A breakthrough study using ground-penetrating radar (GPR) has identified the optimal planting density for Mongolia pine plantations in China's arid regions, offering a science-backed solution to combat tree die-offs threatening anti-desertification efforts.

The research, conducted on a long‐term trial in Zhanggutai, Liaoning Province, challenges traditional methods based solely on aboveground indicators and reveals the crucial role of belowground interactions in plantation health.

Conducted by researchers from the Institute of Applied Ecology of the Chinese Academy of Sciences, this study pinpoints the root cause behind the mysterious decline of Mongolia pine (Pinus sylvestris var. mongolica) plantations—a cornerstone of China's "Three-North Shelterbelt" reforestation project. 

The findings, published in Forest Ecology and Management, reveal that overcrowded tree stands lead to intense underground competition for water, accelerating forest degradation in semi-arid zones like the Horqin Sandy Land.

Mongolian pine was introduced in 1955 to combat desertification along the southern edge of the Horqin Sandland. However, after 35–40 years, many plantations began to decline, exhibiting symptoms such as leaf yellowing, weakened growth, and increased pest infestations. These issues have largely been attributed to high initial planting densities and insufficient thinning practices. Recognizing that survival in arid and semi-arid regions depends on root-mediated water and nutrient uptake, this study reconsiders stand density from a belowground perspective.

Using the Noggin 1000 GPR system alongside traditional soil coring, the researchers examined coarse roots (>5 mm) and fine roots (<2 mm) across 25 trial plots with planting densities ranging from 1,111 to 10,000 trees per hectare. Over 40 years of natural thinning resulted in six distinct density gradients, from 383 to 2,367 trees per hectare. The GPR allowed the team to quantify the spatial distribution of coarse roots without disturbing the soil.

The results showed that as stand density increased from 383 to 2,367 trees per hectare, coarse roots shortened horizontally from 3.65 meters to 1.81 meters and became more densely clustered. Root depth also decreased by 20%, from 95.3 cm to 75.8 cm, pushing trees into shallower, drier soil layers. Simultaneously, fine roots concentrated near the surface (0–20 cm), intensifying water competition. "Even the lowest density plot still showed root overlapping and intense root competition," said lead researcher Dr. LI Mingjuan. "This proves why thinning is essential."

By establishing quantitative relationships between stand density and root competition intensity indicators，the researchers identified the optimum stand density as being between 177 and 214 trees per hectare for 45-year-old plantations.

A notable advancement of this study is the use of GPR, which overcomes the limitations of earlier, labor-intensive, and destructive methods. GPR not only provides a fast, non-invasive means of mapping root distribution but also offers a more accurate understanding of the belowground interactions that drive tree competition and forest stability. This technological breakthrough marks an important step forward in forest management, particularly in arid regions where water availability is a critical limiting factor.