Abundant nitrogen improves seedling growth and establishment. Vigorous growth brings about changes in rates and patterns of plant development and changes in the relationship between primary and secondary metabolism, which may make seedlings more susceptible to herbivores and pathogens than are slow-growing seedlings. We studied how nitrogen fertilization and manual defoliation of source leaves affect growth, carbon allocation, and developmental instability in cloned seedlings of white birch (Betula pubescens Ehrh.). Biomass was higher, whereas concentrations of most classes of phenolic compounds were lower in the nitrogen-rich environment. Interestingly, fertilization did not change the concentrations of cell wall–bound proanthocyanidins, which represent an important fraction of the group of phenolic compounds. Nitrogen enrichment increased levels of fluctuating asymmetry, an index of developmental instability. This result confirms that not only stress but also any deviation from normal resource availability may increase leaf developmental instability in birches. In contrast to fertilization, a one-time defoliation of source leaves did not shape seedling growth, development, or carbon allocation. This could be the result of compensatory growth or of the fact that the defoliation treatment was not strong enough to induce detectable effects until the end of the growing season.