A new drain that had been proposed for the Kranji development was the reason that environmental studies - including a biodiversity survey of the area - were required before land clearance.
The drain was part of a revised submission from the design and planning consultants for the Agri-Food Innovation Park to the National Parks Board (NParks) last August, said Dr Leong Chee Chiew, NParks' commissioner of parks and recreation.
The drain would discharge into the Pang Sua river, which leads to the Mandai Mangrove and Mudflat - a migratory bird stopover site and a habitat considered the main "seeding source" for mangrove propagules for many other mangrove areas.
Dr Leong said: "NParks was concerned with the potential impacts of discharge into Sungei Pang Sua towards Mandai Mangrove and Mudflat, and displacement of wildlife if appropriate measures are not put in place."
And so, under the Wildlife Act, NParks imposed a baseline fauna survey, an environmental monitoring and management plan, as well as a sediment load study for drainage run-off into Sungei Pang Sua, he added.
A full environmental impact assessment - which entails far more rigorous studies on how the development could affect biodiversity and the environment in a developmental site - had not been deemed necessary as the site was not near any sensitive nature area.
Dr Leong also said the site at the intersection of Kranji Road and Kranji Close had largely been cleared in 2008 and was dominated by scrubland. It is now dominated by non-native Albizia trees.
The affected site was occupied by the former railway line operated by the Malaysian company Keretapi Tanah Melayu (KTM), but was returned to Singapore on July 1, 2011.
With the rail line no longer in use and the site left unmanaged pending redevelopment works, vegetation had sprouted up over the past few years.
JTC Corporation - the project and site developer - said the fauna baseline study and environmental monitoring and management plan are estimated to be completed in April. JTC said the results of the baseline study and consultation will be publicly shared after that.
Nature Society (Singapore) president Shawn Lum said that JTC setting out the full timeline of events provided more clarity to the incident. Last week, people had not even been sure about the size of the plot that was cleared, he said.
People might be more understanding of the situation, and this could help members of the public engage with the authorities more constructively on the issue, he added.
But Dr Lum, who is a botanist at the Nanyang Technological Uni-versity, noted that there were other "spontaneous forests" in Singapore that had become important sites for animals.
For example, the former Bidadari forest - now a Housing Board estate - had been full of Albizia, but was an important stopover for migratory birds.
"It will be interesting to see what sorts of wildlife are attracted to these young forests," said Dr Lum.
"It could help to fine-tune plans for Kranji, but could also inform how we think about other spontaneous forest sites in Singapore."