ECOPHYSIOLOGICAL RESPONSES TO TREE SEEDLINGS INVADING DIFFERENT PATCHES OF OLD-FIELD VEGETATION

1 The performance of seedlings of Acer saccharum, Fraxinus americana, Gleditsia triacanthos and Prunus serotina was examined in eight old-field vegetation patch types and in control plots with no competition. Measurements of seedling gas exchange, water potential, foliar N, and specific leaf area were related to microenvironmental conditions and to seedling growth and survival. 2 Among vegetated patch types, predawn leaf water potentials and midday photosynthetic rates of the tree seedlings were highest in the short-statured and shallow-rooted patches of Poa pratensis, and lowest under established trees and shrubs where both soil moisture and subcanopy irradiance are reduced. 3 For seedlings of all tree species, photosynthetic rates per unit leaf area averaged 21-28% of that measured in control seedlings free of competition. Irradiance and photosynethesis both varied significantly among patch types, and photosynthesis was more strongly correlated with irradiance than with other variables. 4 In all species, photosynthesis under full light was strongly correlated with photosynthesis measured under ambient light. Ambient photosynethesis most closely approached photosynethetic capacity in late-successional species (Acer > Fraxinus > Prunus > Gleditsia). Shade-tolerance thereby enhances the potential for late-successional species to invade established plant communities. 5 The ratio of net photosynthesis to stomatal conductance in tree seedlings was decreased in competitive environments dominated by Solidago and other fast-growing herbs. Reduced water use efficiency appears to be caused by competitively induced nitrogen limitations. Some competitive environments thus reduce both soil moisture and photosynthetic water use efficiency of tree seedlings. 6 Measurements of photosynthesis and foliar nitrogen are good indicators of long-term seedling growth, but not of survival. The complex interaction of light, moisture and nitrogen availability varies distinctively among old-field vegetation patches, and invading tree species differ in their ability to cope with these compound stresses. Such differences can explain differential tree seedling growth rates in old-field vegetation, but the densities of trees found germinating and surviving from year to year depend on other factors.