Abstract. All vegetation on Watershed 2 of the Hubbard Brook Experimental Forest was cut during November and December of 1965, and vegetation regrowth was inhibited for two years by periodic application of herbicides. Annual stream-flow was increased 33 em or 39% the first year and 27 em or 28% the second year above the values expected if the watershed were not deforested.Large increases in streamwater concentration were observed for all major ions, except NH 4 +, S0 4 = and HC0 3 -, approximately five months after the deforestation. Nitrate concentrations were 41-fold higher than the undisturbed condition the first year and 56-fold higher the second. The nitrate concentration in stream water has exceeded, almost continuously, the health levels recommended for drinking water. Sulfate was the only major ion in stream water that decreased in concentration after deforestation. An inverse relationship between sulfate and nitrate concentrations in stream water was observed in both undisturbed and deforested situations. Average streamwater concentrations increased by 417% for Ca++, 408% for Mg++, 1558% forK+ and 177% for Na+ during the two years subsequent to deforestation. Budgetary net losses from Watershed 2 in kg/ha-yr were about 142 for N0 3 -N, 90 for Ca++, 36 forK+, 32 for Si0 2 -Si, 24 for AI+++, 18 for Mg++, 17 for Na+, 4 for C!-, and 0 for SOrS during 1967-68; whereas for an adjacent, undisturbed watershed (W6) net losses were 9.2 for Ca + +, 1.6 for K +, 17 for Si0 2 -Si, 3.1 for AI+++, 2.6 for Mg+ +, 7.0 for Na +, 0.1 for Cl-, and 3.3 for SO~-S. Input of nitrate-nitrogen in precipitation normally exceeds the output in drainage water in the undisturbed ecosystems, and ammonium-nitrogen likewise accumulates in both the undisturbed and deforested ecosystems. Total gross export of dissolved solids, exclusive of organic matter, was about 75 metric tons/km2 in 1966'-67, and 97 metric tons/km2 in 1967-68, or about 6 to 8 times greater than would be expected for an undisturbed watershed.The greatly increased export of dissolved .nutrients from the deforested ecosystem was due to an alteration of the nitrogen cycle within the ecosystem.The drainage streams tributary to Hubbard Brook are normally acid, and as a result of deforestation the hydrogen ion content increased by 5-fold (from pH 5.1 to 4.3).Streamwater temperatures after deforestation were higher than the undisturbed condition during both summer and winter. Also in contrast to the relatively constant temperature in the undisturbed streams, stream water temperature after deforestation fluctuated 3-4 o C during the day in summer.Electrical conductivity increased about 6-fold in the stream water after deforestation and was much more variable.Increased streamwater turbidity as a result of the deforestation was negligible; however the particulate matter output was increased about 4-fold. Whereas the particulate matter is normally 50% inorganic materials, after deforestation preliminary estimates indicate that the proportion of inorganic materials increased to 76...

Stream water chemistry varies hyperbolically with stream discharge through four decades of discharge change within the Hubbard Brook Experimental Forest, New Hampshire. This dilution process is most simply explained by the mixing of rain water or surface water with deeper soil water. The resultant mixture of waters subsequently appears as stream water. Sodium and silica concentrations in stream water are markedly diluted during high discharge periods while hydrogen ion, aluminum, and nitrate concentrations are increased. Magnesium, calcium, sulfate, chloride and potassium concentrations are changed very little by stream discharge variations. During the summer, biologic activity measurably reduces the concentration of nitrate and potassium in stream water.

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The balance of calcium, magnesium, potassium, and sodium was studied in six watersheds of the Hubbard Brook Experimental Forest at West Thornton, New Hampshire. These watersheds are characterized by a near climax, northern hardwoods ecosystem, humid continental climate, and acid metamorphic rock. Bedrock in the area is considered to be watertight. Weekly samples of precipitation and stream water were collected from June 1963 through May 1965. Cation analyses were made by atomic absorption spectrophotometry. Precipitation was about 117 em during 1963-64 and 95 em during 1964-65 and did not differ significantly in amount with elevation. Loss of water by runoff from the system was about 68 em during 1963-64 and 49 em during 1964-65. Evapotranspirational losses were calculated at 49 em in 1963-64 and 46 em in 1964-65. About 37% of the total annual runoff occurred in April and less than 0.2% in September. Concentration of Ca++, Mg++, K+, and Na+ in weekly precipitation samples was variable. The annual weighted average values (mg/liter) were: 1963-64, Ca 0.26, Mg 0.06, K 0.21, Na 0.09; 1964-65, Ca 0.30, Mg 0.12, K 0.19, Na 0.22. Differences were not related to elevation. The contribution of cations from dry fallout was negligible. Cation content of stream water was relatively very low. The weighted average content for the entire area (mg/liter) was Ca 1.18, Mg 0.38, K 0.26, Na 0.87 during 1963-64; and Ca 0.80, Mg 0.38, K 0.22, Na 0.91 during 1964-65.In general, the cationic concentrations in drainage water were constant although discharge of water varied greatly. Statistically, the relationship between concentration and discharge was not significant for Ca + + and Mg+ + in any of the watersheds, but was inversely significant for N a+, and was possibly direct for K + in a few instances. The budget for dissolved cations was calculated from determinations of input (cation concentration times volume of precipitation) and output (cation concentration times volume of runoff.) During 1963-64 the average inputs in kg/ha were 3.0 Ca++, 0.7 Mg++, 2.5 K+, and 1.0 Na+, while the output averaged 8.0 Ca++, 2.6 Mg++, 1.8 K+, and 5.9 Na+. During 1964-65 the values were: input, 2.8 Ca++, 1.1 Mg++, 1.8 K+, and 2.1 Na+; and output, 3.9 Ca++, 1.8 Mg++, 1.1 K+, and 4.5 Na+. Assuming that the vegetation is near dynamic equilibrium, the net losses of Ca + +, Mg + +, and N a+ represent conservative measurements of geochemical weathering in the system. The K + budget is probably near balance.1 Financial support was provided by the N a tiona! Science Foundation (Grants No. GB 1144 and GB 4169). We acknowledge and appreciate assistance from G. Furnival with statistical analyses. We are grateful to J. Brown and P. Hoekstra for critical comments and suggestions during the preparation of this manuscript. This is contribution No.Abstract. The hypothesis that variation in dates of the beginning and end of growing seasons of eastern cottonwood (Populus deltoides Bartr.) and American sycamore (Platanus occidentalis L.) can be attributed to variation in...

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