Spatial patterns of throughfall and mineral ion deposition in a lowland Norway spruce (Picea abies) plantation at the plot scale

Spatial variability of throughfall depth and mineral ion composition was investigated in a Norway spruce (Picea abies (Karst) L.) plantation at the small plot scale. A regular array of 39 funnel collectors was set up on the forest floor and throughfall volume and concentrations of nitrate, ammonium, chloride, potassium and sulphate were measured on six sampling occasions over a period of 81 days in winter. Considerable spatial variability was found in the patterns of throughfall depth and ion concentrations but for each sampling period there were consistent features in the spatial patterns of these inputs. Throughfall depth increased significantly as a function of increasing distance from the nearest tree bole while ion load was negatively correlated with distance from the tree. The magnitude of the mean ion load for each period depended on mean throughfall depth. Throughfall depth and deposition of ions were also correlated with spatially referenced values of canopy interception capacity estimated using a deterministic model of the canopy hydrology above each collector. Although linear correlations between the canopy cover above each collector (estimated from image analysis of photographs) and the spatial patterns of throughfall volume, concentration and load were significant for only 17% of the time, least-squares fitting of quadratic equations (input vs cover with a maximum input at some intermediate value of cover) were significant for more than half of the cases examined. This study highlights the need for a better characterisation of canopy architecture in order to improve understanding of its role in affecting water and mineral ion deposition on the forest floor.