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-fi eld vegetation patch types and in control plots with no competition. M easurements of seedling gas exchange, water potential, foliar N, and s pecific 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 seed lings were highest in the short-statured and shallow-rooted patches of Poa pratensis, and lowest under established trees and shrubs where bo th 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. Irr adiance and photosynethesis both varied significantly among patch type s, and photosynthesis was more strongly correlated with irradiance tha n with other variables. 4 In all species, photosynthesis under full li ght was strongly correlated with photosynthesis measured under ambient light. Ambient photosynethesis most closely approached photosynetheti c capacity in late-successional species (Acer > Fraxinus > Prunus > Gl editsia). Shade-tolerance thereby enhances the potential for late-succ essional species to invade established plant communities. 5 The ratio of net photosynthesis to stomatal conductance in tree seedlings was de creased in competitive environments dominated by Solidago and other fa st-growing herbs. Reduced water use efficiency appears to be caused by competitively induced nitrogen limitations. Some competitive environm ents thus reduce both soil moisture and photosynthetic water use effic iency of tree seedlings. 6 Measurements of photosynthesis and foliar n itrogen are good indicators of long-term seedling growth, but not of s urvival. The complex interaction of light, moisture and nitrogen avail ability varies distinctively among old-field vegetation patches, and i nvading tree species differ in their ability to cope with these compou nd stresses. Such differences can explain differential tree seedling g rowth rates in old-field vegetation, but the densities of trees found germinating and surviving from year to year depend on other factors.