Survival, growth, and photosynthesis of tree seedlings competing with herbaceous vegetation along a water-light-nitrogen gradient

In herbaceous dominated patches and ecosystems, tree establishment is influ enced partly by the ability of woody seedlings to survive and grow in direc t competition with herbaceous vegetation. We studied the importance of seas on long wet and dry spells on the competitive interactions between herbaceo us vegetation and oak seedlings along a light and nitrogen gradient in an i nfertile secondary successional grassland in central North America. We cond ucted a field experiment in which seedlings of bur oak (Quercus macrocarpa) and northern pin oak (Q. ellipsoidalis) were exposed to two levels of ligh t (full sun and 80% shade), three levels of nitrogen input (0, 5, 15 g m(-1 ) yr(-1)), and three levels of water input (low, medium and high). In addit ion, seedlings were grown with and without the presence of surrounding herb aceous vegetation under both light and all three water levels. Seedling sur vival, growth, and rate of photosynthesis were significantly affected by co mpetition with herbaceous vegetation and these effects varied along the mul tiple resource gradient. Overall, seedling survival of both species was sig nificantly greater in wetter and shaded plots and when surrounding herbaceo us vegetation was removed and was lower in nitrogen enriched plots. We foun d that soil water was significantly affected by varying inputs of water, li ght, and the presence or absence of herbaceous vegetation, and that seedlin g survival and rate of photosynthesis were highly correlated with available soil water. Our findings show that the impact of season long wet and dry s pells on tree seedling success in grasslands can be affected by light and s oil nitrogen availability.