Life is not easy for the trees in a forest: To avoid being edged out by other trees, they must prevail in the struggle for resources. Ultimately, the successful trees are those whose traits are best suited to their location. An international team, which included researchers from the Max Planck Institute for Biogeochemistry in Jena, has now discovered how three characteristic traits – wood density, maximum height and specific leaf area – decide which trees will emerge victorious in the competition between neighbouring individuals.
Surprisingly, these correlations are consistently applicable throughout the world. Moreover, it emerged from the study that the competition between trees of the same species is always more intensive than that between trees of different species.
Almost one-third of the Earth’s land surface – from the polar circles to the tropics – is under forest cover. The forests accommodate an astonishing variety of tree species with equally wide-ranging forms and strategies. In the tropics alone, mixtures comprising up to 53,000 different tree species are found in the same ecosystems.
Ecologists have long been seeking a general approach that would enable them to predict which of the tens of thousands of tree species that exist in the world can grow next to each other, which ones compete with each other and which will prevail in the competition for survival. This depends on how well individual trees grow and, ultimately, which species can stand their ground in the forest.
Species with greater wood density can well tolerate their neighbours
“Foresters have known for centuries which tree species dominate in their forests,” says Jens Kattge, a Research Group Leader at the Max Planck Institute for Biogeochemistry. “For a long time it was assumed that tree species should be as mixed as possible so that they can occupy many ecological niches and co-exist side-by-side in a location.” In a study headed by George Kunstler from the French National Research Institute of Science and Technology for Environment and Agriculture (IRSTEA) in Grenoble, scientists from all over the world have now systematically examined whether the competition between the trees is subject to certain rules, whether these rules apply consistently at global level and what form they take. Given the incalculable number of tree species that exist, this was clearly an enormous task.
In a study currently published in Nature, rather than focusing on taxonomic species as was the standard approach adopted up to now, the researchers identified such globally valid rules by comparing the functional traits of the species with each other. In an analysis of all of the Earth’s forest ecosystems, the researchers discovered how the competition between the trees is affected by three traits: wood density, specific leaf area and maximum height. These traits were examined in detail in the study. It was already known that they have a similar influence on the physiological functions of individual plants all over the world. “It now emerges that these traits also determine the competition between neighbouring trees on a global level,” says Jens Kattge. “Given the vast differences in the environmental conditions found in the world’s forests, this is astonishing.”
Thus, according to the study, it is not only important that the trees in a location are as diverse as possible so that they can survive alongside each other, they must also differ from each other in respect of one or more of the three traits. These traits also provide a good indication of which species are most resilient to competition. For example, species with a high wood density appear to be particularly tolerant to their competing neighbours and this enables them to prevail in the long term. In contrast, trees with lower wood density grow faster, and for this reason lead the field in the early stages of the competition at least, when trees colonize new area.
Competitive pressure is greatest between trees of the same species
However, a tree must not outperform its neighbours in relation to all traits to be a successful competitor. For example, the native beech tends to grow rather slowly and does not reach a particularly impressive height; it has, however, a high wood density and shade-tolerance, which gives it a major advantage in the ecological competition during the second phase of forest colonization after around 50-100 years. With its traits, it comes very close to a combination identified by the study as being very successful in the long-term competition for survival in global forests. Therefore, from a long-term perspective, shade-tolerant high-growing trees with medium to high wood density enjoy competitive advantages all over the world.
It also emerged from the study that the competitive pressure within one and the same species is always greater than that between trees of different species. This is also rather obvious, as trees from the same species have more or less identical traits. They thus occupy the same ecological niche and compete for the same resources in a location. The fact that one of two trees will ultimately prevail is above all due to the fact that even trees of the same species will not have identical locations and traits. Experts refer here to intra-species variability, which can have advantages for individual trees when it comes to obtaining the resources they need.
Furthermore, the fact that the role played by chance in the ecological competition is far from insignificant, should not be forgotten here – irrespective of whether the competitors are from the same species or not. For example, plants can be damaged by disease or wild animals and this impairs their chances of survival in the battle with their competitors.
More accurate climate forecasting thanks to more detailed vegetation models
For this study on competition in forests, almost 40 scientists from all over the world collated the results of national forest inventories and monitoring data from test sites containing a total of three million trees representing over 2,500 species from over 140,000 locations. The data were evaluated by researchers from Macquarie University in Sidney, Australia. The global plant trait database TRY, which is operated at the Max Planck Institute for Biogeochemistry in Jena and to which the German Centre for Integrative Biodiversity Research (iDiv) also contributes, made a crucial contribution to the study.
The new findings of the study highlight the fact that the competition for resources can be largely explained by a few functional traits. The comparison of functional traits can also provide information about which tree species can survive together in a location. Based on this study, for example, forest managers can improve their planning of tree species mixtures. Given the need to estimate the impacts of climate change on forest ecosystems and vice versa, there is a greater demand now for predictions regarding the composition of plant communities and the competition between the species they contain. The study’s findings can be incorporated into vegetation models and therefore also in Earth system models which take the influence of climate on vegetation into account and vice versa. In this way, they can contribute to improving the forecasting of changes in the climate over the next century.