What are the contributions of landscape composition, weather, and seasonality to the overall variation in surface albedo and the albedo-induced warming or cooling climate benefit potentials across multiple spatial and temporal scales? What are the main key influences to the growing season net ecosystem exchange fluxes at major bioenergy crops under different land use history and land management? These are the main questions of this dissertation, addressed through three complementary studies focused on climate benefit potentials (i.e., cooling and mitigation effects) of biogeophysical and biogeochemical mechanisms in the context of global warming research, across managed agricultural landscapes in southwestern Michigan, United States. Chapter 1 provides an introduction of the overall research work by introducing the two mechanisms examined involved in global warming research, as well as the knowledge gaps and future research needs. Chapters 2 and 3 focus on the climate benefit potentials of the biogeophysical mechanisms, hence the cooling and mitigation effects induced by changes in surface albedo. In particular, Chapter 2 provides a proof of concept to quantify the ecosystem and landscape contribution to local and global climate through the analysis of spatiotemporal changes of surface albedo across five equal area landscapes, each within an individual ecoregion, in southwestern Michigan, USA, and during different weather conditions (i.e., three precipitation scenarios). Results showed that ecoregions, land mosaic, and seasonality contributed to the variation of surface albedo. Different was the response to changes in weather conditions in changes of surface albedo at forest- and cropland-dominated landscapes. The five ecoregions were characterized by cooling effects, with higher magnitudes in forest-dominated landscapes (i.e., higher difference between forest and cropland albedos). Chapter 3 extends the analysis in Chapter 2 to a broader landscape (i.e., watershed level) and over a period of 19 years, by looking at the contribution of major cover types, compared to original land uses, during both growing season and non-growing season (i.e., the effects of snow vs snow-free surface albedo). The 19-year analysis showed that land mosaic (with respect to the original forest cover type) exhibited net cooling effects, varying by cover types and ecoregions considered. Croplands contributed the most to cooling the local climate, with seasonal and monthly offsets of 18% and 83%, respectively, of the annual greenhouse gas emission of maize fields in the same area. On the other hand, urban showed both cooling and warming effects. Overall, landscape composition produced different landscape climate benefit potentials. Chapter 4 focuses on the climate benefit potentials of biogeochemical mechanisms, by looking at investigating the main key influences to the net ecosystem exchange (NEE) fluxes of three major bioenergy crops (viz., no-till continuous maize, restored native prairie, and switchgrass) under different land use history and management. The interannual variations of hypothesized main key influences to the net carbon (C) fluxes varied by cover types and growingseason considered, with maize showing a unique pattern. Net C uptakes were higher within maize, with magnitudes between -9.4 and -22.8 gC m-2 d-1. Results also showed that number, importance, and magnitude of main influences to C uptakes varied by cover type, highlighting the different nature of bioenergy crops (annual vs perennial; monoculture vs polyculture). Lastly, I show that the use of fine-resolution optical and radar remote sensing can improve forecasts of growing season C uptakes at maize cover type, depending on certain remote sensing variables and stage of maize’s growth. Recommendations for further research needs are discussed and include coupling my estimates with emissions from other greenhouse gasses (GHGs) and extending the analysis to non-growing season period for a more comprehensive understanding of C uptake dynamics under different land use history. These analyses of biogeophysical and biogeochemical mechanisms across contrasting landscapes and ecoregions fill important gaps in landscape ecology and ecosystem science as well as global warming research.
Read
