Abstract. Hydraulic conductivity of the xylem is computed as the quotient of mass fiow rate and pressure gradient. Measurements on excised plant stems can be difficult to interpret because of timedependent reductions in fiow rate, and because of variable degrees of embolism. Using Acer saccharum Marsh, stems, we found that certain perfusing solutions including dilute fixatives (e.g. 0.05% formaldehyde) and acids with pH below 3 (e.g. 10 mol m~^ oxalic) prevent long-term decline in conductivity. Xylem embolism can be quantified by expressing the initial conductivity as a percentage of the maximum obtained after fiow-impeding air emboli have been removed by repeated high-pressure (175 kPa) fiushes. Correlation between microbial contatnination and declining conductivity suggests that long-tenn (>4h) declines are caused by microbial growth within the vessels. Unpredictable trends in short-term (<4h) measurements may be caused by movements of air emboli in vessels and/or particulate matter.

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We evaluated factors influencing the development of autumn red coloration in leaves of sugar maple (Acer saccharum Marsh.) by measuring mineral nutrient and carbohydrate concentrations, water content, and phenology of color development of leaves from 16 mature open-grown trees on 12 dates from June through October 1999. Mean foliar nutrient and carbohydrate concentrations and water content were generally within the range published for healthy sugar maple trees. However, foliar nitrogen (N) concentrations were near deficiency values for some trees. The timing and extent of red leaf coloration was consistently correlated with both foliar N concentrations and starch or sugar concentrations, which also varied with N status. Leaves of trees with low foliar N concentrations turned red earlier and more completely than those of trees with high foliar N concentrations. Low-N trees also had higher foliar starch concentrations than high-N trees. During the autumn development of red leaf coloration, foliar starch, glucose and fructose concentrations were positively correlated with red leaf color expression. At peak red expression, the concentrations of glucose, fructose, sucrose and stachyose were all positively correlated with red color expressed as a percent of total leaf area.

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