Abstract
In topographically complex terrain, the spatial heterogeneity of carbon cycling is affected by vegetation heterogeneity and landscape position through the lateral and vertical redistribution of soil water. Vegetation affects carbon cycling directly through photosynthesis, tissue allocation, respiration, and litter production, but also by mediating abiotic environmental factors such as soil temperature, soil moisture, and available light. In areas of topographic complexity, such as mountain catchments, the landscape can laterally redirect soil water, resulting in areas of convergent and divergent soil moisture that create spatial heterogeneities in carbon fluxes. Our knowledge of how vegetation interacts with landscape position to affect carbon cycling within areas of cool, humid watersheds is lacking. Further, we must also understand how these systems are changing and the possible impacts of climate and climatic variability. Here I address this knowledge gap in three ways: 1) by examining landscape controls on surface soil carbon fluxes and response to inter-annual climate variability; 2) examining the landscape controls on litter decomposition, nitrogen availability, and the organic layer; 3) characterizing the spatial and temporal dynamics of the evergreen shrub community. The major findings of my dissertation are that 1) surface soil CO2 fluxes are greater beneath evergreen shrubs across all elevations for the three years measured—a result driven not only by differences in soil moisture and soil temperature beneath shrubs, but also by differences in soil chemical and physical properties; 2) fluxes are constrained during periods of greater soil moisture, with soil moisture responding strongly to varying annual precipitation, resulting in variability of soil CO2 fluxes; 3) litter decomposition is predominately controlled by elevation with seasonal differences possibility related to snow cover; 4) nitrogen availability is controlled by vegetation, with areas of greater availability beneath closed forest canopies—driven by differences in nitrate abundance; 5) evergreen shrub cover in the study watershed is increasing, with the greatest increases at low and mid elevations and along southerly aspects—areas that were once water-limited on the landscape.
