An Overview of the Implications of Global Change for Natural and Managed Terrestrial Ecosystems

Abstract

Global change is the net effect of individual and interactive effects of changes in land use, atmospheric composition, biological diversity, and climate. A synthesis of the past six years' activities of the Global Change and Terrestrial Ecosystems project of the IGBP (International Geosphere-Biosphere Programme) deals with global change effects as ecosystem responses and living with global change. Ecosystem responses are considered in terms of changes in function and vegetation composition/structure. Field experiments of elevated CO2 effects on aboveground biomass show, on average, a positive effect on biomass, ranging from -20% to +80%. Some early predictions of CO2 effects (C3 vs. C4 plants, N-fixers, C:N in litter) are not generally supported, and it is necessary to consider the interactive effects of changes in CO2, temperature, and nitrogen. Dynamic global vegetation models involving transient changes show that biomes will not shift as intact entities. Significant changes in vegetation, especially in high latitudes, are likely over the next century, and changes in disturbance regimes will be most important. Based on forecast changes in land use, vegetation structure, and ecosystem physiology, the terrestrial biosphere will probably become a source rather than a sink for carbon over the next century. Because of land use change, the terrestrial biosphere of the 21st century will probably be further impoverished in species richness and substantially reorganized. More natural ecosystems will be in an early successional state or converted to production systems. The biosphere will be generally weedier and structurally simpler, with fewer areas in an ecologically complex old-growth state. Temperate crop production will probably increase slightly because of CO2 increases (5-7% for wheat for average field conditions), but crop production in the tropics may decline in some areas. Land use change will have the greatest effect on pastures and rangelands; due to a required >2% annual increase in crop production to meet the expanding human population, there will be increased incursion of cropland into rangelands.