Oecologia (1989) 80:356-367 Oecologia 9 Springer-Verlag1989 Ecophysiology of exotic and native shrubs in Southern Wisconsin I. Relationship of leaf characteristics, resource availability, and phenology to seasonal patterns of carbon gain Robin A. Harrington, 1' * Becky J. Brown, z and Peter B. Reich 3 1 Institute for Environmental Studies, ZDepartment of Botany and 3Department of Forestry University of Wisconsin-Madison, Madison, WI 53706, USA Summary. We compared seasonal trends in photosynthesis of two naturalized exotic shrubs (Rhamnus cathartica and Lonicera X bella) and two native shrubs (Comus racemosa and Prunus serotina) in open and nnderstory habitats in southern Wisconsin. We examined the relationships be- tween resource availability and leaf photosynthetic perfor- mance in these four species. All four species had similar relationships between leaf nitrogen (N) content and photo- synthetic rate, but the species differed in absolute leaf N content and therefore in photosynthetic rates. Maximum daily photosynthetic rates of all species were significantly correlated with leaf N content in the open habitat, but not in the understory, where low light availability was the major limitation to photosynthesis. Extended leaf longevity was important in the forest understory because it allowed shrubs to take advantage of high light availability at times when the overstory canopy was leafless. Early leaf emergence was more important than late senescence: from 27% to 35% of the annual carbon gain of P. serotina, R. cathartica, and L. X bella occurred prior to leaf emergence of C. race- mosa, the species with the shortest leaf life span. Extended leaf longevity of exotic shrubs may help explain their persis- tence in the understory habitat, but it contributed relatively less to their annual carbon gain in the open habitat. Key words: Carbon gain - Exotic shrubs - Leaf longevity Photosynthesis - Understory Two exotic shrubs, Rhamnus cathartica and Lonicera X bella have become naturalized in southern Wisconsin for- ests. These naturalized exotic shrub species differ from na- tive species in phenological patterns, with average leaf lon- gevities exceeding those of native shrubs by up to 58 days (Barnes 1972; Brown unpublished data). Although in- creased leaf longevity is often associated with decreased photosynthetic rates (Chabot and Hicks 1982) and longer amortization times (Chabot and Hicks 1982; Kuppers * Present address and address for offprint requests: Forestry/Fuel wood Research and Development Project (F/FRED), Department of Agronomy and Soils, University of Hawaii, 1910 East-West Road, Honolulu, HI 96822, USA 1984a), a longer growing season could increase seasonal carbon gain of these exotic species and may help explain their strong competitive ability. Extended leaf longevity may be particularly important in understory species grow- ing beneath a deciduous forest canopy. In this habitat, early leaf emergence and/or late senescence may allow plants in the understory to exploit periods of high light availability prior to canopy leaf emergence and following canopy leaf fall. In open and understory habitats, exotics with extended leaf longevity have more growing days than native shrubs, provided that environmental conditions, such as low tem- perature in early spring and late fall, are not limiting. The extended growing season of Australian acacias was a factor contributing to their successful invasion into South African coastal shrub communities (Milton 1981). Autumn photo- synthetic rates were high in an exotic hybrid Populus that retains green leaves ~6 weeks longer in the fall than native Populus species (Nelson et al. 1982). In understory habitats, spring and fall photosynthesis is especially important due to increased light availability at these times. Photosynthetic rates were high for understory Ribes uva-crispa in early spring before canopy closure, but were low throughout the summer (Kuppers 1984b). Early spring and late autumn photosynthesis was also important to the survival of eastern red cedar (Juniperus virginiana) growing under a deciduous oak canopy (Lassoie et al. 1983). Peak photosynthetic rate is highly correlated with leaf nitrogen (N) content for a number of species (Mooney and Gulmon 1979; Gulmon and Chu 1981; Field et al. 1983; Field and Mooney 1986), and seasonal trends in leaf N content may affect seasonal patterns in photosynthesis. If foliar N content of the exotics remains high in the fall due to their late senescence, then late season photosynthetic rates of the exotics may be high relative to the native shrubs. Temporal changes in environmental factors also affect sea- sonal photosynthetic patterns (Lassoie et al. 1983). The objectives of this study were (1) to assess the extent to which the extended growing season and physiological traits of naturalized exotic shrubs contribute to their com- petitive abilities, and (2) to characterize relationships be- tween resource availability and leaf photosynthetic perfor- mance. We compared diurnal and seasonal timecourses of net photosynthesis and water use for two exotic shrubs,