Growth of spruce and fir benefits from mixed-species neighbourhoods of these species

Interactions between plant species can be dynamic, changing spatially and temporally with variability in climatic, soil and stand conditions. We examined how inter- and intra-specific interactions between Abies alba Mill. and Picea abies (L.) Karst. varied with climate, site quality and stand density in the Black Forest of south-western Germany, using spatially explicit neighbourhood indices. The mixing response, a measure of complementarity, was quantified as the increase in growth of individual trees in a mixed-species neighbourhood compared to a mono-specific neighbourhood. Both species benefited from growing in mixed-species neighbourhoods, but this complementarity effect (60% to >200%) depended on climatic conditions, site quality and stand density. Complementarity increased for A. alba with increasing mean maximum temperatures in August, those for P. abies increased with mean minimum temperatures in May and site quality, and in each case the magnitude of the effect was amplified with increasing stand density. Complementarity is often considered to become more important in less productive ecosystems, but this study showed that for the given pair of species, complementarity effects can increase as growing conditions improve. A simple model is proposed that describes how relationships between productivity and complementarity change depending on the resources limiting productivity.