Evaluating the impacts of nearly 30 years of conservation on grassland ecosystem using Landsat TM images

摘要

When grassland degradation became a global issue because of overgrazing and other human activities, grassland managers implemented different management methods in an attempt to restore grassland ecosystems (e.g. conservation actions). Few studies have investigated the impacts of conservation actions (removing large grazers and conserving biodiversity) on grassland ecosystems, therefore, this study aims to evaluate the impacts of conservation actions and to measure the time lag of the significant influences of grassland conservations on mixed grasslands that were assessed by five different biophysical parameters (biodiversity, soil organic matter, fresh biomass, litter cover and green cover). Instead of measuring the biophysical parameters from field data and remotely sensed images, the methodology of this study focused on the difference in biophysical parameters between the ecological comparison sites (grazing sites and conserved sites), which enhance the impacts of grassland conservation on grasslands comparing to grazing management. The results show that: (i) biodiversity in conserved grasslands increased gradually in the first 3-5 years, decreased gradually over the next 4 or 5 years, and then stabilized after that; (ii) soil organic matter increased and reached its maximum value within 7-9 years of conservation and then remained at this level, while litter accumulated the maximum level one year later than soil organic matter; (iii) soil organic matter is the primary factor of biodiversity in the grasslands with low litter accumulation, while high-density litter layers are the primary cause of decreases in biodiversity; (iv) fresh biomass decreased over a period of 7-10 years under conservation and remained nearly unchanged after that; (v) green vegetation fraction was also increased by conservation action in about 6-7 years. The result of this study provides the fundamental information for implementing and adjusting grassland management policies.