Maize Cultivation Three Hundred Years Ago Triggered Severe Rocky Desertification in Southwest China

Understanding the forest evolution is vital to answering the reforestation potential in karst areas. Here, we present the first-ever pollen record in karst depression sediment, combined with comprehensive dating methods (137Cs, 210Pbex, and 14C) and historical documents, to reveal plant change history in southwest Guangxi, a severe rocky-desertification region. We inferred three stages of "virgin forest-deforestation-sparse tree planting" over the past three centuries. Before the 1780s, the barren mountains used to be a lush mixed broadleaf forest probably. However, maize cultivation, along with explosive population growth and migration, accelerated mountain reclamation and deforestation, leading to severe rocky desertification since the 1780s, featured by the co-occurrence of Zea pollen appearance and Dicranopteris spore surge from 0.92% to 12.18%. Since the 1930s, sparse tree planting began, as Cupressaceae/Taxodiaceae pollen abruptly increased by 32%. Our study is significant in understanding the rocky desertification causes and guiding ecological restoration in the region. Rocky desertification refers to the transformation of a karst area once covered with vegetation into rock-baer landscapes. In southwest China, rocky desertification is the greatest ecological degradation problem. Although hundreds of billions were invested to carry out ecological restoration, it is uncertain whether the region has reforestation potential. Therefore, we attempted to use pollen record in a closed-environment sediment (karst depression) in southwest China to reveal the plant changes in the past three centuries. Combined with isotopic methods, we found that before the 1780s, tree pollens were high, implying the site used to be a lush forest. However, since the 1780s, maize pollen occurred, coinciding with the pollen percentage surge of another species indicative of ecological degradation. We inferred that maize cultivation, along with explosive population growth and migration, accelerated mountain reclamation and deforestation, leading to severe rocky desertification there. Since the 1930s, cypress and cedar pollens were abruptly increased, indicating that sparse tree planting began. These results are vital to supporting rocky desertification control and reforestation sustainability in the region. The first-ever pollen record of a karst depression in southwest China is presented Three stages of "virgin forest-deforestation-sparse tree planting" over the past three centuries were inferred Maize cultivation since the 1780s triggered severe rocky desertification in the region