Stomatal conductance and transpiration of co-occurring seedlings with varying shade tolerance

Forest ecosystems exert a large influence on the hydrologic cycle through transpiration, with the majority supplied by saplings and adult trees. However, a measurable amount is also supplied by seedlings. The contribution of seedlings is dependent upon species, which is ultimately controlled by microclimate. The objectives of this study were to (1) observe meteorological conditions of two forest microclimates; and (2) assess the intra- and interspecific stomatal conductance and transpiration responses of naturally occurring seedlings of varying shade tolerance. Naturally established seedlings in a deciduous forest understory and an adjacent clearing were monitored throughout the 2008 growing season in southeastern Pennsylvania (39°49′N, 75°43′W). The understory microclimate conditions overall had a lower degree of variability and had consistently lower mean quantum flux densities, air temperature, vapor pressure deficit, volumetric water content, and soil temperature than the clearing plot. Understory seedlings of Fagus grandifolia Ehrh. (American beech) and Prunus serotina L. (black cherry) had significantly lower mean monthly rates of water loss (p = 0.05) than clearing seedlings P. serotina and Liriodendron tulipifera L. (yellow poplar)]. Additionally, water loss by shade-grown P. serotina seedlings was significantly lower (p = 0.05) than by sun-grown seedlings. Physiological differences, specifically shade tolerance, played an important role in determining the rates of stomatal conductance and transpiration in the seedlings. To a lesser degree, microclimate variability also influenced water loss. The results of this study validate results obtained in previous studies conducted largely under controlled conditions. Field validations are critical to developing better models and forest management strategies.