Soluble nonprotein nitrogen compounds indicate changes in the nitrogen status of beech seedlings due to climate and thinning

Abstract

Summary • We assessed the effect of climatic and canopy density changes on the seasonal patterns of total soluble nonprotein N (TSNN) in naturally regenerated beech (Fagus sylvatica) seedlings grown on limestone. • Leaves, roots, wood and phloem exudates from seedlings grown in control and thinned stands on a dry, warm SW-exposed site and a moist, cooler NE-exposed site were examined. The concentrations of amino compounds, ammonium and nitrate, comprising TSNN, were determined in May (new leaf expansion), July (mid-summer) and September (end of the growing season). • In May, Asn was augmented in leaves and roots at the NE site, whereas Arg dominated in leaves and phloem at the SW site. In July, all TSNN compounds declined, independent of site and treatment. In September, TSNN, and particularly Arg, increased in roots, phloem and wood at the SW site, compared with the NE. • TSNN indicates changes in the N status of beech seedlings, due to altered growth conditions. The drier and warmer climate at the SW site, relative to the NE, resulted in earlier N remobilization in spring and storage in autumn. Thinning improved the N status at the NE site, but impaired it at the SW site, by affecting differently the climatic conditions and soil nutrient balance of each site.