Influence of resource availability on growth and foliar chemistry within and among young white spruce trees

Abstract

We increased and decreased the growth of young white spruce through fertilization and root-pruning during three years in a manipulated field study to investigate the influence of variations in resource availability on shoot growth and foliar chemistry within and among trees. As predicted by the carbon/nitrogen balance (CNB) and by the growth differentiation balance (GDB) hypotheses, increases in growth rate caused by fertilization were generally associated with increases in foliar nitrogen and water content and decreases in most C-based secondary compounds (phenols and tannins but not monoterpenes). The CNB and GDB hypotheses also correctly predicted that decreases in growth rate caused by root-pruning would be associated with decreases in foliar N and water content and increases in phenols and tannins. A negative relationship between foliar nitrogen and water content and foliar phenol and tannin content was also observed within trees, again supporting the CNB and GDB hypotheses. In spring, foliar N and water content was highest and foliar content of phenols and tannins lowest, in the upper crown. However, this intra-tree variation changed seasonally, and the highest foliar N and water content and lowest phenol and tannin content was found in the lower crown in fall. Seasonal variations in foliar nutrient content can be best explained by the source/sink hypothesis, as larger buds in the upper crown probably import more nutrients from foliage than smaller buds in the lower crown, resulting in the lower foliar nutrient content in the upper crown in fall. Interactions between whorl and treatment did not significantly influence shoot length, but non-significant trends were in agreement with predictions from the source/sink hypothesis. Thus CNB and GDB hypotheses could adequately explain among-tree variations in growth rate and foliar chemistry when resource availability was manipulated, but only the source/sink hypothesis could explain between-season differences within trees.