Vertical analysis of dual-layer (canopy and forest floor) rainfall interception dynamics in a mature lodgepole pine forest

Rainfall interception is a key component regulating the water balance of lodgepole pine forests across broad regions of western North America. However, information on key rainfall interception processes across the full vertical profile of both canopy and forest floor layers have not been previously described in this important forest biome. This information is necessary to predict the impact of a broad range of disturbance pressures currently facing lodgpole pine forests in North America (e.g., Mountain Pine Beetle (MPB), wildfire, harvesting, etc.). The objectives of this study were to characterize relationships between precipitation (P) and the major components of canopy interception (stemflow, throughfall, canopy interception storage) and forest floor interception (storage, recharge into the mineral soil) in mature lodgepole pine stands at risk from disturbances such as MPB attack. Components of canopy and forest floor interception dynamics were measured in 4 large stands of 100-120 yr. old lodgepole pine during the 2008-2010 rainfall seasons in west-central Alberta, Canada. Rainfall season precipitation ranged from 318-397 mm (~65% of total annual P), which encompassed 149 discrete rainfall "events" over the 3 seasons. While the vast majority of the rainfall events (93%) were less than 12 mm, infrequent storms (> 12 mm) accounted for half of the total growing season rainfall. Throughfall accounted for 53% of season rainfall, while stemflow accounted for ~ 0.5%. Total canopy interception losses ranged from 44-48% of growing season rainfall. While interception losses by the forest floor have received little previous study, forest floor interception losses comprised an additional 22% loss of seasonal rainfall. The combined rainfall interception losses by the forest canopy and forest floor layers averaged 240mm (70 % of season rainfall) which resulted in only 34% of season rainfall being recharged into the mineral soil beneath these two forest layers. A process based model was modified to describe the integrated forest canopy and forest floor layers as a dual-layer model linking these components in series. The model adequately predicted field measured interception dynamics of both of these components. Previous research on rainfall interception dynamics in other forest types has usually neglected the role of the forest floor layer and focused primarily on the canopy layer, however, this research suggests that the interception losses by the forest floor are very important and can account for 1/3 of total interception losses in mature lodgepole pine forests.