Uncovering the mechanisms behind positive diversity-productivity relationships in forests.

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Globally, forests play a critical role in mitigating climate change by removing CO2 from the atmosphere and storing it in wood. The strength of this carbon sync depends on a number of factors, such as climate, availability of nutrients and disturbance. On top of this, it is becoming increasingly apparent that diverse forests tend to be more productive than monocultures. But why? What are the mechanisms that enable diverse forest to outperform ones which are species poor? This is what we aim to understand. 

Several mechanisms have been proposed to explain the positive relationship between diversity and productivity. Among these, complementarity effects are particularly intuitive. The idea is that by mixing species with contrasting ecological strategies we 1) increase the efficiency with which resources are used and 2) lower competition among neighbouring trees. For example, mixing light demanding and shade tolerant tree species can create vertically structured canopies that intercept more sunlight, thereby resulting in faster growing forests. 

We tested the role of complementarity for light in Mediterranean forests. The forests we are studying present two ecologically contrasting types of trees, pines and oaks. Mediterranean pines and oaks differ in a number of key ecological traits, such as height, crown size and rooting depth. As a result pines tend to be light demanding, while oaks are better at coping with shade. We wanted to know whether mixing pines and oaks would increase productivity, and whether this results from a more efficient use of light.

To do this we used wood cores to reconstruct the growth rates of trees within the FunDivEurope plots in Spain. Every year trees leave behind a record of their growth in the form of a tree ring. Using wood cores we are able to measure the distance between tree rings and quantify how fast a tree has been growing.

We found that pines growth much faster in mixture because they have access to more light. Oaks instead maintain consistent growth rates even in the shade of taller pines. As a result, mixed species stands produce around 50% more wood each year compared to monocultures. Our work suggests that light is a key factor in driving the positive association between diversity and productivity in Mediterranean forests. 

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