What’s the best way to rewild Hong Kong?

22 June 2023

New research is investigating different methods of restoring Hong Kong’s unique forests

Rewilding involves returning land to its natural condition, but there are several ways to go about it. These are broadly in two categories: passive rewilding, which involves leaving the land to recover by itself, and active rewilding, which involves conducting a baseline to study to identify species which, according to our best estimate, would have been a part of our prehistoric ecosystem, and purposefully planting these.

Hong Kong’s unique history and abundant biodiversity mean it’s a good place to study different ways of rewilding. With funding from Croucher, University of Hong Kong PhD student Coşkun Güçlü is studying several sites in Hong Kong to find out what will work best in a local context.

An unnatural history

Forests have declined across the world, but how they have been reduced varies a lot. In the UK, for example, woodlands were often fractured but not completely destroyed, leaving corridors like field margins and stream-sides intact. These areas can act as reservoirs, so that if a nearby field is abandoned, seeds can disperse on wind or water and passively rewild the field.

In Hong Kong, much of the forest destruction was more comprehensive, particularly pre-1600s and during World War II. Forests left intact were distant from each other, meaning passive seed dispersal is less likely to help deforested areas regenerate. Subsequent reforestation efforts introduced many non native species.

Soil sampling

To determine which method is best, Güçlü is sampling biological diversity and carbon storage at distinct sites in Hong Kong which are representative of passive and active rewiilding. The amount of carbon in an individual soil sample can be measured relatively simply. Finding out which species are present requires a relatively new technique called environmental DNA, or eDNA.

eDNA involves stripping all of the DNA out of the soil sample. This DNA comes from everything any organism has dropped in the normal course of moving around. Sequencing this mixture of DNA produces strands that can be compared to a database of known DNA sequences from whole organisms, allowing matches to be made. It’s fiddly and painstaking work – Güçlü calls it “a very complex recipe for a difficult cake” – but it can allow a snapshot of fungal, microbial and animal abundance to be determined from a simple soil sample.

Güçlü is taking these samples in various forests, bare grasslands abandoned since the 1940s, and at several spots in Kadoorie Farm and Botanic Garden (KFBG), an environmental NGO that is the custodian of a 148-ha nature reserve in Hong Kong’s New Territories and his collaborators in the research. Areas of KFBG have been actively rewilded in various intensities over the past 20 years, and passively rewilded for 40-50 years, providing good comparison points.

While the first results are being developed in the lab, Güçlü can’t make any conclusions yet. But he has a hypothesis based on his previous work with the fungi and bacteria that help trees grow and store carbon. He thinks actively rewilded areas will have a greater diversity of these helpful microbes as dispersal will limit their number in passively rewilded areas. More microbes mean more diverse services provided to the trees, bolstering their ecological function.

Studies in other parts of the world back this up: some areas are reforested with single species of fast-growing trees that suck in a lot of carbon, but the range of microbe interactions is limited, and growing one species may deplete the soil of vital nutrients. Comparative studies of these kinds of interactions, however, are rare in tropical settings.

What to preserve?

All rewilding faces a question: what should the results look like? “There are ingrained ideas that we should maintain forests to a certain standard,” says Güçlü. “A lot of it is based on culture – what aesthetics we want to preserve.” This can lead to projects where repeated human interventions are required to preserve a desired aesthetic.

The active rewilding approach being developed at the Kadoorie Farm and Botanic Garden is instead scouring historical records and exploring remote, remnant patches of old-growth forest in Hong Kong and southern China to determine which native species to reintroduce, This includes paying attention to species interactions, such as making sure to include the plants that grow on trees, as well as the tree species themselves. The success of this approach is down to a combination of science and horticulture.

“It takes a very long time to actually get everything in that used to be there… it really highlights the value of old growth forests” commented Güçlü.

While we may not know now exactly how we can replace our prehistoric forests, understanding how in time we might arrive at an approximation of what was once here is an important step. Something which future generations may appreciate very much.