Effects of repeated fertilization of young Norway spruce on foliar phenolics and arthropods: Implications for insectivorous birds’ food resources

Repeated fertilization of young forests is a promising concept to increase the production of wood fiber, but the consequences of intense fertilization regimes on forest birds and their food resources, mediated through changes in the foliar chemistry are inadequately known. We assessed the effects of repeated fertilization in stands of young Norway spruce (Picea abies) on foliar phenolics and arthropods in an intensively managed forest area in southern Sweden in relation to the abundance of arboreal feeding birds breeding in the same stands. We anticipated leaf-sucking arthropods (viz. Hemiptera (aphids)) to react more strongly to changes in foliar phenolics than other invertebrate groups. Overall, we found small effects of fertilization on abundance and composition of different groups of foliar arthropods. However, the abundance of Hemiptera was much higher in early spring in fertilized stands than in unfertilized stands, whereas mites (Anactinotrichida and Actinotrichida) were more abundant in late spring in the fertilized stands. On the contrary, springtails (Collembola) were more abundant in unfertilized stands than in fertilized stands the late spring. The concentrations of two of the most abundant phenolics (hydroxycinnamic acid derivatives) as well as the sum concentration of HPLC-phenolics, were consistently lower in the fertilized stands. Positive effects on arthropod abundance mediated trough changes in the foliar chemistry following fertilization could help to explain why resident Goldcrest (Regulus regulus), which feeds on folivorous arthropods (i.e. aphids) during winter, was found to be more common in the fertilized stands (Edenius et al., 2011). Our results suggest that in intensively managed forest areas repeated fertilization of young spruce has the potential to enhance the suitability of these stands as winter feeding habitat for coniferous foliage-gleaning insectivorous birds compared to unfertilized stands in the same developmental stage. However, intensification of forest management further impedes habitat quality for more specialized species and generally reduces the diversity of forest birds by simplifying structure and composition of forest stands and shortening the rotation period. Therefore, careful planning of spatial arrangement of treatment units with different management, retention of natural forest and/or structures and legacies such as dead wood and deciduous trees are necessary to promote avian diversity in intensified forest management schemes.