Ecosystem evaporation, transpiration and sub-canopy vapor fluxes at the Morgan Monroe State Forest, Indiana

In this paper, daily, and annual fluxes of evapotranspiration (ET) and transpiration (T) at an AmeriFlux site in the Morgan-Monroe State Forest (MMSF), Indiana, USA for two growing seasons (1998, 1999) are presented. The site is surrounded by an extensive secondary successional broadleaf forest, in the maple-beech to oak-hickory transition zone. Turbulent flux measurements are obtained by an eddy covariance system at 46 m (1.8 times the canopy height) and at 2 m. Sapflow measurements of T, using the Granier technique, are conducted in 9 representative canopy trees in the area around the tower.

For 1998, when ET46m rates are compared with those for T, obvious differences in the magnitude of the fluxes are evident. However, both fluxes exhibit similar patterns in terms of day-to-day variability and seem to be responding to the same local/meso-scale environmental forcing. Such differences between ET and T have been documented previously. The magnitude by which ET exceeds T is dependent on the contribution of interception losses, soil evaporation and transpiration to total E.Despite considerable (but unknown) uncertainties associated with precipitation measurements on top of a 46 m tower and below canopy, such measurements serve to elucidate the differences in ET46 and sapflow T, at least in part. During the growing season, approximately 60% (1998) and 50% (1999) of P46m was recorded at 2 m. Thus during the growing season, evaporation of intercepted water clearly is a significant component of the above canopy measurements of ET and a partial explanation for the ET46m and T differences. Further explanation is attributed to differences in the species composition of the sapflow sites and the overall composition of the forest, specifically in the turbulent footprint of the tower.