3.1 AGB of grasses and plant community
AGB of grasses and community decreased significantly with land degeneration intensifying. Compared to ND grassland, AGB of grasses and community decreased by 21.3% and 9.6%, 66.2% and 32.6%, 75.9% and 54.8%, and 95.0% and 69.2% in the SLD, MD, HD and ED grasslands, respectively (Fig.1a, 1c). Meanwhile, The AGB of grasses decreased significantly with the increase of the replanting time in active restoration grasslands. Compared to 5Ys, AGB of grasses decreased by 8.3%, 32.9%, 65.5% and 81.3% in the 9Ys, 11Ys, 14Ys and 17Ys grasslands, respectively (Fig.1b). Compared to 5Ys, AGB of community kept stable in 9Ys grasslands and then decreased with the increase of replanting time (Fig. 1d).
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3.2. Photosyntheticcapacity and its relationship with AGB of grasses
In native grasslands, Pn decreased with land degradation intensifying (Fig. 2a). Compared to control (ND grassland), Pn significantly decreased by 4%, 18.7%, 35.8% and 39.6% in the SLD, MD, HD and ED grasslands, respectively. In addition,gs and WUEi initially kept stable and later decreased with land degradation intensifying (Fig. 2c, 2e). Intercellular CO2 concentration initially increased and later decreased with land degeneration (Fig. 2g). In active restoration grasslands, Pn (Fig. b),WUEi (Fig. 2f) andCi(Fig. 2h) significantly decreased with the increase of replanting time. Compared to 5Ys grassland, Pn,WUEi and Ci significantly decreased by 0.6%, 4.0%, and 4.7%, 15.1%, 23.0% and 9.1%, 24.3%, 23.0% and 12.2%, 31.4%, 35.3% and 13.6% in the 9Ys, 11Ys, 14Ys and 17Ys grasslands, respectively. Stomatal conductance increased first and then decreased with the increase of replanting time.
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Net photosynthetic rate (R2 =0.938, P <0.001) (Fig. 3a) and WUEi (R2 =0.302, P =0.007) (Fig. 3c) were positive correlated with AGB in native grasslands. Net photosynthetic rate (R2 =0.881,P <0.001),WUEi(R2 =0.839, P<0.001), gs(R2 =0.803, P <0.001) andCi (R2 =0.665, P <0.001) were highly positive correlated with AGB in active restoration grasslands (Fig. 3). Plant photosynthetic capacity in active restoration grasslands shows stronger association with its AGB than native grasslands.
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