Seasonal changes rather than competition drive inorganic and organic nitrogen acquisition and internal allocation in temperate trees

For trees, soil nitrogen (N) acquisition and its internal allocation to different tissues are driven by species’ properties (e.g. growth rate, nutrient demand), the interactions among tree species, and their abiotic environment. Over the growing season, abiotic changes in, e.g. temperature and rainfall affect biogeochemical N cycling as well as N allocation and remobilisation processes in trees. We studied the influence of seasonality and developmental variation in woody seedlings, focussing on tree-tree interactions. We investigated the interaction effects among seven temperate European tree species on their inorganic and organic net N uptake capacity, internal allocation of N to metabolically active and storage tissues, and morphological traits at four leaf developmental stages over the growing season (i.e. before bud break, after leaf development, before / after leaf senescence). Seedlings of seven temperate European tree species were grown in mesocosms under natural forest conditions in intra- or interspecific competition. At the beginning of the growing season, organic N was favoured over inorganic N, whereas in autumn all N sources were taken up equally. Within species, N uptake and internal allocation generally varied over the growing season, except for N acquisition in Carpinus. Among tree species, the differences in N uptake and allocation were more related to species-specific morphological and physiological root traits, rather than functional properties. The outcome of the interactions was not generally affected within a tree community over the growing season but depended on the individual species. Thus, seasonal variation and species-specific properties should be considered for mixed species forests.