Slope-related variations in twig and leaf traits of Robinia pseudoacacia in the northern mountains of Lanzhou

Aims The growth relationship between twigs and leaves is a strategy that plants enhance the ability to use space resources under environmental stresses, and elucidation of this characteristics is important for understanding the phenotypic plasticity of plants in coping with environmental heterogeneity. Our objective was to examine how Robinia pseudoacacia would vary in twig and leaf configuration in response to changes in slope aspect. Methods In the northern mountains of Lanzhou in Gansu Province, China, 20 transects were laid out horizontally along the contour at intervals of 50 m from an elevation of 1 550 m upward in four different slope aspects, and 12 plots were set up along each transect at intervals of 5 m. A handheld GPS was used to measure latitude, longitude and altitude of each plot. Community traits were investigated and all individuals of R. pseudoacacia were used for measurements of the cross-sectional area of twigs, total leaf area, leafing intensity, and average area of a single leaf on each twig. ArcGIS was used to construct the digital elevation model (DEM). The 240 plots were categorized into groups of northern, eastern, western and southern aspects, and the standardized major axis (SMA) estimation method was then used to examine the allometric relationship between the cross-sectional area of twigs, total leaf area, leafing intensity and average area of a single leaf. Important findings With changes in the slope aspect from north to east, south and west, the crown density, average tree height and soil moisture of the plant community displayed a pattern of decrease—increase, and the cross-sectional area of twigs, total leaf area and average area of a single leaf of R. pseudoacacia displayed a pattern of decrease—increase and the leafing intensity displayed a pattern of increase—decrease. Significant positive relationships between the cross-sectional area of twigs and total leaf area were found in R. pseudoacacia in all slope aspects (p < 0.05), and the common slope of the regressions was significantly greater than 1; significant negative relationships between leafing intensity and average area of a single leaf were found in all slope aspects (p < 0.05), and the common slope of the regressions was significantly close to –1. In addition, when the slope aspect changed from north to east, south and west, the y-intercepts in the scaling relationships of the cross-sectional area of twigs vs. total leaf area and the leafing intensity vs. individual leaf area both displayed a pattern of decrease—increase. The allometric relationship between twig and leaf with changes in slope aspect of the habitat reflected the response and adaption of plant functional traits to their biotic and abiotic environments and the investment balance mechanism of plant architecture construction.