Abstract
► Basal area removal of greater than 40% may lead to negative effects on the forest ecosystem.
► Above 40% basal area removal stream water NO
3
− concentrations increased substantially.
► Increased basal area removal leads to longer periods of elevated chemical concentrations in streams.
Previous studies have shown that clearcutting of northern hardwood forests mobilizes base cations, inorganic monomeric aluminum (Al
im), and nitrate (NO
3
−-N) from soils to surface waters, but the effects of partial harvests on NO
3
−-N have been less frequently studied. In this study we describe the effects of a series of partial harvests of varying proportions of basal area removal (22%, 28% and 68%) on Al
im, calcium (Ca
2+), and NO
3
−-N concentrations in soil extracts, soil water, and surface water in the Catskill Mountains of New York, USA. Increases in NO
3
−-N concentrations relative to pre-harvest values were observed within a few months after harvest in soils, soil water, and stream water for all three harvests. Increases in Al
im and Ca
2+ concentrations were also evident in soil water and stream water over the same time period for all three harvests. The increases in Al
im, Ca
2+, and NO
3
−-N concentrations in the 68% harvest were statistically significant as measured by comparing the 18-month pre-harvest period with the 18-month post-harvest period, with fewer significant responses in the two harvests of lowest intensity. All three solutes returned to pre-harvest concentrations in soil water and stream water in the two lowest intensity harvests in 2–3 years compared to a full 3 years in the 68% harvest. When the results of this study were combined with those of a previous nearby clearcut and 40% harvest, the post-harvest increases in NO
3
−-N concentrations in stream water and soil water suggest a harvesting level above which the relation between concentration and harvest intensity changes; there was a greater change in concentration per unit change in harvest intensity when basal area removal was greater than 40%. These results indicate that the deleterious effects on aquatic ecosystems previously demonstrated for intensive harvests in northern hardwood forests of northeastern North America that receive high levels of atmospheric N deposition can be greatly diminished as harvesting intensity decreases below 40–68%. These results await confirmation through additional incremental forest harvest studies at other locations throughout the world that receive high levels of atmospheric N deposition.