Recently, the increasing shrub-encroached grassland in the Mongolian Plateau partly indicates grassland quality decline and degradation. Accurate shrub identification and regional difference analysis in shrub-encroached grassland are significant for ecological degradation research. Object-oriented filter (OOF) and digital surface model (DSM)-digital terrain model (DTM) analyses were combined to establish a high-accuracy automatic shrub identification algorithm (CODA), which made full use of remote sensing products by unmanned aircraft systems (UASs). The results show that: (1) The overall accuracy of CODA in the Grain for Green test area is 89.96%, which is higher than that of OOF (84.52%) and DSM-DTM (78.44%), mainly due to the effective elimination of interference factors (such as shrub-like highland, well-grown grassland in terrain-depression area, etc.) by CODA. (2) The accuracy (87.5%) of CODA in the typical steppe test area is lower than that (92.5%) in the desert steppe test area, which may be related to the higher community structure complexity of typical steppe. Besides, the shrub density is smaller, and the regional difference is more massive in the typical steppe test area. (3) The ground sampling distance for best CODA accuracy in the Grain for Green test area is about 15 cm, while it is below 3 cm in the typical and desert steppe test area.

Phosphorus (P) is a fundamental component of plant organisms. Most of the Earth System Models (ESMs) project increases in future Net Primary Productivity (NPP) due to climate warming. However, large uncertainty exists in projected NPP due to future P limitation. Subtropical China is a region with high vegetation NPP, but its forests are mostly P limited. In this study, we used the simulations of Atmospheric-Vegetation Interaction Model 2 (AVIM2) to diagnose the P limitation in forests in this region, and found that climate warming in the period of 1951–2010 had enhanced P limitation. The P demand during 1981–2010 for Evergreen Broad-leaved Forest (EBF) and Evergreen Needle-leaved Forest (ENF) are 1.67 and 1.8 times than that during 1951–1980, respectively. The observed current Available Soil P (ASP) density in 4 representative forest sites in subtropical China varied between 940 $\mathrm{mg}·{\mathrm{m}}^{-2}$ and 2365 $\mathrm{mg}·{\mathrm{m}}^{-2}$ , and the P demands account for 0.86% to 25.5% of the ASP for the period of 1951–2010. Future P demands are estimated to account for 3.2% to 68.3% of the current ASP at the end of this century for RCP8.5. Therefore, forests, especially plantations, in subtropical China are facing high risks of P limitation.