Plants play a key role in the hydrologic cycle and serve as a significant source of water vapor through the process of transpiration. The contributions of saplings and mature trees have been well documented and are sufficiently understood, including responses to changing meteorological conditions. However, the role and magnitude of contribution of seedlings to the transpiration stream is less well known. In situ seedling data are needed to validate controlled experiments performed in laboratory settings and to develop more robust understanding of understory contributions to forest systems. In this study, the impacts of microclimate on intra- and interspecific differences of stomatal conductance and transpiration in three co-occurring seedling species of vary shade tolerances were investigated. Water loss in Fagus grandifolia Ehrh. (American beech), Prunus serotina Ehrh. (black cherry), and Liriodendron tulipifera L. (yellow poplar) seedlings were monitored in two study plots, a dense forest understory and an adjacent field clearing, in southeastern Pennsylvania (39°49'N, 75°43'W) beginning 22 May through 6 October 2008. Seedling observations were coupled with five-minute meteorological data. This thesis summarizes the findings of the 2008 growing season.

Shade tolerant species (F. grandifolia and P. serotina) growing in the understory had significantly lower mean rates of stomatal conductance and transpiration than shade intolerant species (P. serotina and L. tulipifera) growing in the clearing (α=0.05). Additionally, water loss by shade grown P. serotina seedlings was significantly lower than sun grown P. serotina seedlings (α=0.05). Physiological differences, specifically shade tolerance, played an important role in determining rates of stomatal conductance and transpiration in seedlings. To a lesser degree, changes in microclimate also influenced water loss. Statistical analysis revealed larger variation of microclimate conditions in the clearing plot and consequently a larger range of response in stomatal conductance and transpiration by P. serotina and L. tulipifera clearing seedlings, in comparison to understory seedlings, which maintained more similar rates of water loss throughout the day as well as throughout the growing season.