Spatial distribution of root systems of Pinus sylvestris var. mongolica trees with different ages in a semi-arid sandy region of Northeast China

Abstract

Mongolian pine (Pinus sylvestris var. mongolica), the most important tree species for sand fixation in semi-arid sandy lands, experiences dieback in plantations after approximately 35–40 years since planting. Water limitation was considered as an important cause for the dieback. The spatial distribution of root systems plays a key role in determining water resources used by trees. However, it is still unknown for the spatial distribution of Mongolian pine trees with different ages, which constrains our understanding of the mechanism underlying the dieback and management of Mongolian pine plantations. Horizontal and vertical distributions of coarse roots [root diameter (RD) ≥ 5 mm] of Mongolian pine trees with 10, 20, 30, 40, 50 years old were estimated by ground-penetrating radar. Meanwhile, the distributions of fine roots (RD ≤ 2 mm) were simultaneously measured using soil coring method. Results showed that the horizontal distance of coarse roots from trees increased with tree ages up to 30–40 years, i.e., 3.0–4.0 m for 10- and 20-year-old trees, ca. 6.1 m for 30- and 40-year-old trees; but merely 4.85 m for 50-year-old trees. The fine root densities kept relatively even within 4.0 m from the trees for 10- and 20- year-old trees (ca. 0.02 mg cm−3), whereas, they decreased sharply with increasing of distances from the trees for 30-, 40- and 50-year-old trees (0.015–0.025 mg cm−3 at 1.0 m from trees to ca. 0.00 mg cm−3 at 5.0 or 6.0 m from trees). In the vertical direction, the rooting depth, at which 95% of roots located, was 83.5, 99.9, 123.6, 105.2 and 125.9 cm for the coarse roots of 10-, 20-, 30-, 40- and 50-year-old trees, respectively, whereas it was 140.6, 152.2, 135.5, 144.2 and 127.0 cm for fine roots, respectively. These findings indicated that the spatial distribution of roots of Mongolian pine trees shrank for the 50-year old trees, possibly due to being close to mature stage in plantation sites. Based on spatial distribution of roots, we suggested that the stand densities should be determined according to the area occupied by tree root systems in order to avoid the dieback or mortality due to water competition, i.e., 403–515, 321–402, 217–290, 139–179 trees ha−1 for 10-, 20-, 30- and 40-year-old trees, respectively (none for 50-year-old plantations as they were close to mature stage). Our findings provide new insights into stand density optimization of plantations from tree belowground competition for water in semi-arid and arid sandy regions.