The first roads that cut through rainforests are well-known conduits for deforestation. However, new research finds that secondary roads, those that branch off the primary road, cause far more forest loss than the original roads themselves.

The conservation impacts of secondary roads have not been well understood. So, researchers used satellite images and newly available maps starting in 2019 to look at secondary road impacts in three tropical forest regions: the Brazilian Amazon, the Congo Basin and New Guinea.  They examined 92 primary, or first-cut, roads and measured the length of associated secondary roads and the deforestation that resulted from them.

In all three regions, secondary roads caused much more deforestation than the initial roads, but the scale of impact varied widely. On average, for every kilometer (0.6 miles) of first-cut road, the study found an additional 4.8 km (3 mi) of secondary roads in the Congo Basin, 9.8 km (6 mi) in New Guinea, and a striking 49.1 km (30.5 mi) in the Brazilian Amazon.

The deforestation and forest degradation associated with the secondary roads also varied greatly across regions. In the Congo Basin, deforestation associated with secondary roads was 31.5 times greater than that caused by primary roads. In New Guinea, secondary roads contributed to 22.2 times more forest loss, and in the Brazilian Amazon they led to 305.2 times more deforestation and degradation compared with primary roads.

The impacts of secondary roads are not thoroughly considered when countries conduct environmental impact assessments for new road construction, Jayden Engert, lead author of the paper who conducted the study as part of his Ph.D. with James Cook University, told Mongabay by phone. “Not in any impact assessment that I’ve seen or heard discussed,” Enger said. “The typical thing is just to look at the clearing for the construction of the [primary] road,” he added.

The high variability of secondary road impacts between regions can largely be traced back to geography and socioeconomic factors, the authors write.

Roads in the Amazon, for example, were built by the Brazilian government for agriculture; today the region is a breadbasket for commodity crops and livestock, so deforestation is high. In the Congo Basin, much of the forested land in the study is used for selective logging, not agriculture, which leads more to forest degradation than outright deforestation.

Much of New Guinea is mountainous and difficult to develop, so agriculture tends to be industrial plantations near primary roads, although the authors note that new projects are expected to increase road building in the near future.

Engert said he hopes this research will give more context for assessing the likely effects of such projects “to identify what areas are likely to be impacted most by these developments and develop efforts to minimize the impacts,” he said.

Banner image: The Trans-Amazonian Highway cutting across the Amazon has spawned a network of illegal offshoots. Image courtesy of the National Department of Transport Infrastructure.