| Terrestrial vegetation plays an important role in the exchanges of carbon, water, and energy at the land surface. Changes in vegetation productivity at regional or larger scales have important implications for regulating the atmospheric CO2 concentration and climate and affecting food production. The characterization of large-scale increases in vegetation productivity will lead to a better understanding of the distribution and dynamics of carbon sources/sinks and climate change. In this dissertation, I examined the increases in vegetation productivity at multiple scales. First, I examined the increases in vegetation productivity and their associations with climate variability at the global scale over the period 1982 to 1998 using satellite data and ground-based climatology data. Temperature, and, in particular spring warming, was the primary climatic factor associated with greening in the northern high latitudes, Western Europe, U.S. Pacific Northwest, tropical and subtropical Africa, and eastern China. Precipitation was a strong correlate of greening in fragmented regions only. Globally, greening trends are a function of both climatic and non-climatic factors, such as forest regrowth, CO2 enrichment, woody plant proliferation, and trends in agricultural practices. Second, I analyzed the responses of vegetation productivity to climate both within and across biomes for the conterminous US using satellite data and climate data. Forested and non-forested biomes differed sharply in their response to spatial gradients in temperature and precipitation. Precipitation affected the productivity of all biomes, and the influence of precipitation on productivity differed across biomes. The effect of seasonal Tmax and Tmin was only apparent for forested biomes, which are less likely to be water limited. The seasonality of temperature relations, and the relatively greater importance of Tmin for some biomes, suggest that the dominant effect of temperature is through its influence on growing-season length. Third, I looked at the rates of western juniper expansion and the resulting C uptake in central Oregon over the period 1975--2000 using satellite data, forest inventory data, and field measurements. Western juniper expanded at the rate of 0.15% per year over the 25-year period. The results suggest that the expansion of western juniper caused C accumulation of 0.27 Mg C ha-1 yr-1 in aboveground woody biomass. Total C stock in aboveground woody biomass increased from 0.13 Tg in 1975 to 0.21 Tg in 2000. Western juniper expansion may have a significant impact on the regional C budget. |
