Seasonal variation of photosynthesis and respiratory temperature response of invasive Japanese barberry (Berberis thunbergii) and two co-occurring native understory shrubs in a northeastern US deciduous forest

Early leafing and extended leaf longevity can be an important mechanism for the invasion of the forest understory. In this study, we compared the phenology, photosynthetic characteristics, and respiratory temperature responses of Berberis thunbergii (Japanese barberry), an early leafing invasive shrub, and two co-occurring native species, evergreen Kalmia latifolia (mountain laurel) and late leafing Vaccinium corymbosum (high bush blueberry), throughout the 2004 growing season in an oak dominated northeastern US deciduous forest. B. thunbergii leafed out one month earlier than V. corymbosum and approximately two weeks prior to the overstory trees. The photosynthetic capacity (characterized by V_cmax and J_max) of B. thunbergii was the highest in spring open canopy, and declined with canopy closure. The 2003 overwintering leaves of K. latifolia displayed high V_cmax and J_max in spring 2004. In new, 2004-produced new leaves of K. latifolia, the photosynthetic capacity was low at the early stage (June), but gradually increased to a peak in mid September, and reduced in late November. V. corymbosum, by contrast, maintained low V_cmax and J_max throughout the growing season. In B. thunbergii, photosynthetic light acclimation was mediated by adjustment in both LMA and N_m, which was weaker or absent in K. latifolia and V. corymbosum. The seasonal trend of the basal respiration rates (R₀) and E₀, the respiratory temperature coefficient, were also different among the three shrubs. Throughout 2004 growing season, B. thunbergii displayed much higher annual leaf respiration (mass based) than two native shrubs, indicating a higher cost per unit of biomass investment. B. thunbergii’s apparent success over the co-occurring natives therefore, appears related to a high spring carbon subsidy, and subsequent acclimation to varying irradiance through active nitrogen allocation and leaf morphological modifications. In contrast, respiratory properties are not likely to lead to carbon balance advantage of B. thunbergii.