Dendroecological and Ecophysiological Analysis of Gap Environments in Mixed-Oak Understoreys of Northern Virginia

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Ecophysiological, morphological and dendroecological attributes were examined in species growing in understorey and newly formed gap environments in mixed-Quercus forests of northern Virginia. Gap environments were characterized by higher photosynthetic photon flux density (PPFD) but were not different from non-gap control plots in their soil moisture, predawn leaf water potential (Ψ), leaf temperature or leaf to air vapour pressure deficit (VPD). In addition, few significant differences existed between regeneration responses at gap and control sites. Plants in gaps experienced higher rates of net photosynthesis (A) and leaf conductance to water vapour diffusion (g wv ) than control plants, although parameters did not differ between early vs late successional species. Gap plants also produced leaves with smaller leaf area and greater leaf thickness, leaf mass per area (LMA) and stomatal density than control plants. All gap species experienced accelerated height growth compared to non-gap plants, with early successional Sassafras albidum (Sassafras) and Liquidambar styraciflua (Sweetgum) exhibiting the highest rates. In contrast, average radial growth in small, understorey trees was not significantly different between habitat type, although Quercus alba (White Oak) did respond significantly to the most recent gaps. Many small trees (6-12 cm d.b.h.) were surprisingly old - up to 100 years in age. Despite severe suppression periods in these trees, they experienced release events during their lifetime, although not necessarily to the most recent gaps. This may partially explain the lack of correlation between ecophysiological and growth parameters.