Factors regulating throughfall flux in a New Hampshire forested landscape

This study examined the regulation of throughfall flux by six factors: precipitation amount, dry deposition, precipitation acidity, precipitation ion concentrations, forest type, and leaf area index. The influences of these factors were determined by analyzing the spatial and temporal variance in net throughfall flux (throughfall minus precipitation) in the forested landscape of the Hubbard Brook Experimental Forest (White Mountains, New Hampshire, U.S.A.). Throughfall was collected during the growing seasons of 1989-1992 in eight sites encompassing differences in elevation, forest type (including mature northern hardwood, young northern hardwood, and spruce-fir-birch types), and canopy surface area. Regression analysis of single-event data showed that within a forest type, the most important factor regulating the event-to-event variation in net throughfall flux was the amount of precipitation, suggesting that canopy exchange (leaching and uptake) regulated the net throughfall flux for most solutes. The leaching of Ca²⁺ and Mg²⁺ were significantly higher in more acid rain events. Hydrogen ion, NH₄⁺, and NO₃^- were all retained by at least some of the canopies. However, the data indicate that the retention mechanism for NO₃^- was saturated under ambient concentrations in precipitation, whereas the retention of NH₄⁺ and H⁺ was not saturated and increased with increasing concentrations of those ions in precipitation. Duration of the interevent dry periods was not a significant factor in most of the regressions, contrasting with results from studies at other sites. Regressions using precipitation amount, length of the dry period, and precipitation acidity as independent variables explained 50-90% of the variance for most sites and solutes. For SO₄^2-, which had a small net throughfall flux relative to the precipitation flux, the regressions were not significant or explained <50% of the between-event variation in net throughfall flux. For most solutes, forest type was the major factor controlling spatial variation in net throughfall flux among stands. Particularly strong differences between forest types were noted for NO₃^-, SO₄^2-, NH₄⁺, and Cl^-. The contrast in net throughfall flux between two young hardwood stands of different leaf area index indicated that leaf area was a major controlling factor for K⁺, H⁺, and total organic carbon.