Knowledge of whole tree seedling water fluxes is important in ecological and forestry research, especially under conditions with low transpiration, but no standard method has yet been established that provides reliable in situ measurements. The aims were: (1) to assess the performance of two methods for estimating sap-flows in oak seedlings following planting by correlating the data they provided with natural light intensities over a three-week period, and (2) to compare the estimates with transpiration data obtained by weighing pots. Estimates of sap flows obtained from data provided by constant power (Dayau-type) heat balance gauges under low light conditions (100-450 mu mol m(-2) s(-1)) were less variable than estimates from variable power (EMS-type) heat balance gauges. The EMS-type system yielded data with little between-gauge variation, but consistently underestimated transpiration on a daily basis, a systematic error that should be corrected by other methods. The Dayau-type gauges yielded data with substantial variations, and several gauges arc probably needed in research to cover these variations. Further, both systems provide rather uncertain estimates of short-time (hour) transpiration ratcs. However, provided that these considerations arc taken into account, we conclude that it should be possible to use either system in various research contexts.
