For example, in Minqin, an area in north-western China, studies showed that groundwater levels have dropped by 12–19 metres since the advent of the project. Land erosion and overfarming have halted planting in many areas of the project. China's increasing levels of pollution has also weakened the soil, causing it to be unusable in many areas. Furthermore, planting blocks of fast-growing trees reduces the biodiversity of forested areas, creating areas that are not suitable to plants and animals normally found in forests. "China plants more trees than the rest of the world combined", says John McKinnon, the head of the EU-China Biodiversity Programme. "But the trouble is they tend to be monoculture plantations. They are not places where birds want to live." The lack of diversity also makes the trees more susceptible to disease, as in 2000, when one billion poplar trees were lost to disease, setting back 20 years of planting efforts. Liu Tuo, head of the desertification control office in the state forestry administration, is of the opinion that there are huge gaps in the country's efforts to reclaim the land that has become desert. At present there are around 1.73 million sq kilometers that have become desert in China, of which 530,000 km2 are treatable. But at the present rate of treating 1,717 km2 per year, it would take 300 years to reclaim the land that has become desert. China's forest scientists argued that monoculture tree plantations are more effective at absorbing the greenhouse gas carbon dioxide than slow-growth forests, so while diversity may be lower, the trees purportedly help to offset China's carbon emissions. However, a study released in 2016 finds that wild woodlands are much more effective than monocultural forests in storing carbon dioxide, with more resilient and greater tree health, size, lifespan, and depth of organic matter rich soil. Gao Yuchuan, the Forest Bureau head of Jingbian County, Shanxi, stated that "planting for 10 years is not as good as enclosure for one year", referring to the alternative non-invasive restoration technique that fences off (encloses) a degraded area for two years to allow the land to restore itself. Jiang Gaoming, an ecologist from the Chinese Academy of Sciences and proponent of enclosure, says that "planting trees in arid and semi-arid land violates [ecological] principles". The worry is that the fragile land cannot support such massive, forced growth.