In forests, diversity shapes not only the structure of the ecosystem (and its aesthetics) but also determines how trees interact with each other. Contrary to the static and immobile impression of trees, they are remarkably adaptable. Depending on the composition of the forest community, they are able to adjust their individual resource-use and thus growth strategies. However, it is not only the neighbouring trees that play a role in the individual adaptation strategies of trees and the overall ecosystem functioning, but also the diversity and interactions with mycorrhizal fungi. A study by Pablo Castro Sánchez-Bermejo and colleagues explores these fascinating dynamics, revealing the role of tree-tree competition and tree-mycorrhizal associations in shaping the functional traits of trees.
Revealing Strategies through Leaf Traits
By shifting functional traits, trees are able to adapt to competition for limiting resources. This is noticeable in leaf traits, which are quite plastic and reliable proxies for resource-use strategies. Thus, leaf traits change reflecting a trade-off between a leaf’s lifespan and its maximum photosynthetic rate and categorizes trees and other plants as either:
- Acquisitive strategists: Trees with thin, light leaves rich in nitrogen and other nutrients, designed for rapid growth and resource capture.
- Conservative strategists: Trees with tough, long-lasting leaves that are built to conserve resources over time.
Although leaf traits were initially used to compare species, leaf traits also vary within species (intraspecific variation) and even individuals (intraindividual variation). This is especially true for trees, as they are long-living species that particularly need to adapt to changing conditions during their lifetime, including the diversity in the forest stand. Therefore, as in forest monocultures there is typically a higher competition between trees due to similar resource exploitation of resources, trees are characterized by being conservative strategists and showing high trait variability in leaves within the same canopy. However, in mixed forests, the same tree species that had thick leaves in monospecific communities has a higher specific leaf area, thus thinner, lighter leaves designed for fast resource capture – the acquisitive strategy.
Looking beneath the Surface
While competition between trees is a major driver of how they grow, other (micro)organisms, like mycorrhizal fungi, also play a crucial role. As they form a mutualistic, beneficial relationship with the roots of plants and trees, the fungi enhance nutrient and water uptake, while in return, receive carbohydrates produced by the plants and trees during photosynthesis.
Mycorrhizal fungi associated to trees are categorised into two main types and define corresponding type of trees:
- Arbuscular mycorrhizal fungi: This is the most abundant type among trees and is associated with fast-growing trees as they specialize in quickly absorbing nutrients.
- Ectomycorrhizal fungi: These fungi are more associated with slow-growing, resource-conserving trees.
By partnering with fungi, trees also adapt how they use resources: With an increasing diversity of arbuscular mycorrhizal fungi simultaneously interacting with the tree, the nitrogen amount in leaves – crucial for photosynthesis as it is a key component of enzymes that help trees convert sunlight into energy – increases. Again, this means that trees of the same species adopt different strategies in its leaves, this time depending on the belowground diversity of fungi interacting with the tree.
A Peek into the Experiment
To understand all these interactions, the researchers studied 640 trees belonging to 10 native deciduous species in a tree diversity experiment in Central Germany. The trees were planted following a tree species richness gradient from monospecific plots (one species) over two- species mixtures up to four- species mixtures, whereby the mixtures comprised only arbuscular mycorrhizal or only ectomycorrhizal associated tree species or a balanced combination of arbuscular mycorrhizal and ectomycorrhizal associated species.
Over 3,000 leaves were collected from these trees, and their traits were analysed using advanced techniques like spectroscopy, which uses light to measure the chemical and structural properties of leaves. This allowed the researchers to assess key traits such as nitrogen content, leaf thickness, and dry matter content without damaging the samples.
In a Nutshell: Why Does it Matter?
The study has novel implications to understand how trees modify their resource-use strategy in response to biotic interactions. Contrary to previous considerations, the investigations show that tree and mycorrhizal diversity act on different traits and, therefore, suggest that tree diversity is not enough to explain all intraspecific responses in forests. However, more in-depth knowledge on tree-mycorrhiza interactions is needed to more comprehensively understand all these complex dynamics.
These insights are crucial for forest management and restoration. By fostering diversity in both tree species and fungi, healthier, more resilient forests can be established.
If you are interested in the detailed methodology of the project and would like to know more about the exciting world of competition and interaction going on in forests, you can find the research paper here: Tree and mycorrhizal fungal diversity drive intraspecific and intraindividual trait variation in temperate forests: Evidence from a tree diversity experiment – Castro Sánchez‐Bermejo – 2024 – Functional Ecology – Wiley Online Library
