Our latest paper aiming to untangle the mechanisms behind diversity-productivity relationships in forests has just been published in Journal of Ecology. Working in Mediterranean mixed forests in Spain, we find that complementary use of canopy space by oak and pine species means that mixtures of these two functional groups produce around 50% more wood each year compared to monocultures. However, the magnitude of the diversity effect on wood production fluctuates over time, decreasing noticeably in strength during drought years.
The study was conducted in the Alto Tajo Natural Park, in the Guadalajara province of central Spain, as part of the FunDivEUROPE project which aims to determine the functional significance of forest biodiversity. Using tree ring data from permanent forest plots we explored how mixing pine (Pinus sylvestris and Pinus nigra) and oak species (Quercus faginea and Quercus ilex) influences wood production in Iberian forests. We found that pine species receive more light, develop larger crowns and grow 138–155% faster when in mixture with oaks. However, this positive effect of species mixing on growth was severely reduced under drought conditions due to increased competition for water with neighbouring oaks. In contrast to pines, oak trees were less responsive to mixing, primarily as a result of their ability to tolerate shade and water shortage. Our results suggest that competition for light is key in driving positive diversity effects in forests, but also show that the strength of complementarity can change in response to climatic conditions.