In the 1980’s and early 1990’s, the AFCD rapidly reforested cleared and degraded areas in the Sai Kung Country Park with several exotic (non-native) species of trees. These included Acacia Confusa (Taiwan acacia) and Melaleuca quinquenervia (paperbark). The acacia tree is also known as a wattle tree. Both of these species of trees are known to be fast growing, even in infertile soil, so offer a good solution to soil erosion and will quickly enhance forest cover.
However, these trees now pose a threat for the indigenous tree species in the Country Park. Both tree species seed profusely, with the bulk of the fire resistant seed pods sprouting after forest fires. These seed pods also can be dormant for long periods allowing for wide dispersal by rain, wind and animals. Both species have also been declared as being invasive in many areas they have been introduced in the past.
Whilst it can be argued the root systems and leaf litter can improve the condition of the soil under the tree canopy, the invasive root systems and the shading under the trees has been shown to inhibit the natural species from growing there. The left photograph above shows the lack of natural undergrowth beneath an acacia as compared to the native forest assemblage on the right.
In 2009, the AFCD launched the Country Park Plantation Enrichment Project (PEP) to deal with this issue. The idea was simple, to replace exotic species with native ones. In 2015 some 4,000 exotic trees were replaced by 6,500 natives.
These photographs are an example of how AFCD has weeded out acacia. The logs are left where they land to continue protecting the soil from erosion. Meanwhile, newly planted native species are able to repopulate areas.
In the 1980’s almost 70 acacia and paperbark trees were planted at both of the BBQ sites at Hoi Ha. In 2015, 2016 and 2017 tree surveys were carried out around the village of Hoi Ha in part to investigate the current status of these introduced exotic trees. The Hoi Ha Action Group (HHAG) study concluded that the increase in exotic tree cover, by these species, was almost 7% year on year. Furthermore, 13 new trees (>3m tall) were identified as having sprouted in the area towards the village. This would be the natural receiver for the seeds generated by the trees at the northern BBQ site.
The Northern BBQ Site at Hoi Ha
Records show that there has not been a forest fire of significance between the BBQ site and the Hoi Ha village since these trees were planted. A key concern is how many of these types of trees would sprout and take hold afterwards.
An aerial view of the area in early 2017 shows the original acacia and paperbark forest (outlined in red). The blue circles show the extra trees that have been seeded from this original area. There are several of these trees further down towards the village, but these have been planted as part of other restoration works. They are also spreading into the natural secondary forest in that area.
In late 2016 the AFCD’s proposal to build a Visitor’s Centre on the Northern BBQ site was basically finalised. Part of the works will involve cutting down some of the acacia and paperbark trees to make way for the structure.
Surprisingly, some of the local environmental groups objected to the removal of the exotic trees and made this a key objection to the building of the AFCD Visitor’s centre since around 2005. Since these groups claim to have experts in forestry and flora who should know about invasive species, it is unclear exactly what their motives are. Currently they are suggesting that only a few should be cut down.
An ad hoc EIA for the centre, carried out by HHAG, suggested that all of the exotic trees at the BBQ site should be replaced with native species as part of the building work for the centre. We hope that the Authority does its job and removes this invasive threat to the natural forest and take advantage of the situation to promote learning and sustainable forestry.
The EIA also suggested that native fruit trees like the hairy fig should be planted to entice animals to the site. Suitable flowering plants and creepers like the protected Indian birthwort should also be added to encourage rare butterflies like the common bird wing butterfly to use the area more.
Interestingly, during the EIA it was noticed that saplings of some protected native species had been planted at strategic places in the BBQ site. Where these came from we can only guess.
Finally, it is important to realise these invasive tree species have already spread into the lower part of the Pak Sha O river valley near to the Hoi Ha Village. If anything the AFCD should be encouraged to take action to replace them with native species as soon as possible.