Microclimatic effects on Central European deciduous tree species and their interactions with arthropod herbivory

Abstract

Arthropod herbivores act as mediators for effects that cascade up and down the trophic chain. Therefore, herbivory plays an important role for driving ecosystem processes and influencing ecosystem structures and functions. Generally, ecosystem processes are mediated by interactions between organisms. The plant community composition is influenced by competitive interactions among plants, which is affected by herbivore species. Leaf area loss to insects can reduce tree growth, but alters material flows from canopies to forest soils. Therefore, the chemical quality of litter is changed (increases in nitrogen content) through enhanced nutrient cycling rates caused by herbivory. Climate and microclimate can affect insect physiology and behaviour directly or indirectly through climate-induced changes of host plants. Temperature determines the geographical range, site and timing of activities, success of oviposition and hatching, and the duration of developmental stages of arthropod herbivores. The activity of poikilothermic insects increases with temperature, and therefore growth and consumption rates are enhanced. However, morphological and functional leaf traits that determine host plant palatability often mediate indirect environmental effects on herbivory. Leaf palatability is determined by leaf toughness, nutrients, and defence compounds. In warm environments, expected high rates of arthropod herbivory can then be suppressed by negative changes of leaf traits. Microclimate gradients are found across the different strata of forest ecosystems. Abiotic factors change vertically between forest layers due to a micro-environmental gradient. Along the vertical gradient, microclimate is affected by the light regime, with increasing temperatures and decreasing humidity from understorey to upper canopies. Various organisms are distributed along the vertical forest gradient based on changes in environmental conditions and in the quality and quantity of available resources. Temperate deciduous forests reveal highly stratified arthropod communities with vertical and horizontal distribution patterns. Microclimatic requirements and the availability of food resources along the vertical forest gradient can reflect spatial distributions and preferences of arthropods. This research study investigated arthropod herbivory on leaves of deciduous tree species along the vertical gradient of temperate forests. A field study with ten forests sites in Central Germany and an experimental study in greenhouses were conducted, addressing effects of microclimate and leaf traits on arthropod herbivory. Juvenile and adult individuals of Fagus sylvatica L. (European beech), the dominant deciduous tree species in Central Europe, were chosen as main research subjects. Furthermore, Acer pseudoplatanus L. (Sycamore maple) and Carpinus betulus L. (hornbeam), two frequent tree species in the forest understorey, were also surveyed.