Survey characteristics of the sites.

Tree line areas exhibited significant changes in response to climate change, including upward migration. Lower tree line dynamics are rarely studied, but as unique features in arid and semi-arid areas, they may influence forest distribution. Here, eight lower tree line plots in a Picea crassifolia Kom. (Qinghai spruce) forest in the arid and semi-arid Qilian Mountains of northwestern China were used to determine changes in tree line location and relationships with meteorological factors during 1968–2018. The results showed that the lower tree line descended by an average of 9.82 m during 1968 to 2018, and exhibited almost no change after 2008. The change in the lower tree line was significantly correlated with the annual average temperature (°C) and annual precipitation (mm) and may be affected by human activities. In the past 50 years, the lower tree line in arid areas exhibited a downward trend. Our findings indicate that the movement of the lower tree line is also an important aspect of climatic changes in coniferous forest distribution in arid and semi-arid mountains.</div

Characteristics of the sites, in aspect, 1 for shaded slopes (N (0–22.5°, 337.5–360°)), 2 for semi-shaded (NE (22.5–67.5°), E (67.5–112.5°), NW (292.5–337.5°)), and 3 for partly-sunny slopes (SW (112.5–157.5°), SE (202.5–247.5°), and W (247.5–292.5°)).

Characteristics of the sites, in aspect, 1 for shaded slopes (N (0–22.5°, 337.5–360°)), 2 for semi-shaded (NE (22.5–67.5°), E (67.5–112.5°), NW (292.5–337.5°)), and 3 for partly-sunny slopes (SW (112.5–157.5°), SE (202.5–247.5°), and W (247.5–292.5°)).</p

Functional relationship between DBH and age of trees in Guantai sampling.

Functional relationship between DBH and age of trees in Guantai sampling.</p

Changes in position of the lower tree line (1) (distance Y(m) is the distance along the slope direction and distance X(m) is the distance spline perpendicular to the slope direction).

Changes in position of the lower tree line (1) (distance Y(m) is the distance along the slope direction and distance X(m) is the distance spline perpendicular to the slope direction).</p

The average proportion of Picea crassifolia Kom. trees at different ages in 8 plots.

The average proportion of Picea crassifolia Kom. trees at different ages in 8 plots.</p

Decadal and 50-year mean distance and standard deviation of descent elevation of the lower tree line.

Decadal and 50-year mean distance and standard deviation of descent elevation of the lower tree line.</p

Study area in the Qilian Mountains of northwestern China.

Study area in the Qilian Mountains of northwestern China.</p

The relationship between mean distance of descent elevation of the lower tree line (m) and different meteorological factors from 1968–2018.

(M represents mean distance of descent elevation of the lower tree line (m), C1 to C10 represent annual average temperature (°C), annual precipitation (mm), spring average temperature (°C), summer average temperature (°C), autumn average temperature (°C), winter average temperature (°C), spring precipitation (mm), summer precipitation (mm), autumn precipitation (mm), winter precipitation (mm); * represents a significant correlation at the 0.05 level (bilateral)).</p

Investigation of Cyclopropane Stereomutation by Quasiclassical Trajectories on an Analytical Potential Energy Surface

Investigation of Cyclopropane Stereomutation by Quasiclassical Trajectories on an Analytical Potential Energy Surfac

Additional file 4 of Enhancing medical education in respiratory diseases: efficacy of a 3D printing, problem-based, and case-based learning approach

Supplementary Material 4. Questionnaire surve