Nutrient Movement Through Western Hemlock-Western Redcedar Ecosystems in Southwestern British Columbia

Abstract

A calibrated watershed study was established in a Tsuga heterophylla-Thuja plicata-Pseudotsuga menziesii forest in the coastal western hemlock biogeoclimatic zone in southwestern British Columbia to study the effects of clearcut logging and broadcast slashburning on ecosystem biogeochemistry. Water was sampled regularly at the following states in its passage through a forest-watershed ecosystem: (1) precipitation above the forest; (2) canopy throughfall; (3) surface runoff (overland flow); (4) forest floor leachate; (5) mineral soil leachate near the bottom of the rooting zone; (6) groundwater; and (7) stream water. All samples were analyzed for K/sup +/, Na/sup +/, Mg/sub 2//sup +/, Ca/sub 2//sup +/, Fe/sub 2//sup +/, Mn/sub 2//sup +/, Al/sub 3//sup +/, NH/sub 4//sup +/, Cl/sup -/, Po/sub 4//sup -3/, NO/sub 3//sup -/, SO/sub 4//sup =/, dissolved SiO/sub 2/, pH, alkalinity (as bicarbonate), and electrical conductivity. There was a general increase in chemical concentrations as water entered and moved vertically down through the system to maximum values in forest floor leachate. This was followed by a decrease to minimum values in groundwater, with a slight increase again in stream water. The pH increased steadily from a low in throughfall to maximum values in stream water. These changes in solution chemistry are discussedmore » in relation to ecosystem nutrient cycling processes. Chemical concentrations, pH, and electrical conductivity were generally highest in late summer and early autumn and lowest in winter and early spring. This pattern generally occurred throughout the system and was attributed to seasonal cycles of geological and biological activity, and to rainfall patterns. Data from this study are consistent with the current theory explaining the chemistry of solutions passing through soil.« less