Underestimation of dry deposition by throughfall in mixed northern hardwood forests

Abstract

Throughfall and catchment mass balance approaches were used to estimate dry deposition inputs of SO 4 and Cl to the Bear Brook in Maine for the period November 1988–November 1992. For throughfall, dry deposition of SO 4 and Cl averaged 320 and 261 eq ha −1 year −1, respectively, or 0.9 and 1.5 times the input in precipitation, respectively. The equivalent values for the catchment gave dry deposition of SO 4 and Cl as 616 and 380 eq ha −1 year −1, respectively, or 1.7 and 2.2 times the input in precipitation, respectively. The lower estimates obtained by the canopy mass balance method are primarily attributed to an underestimation of the importance of conifers in intercepting dry and occult deposition, particularly in mixed hardwood-softwood canopies such as the Bear Brook. Although net throughfall is commonly used to estimate dry and occult deposition in forested catchments, dry deposition may be underestimated, particularly in uneven-aged, mixed hardwood-softwood forests characterized by occasional superdominant conifers. The uneven canopy architecture of such forests increases its aerodynamic roughness and thus its efficacy in capturing dry deposition. Mixed-species forests, resulting from both natural and anthropogenic disturbance, are a common component of the landscape throughout much of New England.