Searching for Similarity in Catchment Controls on Complex C, N, and P Export Patterns from Forests to Surface Waters across Continental Scale Gradients (Invited)

Inter-catchment comparison of solute export from terrestrial to aquatic ecosystems is challenging due in part to natural sources of variation caused by topographic controls on processes that affect solute movement. Recent advances in digital elevation models (DEMs) enable more precise characterization of the topographic features that define source areas of carbon (C), nitrogen (N), and phosphorus (P) as well as the movement of these elements to the sink areas where they are stored, converted to gaseous forms, and/or exported to aquatic systems. This is particularly true in catchments with dense forest canopies that conceal these topographic features from aerial views. A conceptual model developed from a forested catchment in central Ontario, Canada presented here details the topographically controlled processes regulating source areas of solutes, their movement, and their fate. Statistical models relating topographic attributes representing these processes using digital terrain analysis on a 5 x 5 m LiDAR DEM explained the majority of the natural sources of variation in solute exports for forested catchments within the geographic region. Topographic attributes representing hydrologic flushing areas and the potential rate of expansion of these flushing areas were important in controlling nitrate-N export (explaining 85% of the natural variation in export). In contrast, hydrologically saturated areas that were contiguous and connected to the stream were important in controlling dissolved organic nutrient export (65% dissolved organic N, 90% dissolved organic C, and 87% total dissolved phosphorus). Based on the success from this geographic region, the topographic attributes were applied to forested catchments along an E-W gradient across eastern Canada (from dry to wet climatic conditions, with shallow soils) and along a N-S gradient along eastern North America (from snow to snow-free seasons, with shallow to deep soils), and across forest regions ranging from deciduous to mixed to coniferous stands. The findings reveal the universality of topographic controls on processes affecting solute export and suggest that digital terrain analysis should be used as a precursor to examining catchment responses to global change.