Determination of gas exchange
An LI-6800 portable infrared gas analyzer (LI-COR, Inc., USA) was used to measure gas exchange parameters in the stems and leaves. Measurements of the net photosynthesis rate (P n), stomatal conductance and the dark respiration rate (R d) were conducted between 09:00 and 12:00 h on clear days at 800 and 0 μmol photon m−2 s−1, respectively. Photosynthetically active radiation (PAR) was emitted from a red:blue (9:1) LED light source integrated into the LI-6800 measurement chamber (1×3 cm). The CO2 concentrations flowing into the leaf chamber were controlled at 400 μmol mol–1, the temperature of the plant organ in the leaf chamber was maintained at 27°C, and the relative humidity was 65%. Stems of the third internode were selected to analyze the gas exchange. Before the gas exchange measurements were performed, the diameter of the stems was measured using a 150-mm Vernier caliper (Tricle brand, Shanghai, China) and used to compute the projected area. The total stem surface area was calculated by multiplying the projected area by π (3.14). Because the photosynthesis abilities of stems are low, the measurement precision of the device on the stems was much lower than that for the leaves (Figure S1). To collect more reliable data from the stems, we started automatic logging at 3-5 second intervals for 2 min after photosynthesis had reached a relatively steady state. Every measurement of the stems was based on an average of the 2-min recordings. Since PAR was applied to only one side of the stems in the measurement chamber, the photosynthesis rate of stems was expressed on a half-surface area basis. However, because stomata are randomly distributed on the stem surface, the stomatal conductance of stems was expressed on a total surface area basis. For the leaves, the photosynthesis rate was expressed on a regular leaf area basis. Because the leaves of the invasive species have stomata on both the abaxial and adaxial sides, to facilitate comparisons with stems, the stomatal conductance of the leaves was expressed on a double-leaf area basis. The gross photosynthesis rate (P g) was subsequently calculated asP n + R d.