Abstract
Nitrogen availability limits growth in most boreal forests. However, parts of the boreal zone receive significant levels of nitrogen deposition. At the same time, forests are fertilized to increase volume growth and carbon sequestration. No matter the source, increasing nitrogen in the boreal forest ecosystem will influence the resource situation for its primary producers, the plants, with possible implications for their defensive chemistry. In general, fertilization reduces phenolic compound concentrations in trees, but existing evidence mainly comes from studies on young plants. Given the role of the phenolic compounds in protection against herbivores and other forest pests, it is important to know if phenolics are reduced with fertilization also in mature trees. The evergreen Norway spruce is long-lived, and it is reasonable that defensive strategies could change from the juvenile to the reproductive and mature phases. In addition, as the needles are kept for several years, defense could also change with needle age. We sampled current and previous year needles from an N fertilization experiment in a Norway spruce forest landscape in south-central Norway to which N had been added annually for 13 years. We analyzed total nitrogen (N) and carbon (C), as well as low-molecular phenolics and condensed tannins. Needles from fertilized trees had higher N than those from controls plots, and fertilization decreased concentrations of many flavonoids, as well as condensed tannins in current year needles. In previous year needles, some stilbenes and condensed tannins were higher in fertilized trees. In control trees, the total phenolic concentration was almost five times as high in previous year needles compared with those from the current year, and there were great compositional differences. Previous year needles contained highest concentrations of acetophenone and stilbenes, while in the current year needles the flavonoids, and especially coumaroyl-astragalins dominated. Condensed tannins did not differ between current and previous year needles from control trees. In conclusion, the phenolic defense of current year needles of mature P.abies trees was strongly changed upon fertilization. This may imply that nitrogen deposition and forest fertilization leave forests less robust in a time when pests may take advantages of a changing climate.
Highlights
Boreal and temperate forests soils naturally have a low availability of nitrogen (N) and N is often the primary growthlimiting nutrient in these soils (Tamm, 1991; Binkley and Fisher, 2013)
Control needles from the previous year contained over three times more total low molecular phenolics than did current year needles (Table 1), and the composition differed greatly (Figure 2, Table 1)
Current year needles had more than double concentration of flavonols compared with the older needles, of which 3,6-dicoumaroyl astragalin was the highest compound in both needle types (Table 1)
Summary
Boreal and temperate forests soils naturally have a low availability of nitrogen (N) and N is often the primary growthlimiting nutrient in these soils (Tamm, 1991; Binkley and Fisher, 2013). The increased nitrogen deposition rates over the last 150 years have greatly altered this in parts of these biomes (Galloway and Cowling, 2002; Meunier et al, 2016), and it is wellknown that N deposition often directly enhances forest growth (Meunier et al, 2016).Continental and northern regions of the European boreal forests, are relatively little affected by N deposition (e.g., Gundale et al, 2011). The general power of N is pointed out in the recent review by Meunier et al (2016), who conclude that increased availability of N in the base of forest food webs results in cascade effects, which in turn substantially affect other trophical levels through alterations in food web structure and functioning
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