[揽瓜阁精读] 248. grass

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248 grass
P1: well-drilling and earth-moving equipment were the earliest sources of human intention. Water storage tanks dug in e lake basins, dams on lower reaches of e s to provide water.
P2: shrubs: change landscape hydrology（水文）and demise（使衰退）grass through effects on rainfall and competition for soil water
P3: shrubs invasion followed by a self-reinforcing process as a consequence. Rsources islands underneath the shrubs for further desertification, this makes bare patches（光秃秃的地方）foster further erosion. Shrublands also well adapted to natural perturbations（扰乱）so reinforce their existence. E.g. creosote bushes干旱了五年，一下雨又活了还produced new growth
P4: support bg grasses soil’s integrity（完整性）govern original desert grasslands. Soil damages open up unvegetated spaces so paces can be invaded by shrubs. Soil erosion has a positive feedback system that maintains desertification.