Energy balance and transpiration in an urban tree hedgerow in Mexico City

A series of sap flow (transpiration, TRP) and energy balance components measurements were carried out in a representative tree hedgerow of Fraxinus uhdei in Mexico City (19° 19' N, 99° 11' W, 2250 m ASL). Measurements of sap flow in the trunks of the trees was selected as an alternative tool instead of eddy covariance or Bowen ratio-energy balance methods in order to determine the energy balance components of the tree canopy. These measurements were made in October (end of the rainy season) and December (dry season) in 1997, from 8:00 to 17:00 LST. TRP showed a unimodal pattern during the day. Mean daily transpiration was higher at the end of the rainy season (1506 g m^–2 d^–1) than in the begining of the dry season (972 g m^–2 d^–1) with maxima rates of 0.075 and 0.046 g m^–2 s^–1, respectively. During the rainy season net radiation (Q_N) was mainly dissipated by latent (Q_E) and sensible (Q_H) heat, 60 and 34 %, respectively. Latent heat increased in the day up to 184 W m^–2. Q_N was dissipated by Q_H and Q_E by 75 and 25%, respectively, and Q_H had a maximum hourly value of 298 W m^–2 during the day (8:00-17:00 LST) in the dry season. Seasonal differences in Q_E and Q_H were probably due to differences of water availability. Heat storage of the tree hedgerow canopy was found to be a negligible component of the energy balance in both seasons. Actual transpiration of the system was always lower than transpiration at equilibrium. This was probably due to a low water availability in the substratum, a high atmospheric evaporative demand and an effective stomatal control. Because the tree hedgerow was completely surrounded by paved areas and wind was coming from a drier side, it is possible that transpiration rates were affected by advection probably supressing TRP.