Red spruce soil solution chemistry and root distribution across a cloud water deposition gradient

Abstract

The decline of red spruce (Picearubens Sarg.) at high elevations in eastern North America has been linked in time and space with exposure to acidic cloud water. To investigate the belowground effects of a cloud water deposition gradient between two mature red spruce stands on the summit of Whitetop Mountain, Virginia, the chemistries of precipitation, throughfall, and soil solution were monitored over a 2-year period, and fine-root distributions were characterized. Deposition of water, sulfate, nitrate, and ammonium in throughfall and stemflow was from 15 to 55% greater at the site with greater exposure to cloud water deposition (high cloud site), depending upon the particular ion and year. Soil solution nitrate concentrations were highly variable over time, and base cation, Al, and H ion concentrations were highly correlated with nitrate in both organic and mineral horizons at both sites. Soil solution nitrate, base cation, Al, and H ion concentrations were two to six times greater during periods of low soil moisture in the summer–autumn of 1987 and 1988 than during the remainder of the study period. In the mineral soil solutions, the high cloud site had significantly higher (p < 0.001) concentrations of nitrate and Al, and significantly lower (p < 0.05) Ca:Al and Mg:Al ratios. The high cloud stand also had shallower root systems, with fine-root biomass less than 40% of that of the low cloud stand (p < 0.05) at all depths greater than 18 cm. Soil solutions collected from below 15 cm at the high cloud site had a mean Ca:Al ratio less than 0.5 and Al concentrations that during dry periods, frequently approached or exceeded the literature values for the toxicity threshold for red spruce root growth. Restricted root development in the high cloud stand was apparently the result of this unfavorable chemical environment.