Tree Variability Limits the Detection of Nutrient Treatment Effects on Sap Flow in a Northern Hardwood Forest

Transpiration plays a large role in determining forest water budgets. However, the role of nutrient availability in transpiration rates is not well understood. The objective of this study was to determine whether nutrient fertilization influences sap flow rates in northern hardwood forests. We were unable to detect an effect on sap flow from additions of nitrogen, phosphorus, a combination of nitrogen and phosphorus, or calcium silicate in American beech (Fagus grandifolia, Ehr.), red maple (Acer rubrum L.), sugar maple (Acer saccharum Marsh.), white birch (Betula papyrifera Marsh.), or yellow birch (Betula alleghaniensis Britton.). Tree-to-tree variability was high, with coefficients of variation averaging 39% within treatment plots. A better study design would include pretreatment data as a covariate. Our data could be used in a power analysis to determine the minimum difference in sap flow rates detectable with our observed variability and study design