Climate condition affects foliar nutrition in main European tree species
Foliar nutrient concentrations, contents, and ratios are important indicators for the nutritional status of trees. They depend on the availability of nutrients and the uptake capacities of the trees, which are controlled by forest structure, soil and climate condition. Consequently, accounting for climate conditions can aid the interpretation of foliar chemistry measurements. We applied a moving-window approach to identify the effects of atmospheric temperature and precipitation on tree nutrition at different time intervals based on data collected by institutions of the German Federal States within the ICP Forests Level II network. We studied the main nutrients N, P, K, Ca, and Mg as well as foliar mass for the main temperate tree species (European beech, temperate oaks, Norway spruce, Scots pine). Results show that foliar traits of all main tree species are affected by either current and/or lagged climate condition. Nutrient concentrations are generally less sensitive to climate condition than foliar mass and nutrient contents. Nutrient contents show the same response direction to climate condition as foliar mass, while nutrient concentrations mostly show an opposite response, potentially indicating the existence of dilution effects. Only Ca content in spruce shows weak effects of climate as changes in foliar mass are entirely counterbalanced by opposing changes in Ca concentrations. For spruce, pine, and oak significant climate effects on nutrient ratios were found. In general, N:P, N:K, and P: K are less sensitive to climate variations than ratios including Mg or Ca. In beech, all nutrient concentrations show a similar response to climate condition. Nutrient ratios in beech are thus relatively robust against climate condition compared to concentrations and contents. Our results highlight that intervals of less than three month provide a good indication of the climatic impact on tree nutrition. Longer periods, or means over several years, are less suitable as indicators. Defined periods show, however, a significant role of climate beside soil factors and species on foliar nutrition and should therefore be considered for the interpretation of tree nutrition.