Rather than being decimated, natural forests get shaped by extreme typhoons, creating a patchwork of structurally diverse habitats.
The research was published in the New Phytologist on 14 December 2024.
A project born from a passion to protect local wildlife
Dr Aland Chan, Postdoctoral Research Associate in the Forest Ecology and Conservation group at the University of Cambridge and first author of the paper, spent his PhD studying forest restoration in the mountains around his home in Hong Kong.
Mountains in Hong Kong were almost completely deforested at the close of the second world war, but nature has been coming back ever since. In September 2018, however, a powerful tropical cyclone called Typhoon Mangkhut hit Hong Kong directly, exposing the vegetation to gusts measuring up to 250 km/h.
Dr Chan explains “Typhoon Mangkhut was the strongest storm to hit Hong Kong in over 40 years, causing widespread damage to the city. It is something everyone in the territory will remember vividly for decades to come.”
“As a nerdy plant scientist, I was initially busy running around the mountains in search of rare mistletoes dislodged by the storm. I mean, how often do you get the opportunity to photograph Viscum articulatum and Viscum liquidambaricola up close? Almost never!”
“On the mountains, I could see how the wind had affected patches of forests differently. I wondered why that was. As it turns out, we were incredibly lucky. Repeated LiDAR scans were commissioned before and after the typhoon, giving us the unique opportunity to look at these wind-forest dynamics from a whole new angle.”
Natural forests less susceptible to winds than plantations
The researchers found that while exotic plantations of comparable stature in their study had suffered heavily, the natural rainforests seemed surprisingly well-adapted to the typhoon winds.
“One more reason to opt for natural regeneration of native trees over exotic monocultures.” says Dr Chan.
He explains why this research is so key.
“When we restore forests on degraded landscapes, we often aspire to rebuild structurally complex forests that support biodiversity, and tall trees that act as large carbon sinks. To achieve this, we must first understand what makes forests structurally complex and what limits the size of trees.”
“Wind regimes are changing. Climate scientists have already observed typhoons becoming less frequent but more intense. Our results provide the necessary background understanding of how this may affect forests in typhoon-prone areas in future.”
Wind exposure strongly limits forest height
The results show it is unlikely that a typhoon would destroy a natural forest. Instead, what typhoons do is hit some forests heavier than others which results in a patchwork of different forest types across the landscape.
The team found that typhoons limit forest height by trimming taller forests. Strong winds create a dynamic equilibrium between short trees growing taller and tall trees being more heavily damaged. This wind effect on maximum canopy height was found to be stronger than other variables studied.
The study also showed that typhoons disproportionally affect forests in more sheltered locations. Wind-sheltered forests were more heavily damaged by the typhoon and, in the long term, develop rugged canopies in contrast to flat-topped forests in more exposed sites.
Dr Chan says “We initially expected forests in exposed sites to suffer heavier damages. Afterall, these sites tend to have the highest wind speeds during typhoons. As it turns out, trees tend to adapt structurally to their environments, so it was in fact the sheltered sites that suffered the most.”
Next steps
The team are now looking to evaluate these wind-forest relationships at larger geographical scales. Some of the results differ from those reported in other areas such as the Americas, so it is an interesting next step to investigate why this happens and what other factors are at play.
Reference: Aland H. Y. Chan, Toby D. Jackson, Ying Ki Law, E-Ping Rau, David A. Coomes. Forest dynamics where typhoon winds blow. New Phytologist. https://doi.org/10.1111/nph.20350
Image: Naturally regenerated montane forests left largely unscathed after Typhoon Mangkhut. Photo by Aland Chan.
Funding: The project was funded by Gates Cambridge Trust. The fieldwork was supported by colleagues at the University of Hong Kong.