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What determines competition amongst forest trees?
When trees compete for light, water and nutrients in a forest, their functional traits determine how well they fare against their competitors. This is the conclusion of the most extensive study on tree competition to date, published today in the journal Nature.
An international team of researchers has revealed that three functional traits – wood density, specific leaf area and maximum height – affect competition in predictable ways across all forested biomes worldwide. "This is the first time that we were able to verify key drivers of forest succession globally," explains co-author Niklaus Zimmermann from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL.
Competition between neighbouring trees has a big impact on their growth. Trees have different strategies to deal with competing neighbours. Some grow quickly and tall, overshadowing the others, but die young. Others grow more slowly, but outlive the fast growing ones and cast shade on them over a longer period. These interactions have a strong influence on the dynamics of forests and their functioning as ecosystems.
Forests are crucial elements of the earth system, also for humanity, stretching across ecosystems from boreal regions to the tropics. Therefore, ecologists have long sought an approach that might allow competition to be predicted in a general way across ecosystems and the tens of thousands of different tree species worldwide.
The research published in
Nature achieved this via so-called "functional traits" of species – wood
density, specific leaf area (SLA, area per unit dry mass of leaf) and maximum
height. Trees differ in these traits in a consistent way. Poplar and birch
trees are light demanding species and exhibit comparably high SLA, meaning that
their leaves let a lot of light shine through, while beech trees possess
thicker leaves, exhibit lower SLA and are thus more shade tolerant. Poplar and
birch grow fast, tall and have lower wood density, while beech trees grow more
slowly and with higher wood density.
For this study a team of almost 40 researchers brought together national forest inventory and research plot data spanning 3 million trees in over 140,000 plots across the world. Marc Hanewinkel and Niklaus Zimmermann from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL contributed data from the Swiss National Forest Inventory NFI, which surveys changes of Swiss forest on roughly 6500 plots.