Tree species and fungi – A special experimental design

This question was addressed by Tama Ray and her colleagues (2023), including Andreas Fichtner and Benjamin Delory from the Institute of Ecology at Leuphana University Lüneburg (Delory meanwhile: Utrecht University). For their study, they used data from the myDiv experiment. MyDiv is an experiment that falls under the functional biodiversity research described above. The experiment aims to better understand the relationship between above- and belowground interactions in tree species mixtures. This is based on the assumption that forest stands rich in tree species with different types of mycorrhiza function best. A mycorrhiza is a complex cohabitation between fungi and plants from which both benefit. The fungi are in contact with the fine roots of the plant and improve nutrient and water uptake for the plant and in return receive carbohydrates produced by the plant. There are many different types of mycorrhiza, which can be roughly categorised as arbuscular mycorrhiza (AM) and ectomycorrhiza (EM). Trees at temperate latitudes can be associated with either AM or EM or with both types of mycorrhiza. AM and EM function differently and have different specialisations.

In the MyDiv experiment, both the influence of aboveground and belowground diversity on productivity was analysed together. This means that there were two diversity variables: Firstly, the number of tree species (along the gradient: Monoculture, two tree species mixed, and four tree species mixed). Secondly, the types of mycorrhiza (AM, EM or mixed AM and EM). The trees were planted in 2015 near Bad Lauchstädt in Saxony-Anhalt, Germany.

What did the scientists expect?

Ray and colleagues have hypothesised that the greater diversity of tree species aboveground results in a more complex structure of the forest, i.e. leads to an increase in structural complexity. Structural complexity characterises the differing distribution of tree biomass in the three-dimensional space: different tree species have different tree heights and form differently structured crowns and thus use the space in the canopy more diversely than just one tree species would. This could allow the trees to utilise the available light more efficiently. As all plants need light for photosynthesis, in addition to water and nutrients, such more efficient utilisation of light could result in increased photosynthetic performance and thus increased growth. The assumption was similar for a higher diversity of mycorrhiza types: Due to the different specialisation of the various types, the scientists expected that the belowground resource use of nutrients and water is more efficient in stands with different mycorrhiza types and thus also contributes to increased productivity.

How do you measure the complexity of a forest structure?

Not only the experimental setup but also the measurement method in this experiment is special. Conventional measurement methods analyse structural complexity in two dimensions. In this experiment, however, the structural complexity was recorded three-dimensionally using terrestrial laser scanning, and a structural complexity index (SSCI) was created from this. This means that the specific distribution of leaves, branches, and trunks of the trees in space was recorded by a laser and translated into a numerical value. Productivity was measured annually based on the trunk diameter and tree height of each tree.

Surprising results

The scientists discovered several things:

1. Interestingly, contrary to expectations, the diversity of mycorrhizal types did not influence the productivity of the young forest. The scientists suspect that this effect may only play a role in older forests than the young forest of the project, as other studies have observed such effects in older forests. 
2. With increasing tree species diversity, structural complexity increases.
3. With this increasing structural complexity, productivity increases. In other words, the more complex the structures of the tree stands, the better the forests grow.

In order to test the assumed effect of increased light utilisation, the scientists measured the light incidence on the forest floor. They found that the less light reaches the forest floor, i.e. the more light was previously captured by the tree canopy, the stronger the correlation between structural complexity and productivity. Ray and his colleagues have therefore concluded that increasing the structural complexity of the canopy allows the forest to grow better because the available light is then utilised more efficiently, meaning that more light is available for photosynthesis and thus for growth. The increasing diversity of tree species therefore has an indirect effect on the growth of the forest because it makes the structure of the forest more complex, which in turn allows the forest to grow better (see overview). In the experiment, forests with a more complex structure grew almost twice as fast as forests with a less complex structure.

Miracle tree species or team effect?

In the next step, the scientists wanted to find out whether the observed increase in productivity was due to a specific tree species or whether it was caused by the mixing effect. To this end, they calculated the so-called “overyielding”. This describes the increased growth of a tree species that only occurs when it is mixed with other tree species compared to a monoculture. So, if a beech in monoculture would grow 5 m³ per tree per year, but in a mixture with oaks, for example, it would grow 8 m³ per year, then this additional growth of 3 m³ per year would describe the overyielding. The overyielding therefore describes the net biodiversity effect of the mixture of tree species on the productivity of the individual tree species (which grows in this mixture). This effect can be mathematically divided into the species identity effect and complementarity effects. The former would be an effect for which a particular tree species is responsible, the latter an effect caused by the mixture of tree species. This can be compared to a football team: Does a team win because it has a particular football player on the team who plays much better than the others, or does it win because the team as a whole plays so well together? The effects in this experiment could largely be explained by the team effect, i.e. they are mainly complementarity effects.

Light-loving and shade-tolerant complement each other well

The scientists found that structural complexity explains half of the variation in productivity and that almost two-thirds of this can be attributed to this mixing effect of tree species alone. This strong effect of the mixture of tree species itself could be traced back by the scientists mathematically. They found that the effect can be explained most strongly by different shade tolerances of trees. Birches, for example, do not tolerate shade well, but beeches do and can therefore thrive under the fast-growing birches. Together, this gives them more space in which they can grow optimally and therefore more complexly. Thus, the more diverse the shade tolerances of trees in a forest are, the better and more complex the space can be utilised, which means more light can be captured and thus more light is available for photosynthesis, so the forest grows better (see overview). In addition, taxonomic diversity (i.e. the different ancestral histories of trees) also influences this stronger growth in mixed tree species. The scientists have not yet been able to explain exactly why this is the case, but they believe it could be due to the different shapes and branching patterns of the tree crowns of the various tree species.

How can these findings be used?  – Climate change and drones

These results provide valuable implications for practice, including the restoration of forests: For example, tree stands should be designed in such a way that they contain several tree species that have different light requirements. This would allow the forests to grow as quickly as possible and store larger amounts of carbon aboveground, which is particularly relevant in view of climate change. In addition, the structural complexity could be used as a proxy for productivity in the future based on the results. In the course of new satellite-based methods and drone applications, the structural complexity and thus productivity of a forest could be assessed relatively quickly and over a large area, thus, for instance, improving the predictive power of carbon models.

If you want to delve deeper into the topic, you can find the paper by Ray and colleagues here: Ray, T., Delory, B. M., Beugnon, R., Bruelheide, H., Cesarz, S., Eisenhauer, N., Ferlian, O., Quosh, J., von Oheimb, G., & Fichtner, A. (2023). Tree diversity increases productivity through enhancing structural complexity across mycorrhizal types. Science Advances, 9(40), eadi2362. https://doi.org/10.1126/sciadv.adi2362

Cover picture: Structurally rich beech forest in Sweden (Photo: Andreas Fichtner).

What came to your mind? Was there maybe some kind of picture of a beautiful forest wandering around your head? A lush green canopy of majestic trees, whose miraculous leaves use the sunlight to turn water and CO₂ into carbohydrates through photosynthesis, sequestering carbon, and giving oxygen to the air in return? I can fully relate to that – probably most people can. Trees and forests can be great natural climate solutions and havens of biodiversity, offering a home for many species, so of course, they are very valuable ecosystems. But how does it come that we typically only think of forests here? Only about trees doing photosynthesis, when all plants are capable of doing that? Could it be that we are overlooking something? According to Staude and colleagues, the short answer is: Yes.

We could protect more than three-quarters of threatened plant species by shifting restoration priorities

Yes, we are overlooking something. Something which is actually much too big to overlook as the scientists Ingmar Staude and colleagues (2023), including Vicky Temperton and Emanuela Weidlich from the Institute of Ecology at Leuphana, found out: 82 % of all endangered plant species in Germany are found in high-light ecosystems such as grasslands, and only 1 % in shaded ecosystems such as forests (see figure below). And with this number it becomes very clear that there is a problem: While it is common agreement by now, that the restoration of ecosystems is crucial in our current time of increasing species loss and climate change, such restoration efforts are commonly equated with planting trees and restoring forests – not just by us, but also by policymakers. Yet when we look at the data from Germany (but also increasingly for other regions of the world) we find that the most threatened species are often from the open grassy habitats or biomes (Hoekstra et al. 2004, Jandt et al. 2022, Staude et al. 2023). Conservation, including restoration practice and policy, is globally often strongly focused on forests, mostly neglecting and underfunding or underprotecting open grassy ecosystems (grasslands, savannas, shrubland), although carbon-rich peatlands are now also starting to attract the attention they deserve as the habitat that can store by far the largest amount of carbon (peatlands cover around 3 % of the earth but can store more than double the amount of carbon than forests).

How do the scientists get these numbers?

For their analysis, Staude and colleagues combined overall vegetation data derived from the sPLOT database for plant data across Germany over time with Ellenberg indicator values. Ellenberg indicator values characterize species using different habitat parameters such as light availability, temperature, and nutrient content in the soil. This allowed them to then assess to what extent the species whose abundance has declined over the past century are associated with different habitat types (grasslands, forests, shrubland). The aim of this analysis was to examine how the threat status of the endangered plant species is related to their niches (the specific environmental factors that make up the species’ habitat) and how this developed. For this, the Ellenberg light values representing the light demand of plant species were chosen, as grassy ecosystems offer much light for their plants in contrast to forests which generally are shaded habitats. The resulting threat status for light demand was also compared to the threat status for nutrient demand, as high nutrient inputs mainly caused by agricultural intensification are known to be a primary driver of species loss.

This analysis showed the values mentioned in the beginning: 82 % of all red-listed plant species in Germany are light demanding, which is even more than the 61 % that need nutrient-poor habitats – while this nutrient problem is well known. Indeed, every second plant species of all plant species in Germany that require full light or nutrient-poor soils is endangered and the trend is increasing. Staude and colleagues argue that this is not just the case in Germany, but that these results are well transferable to other regions in Europe and beyond, which they underpin with a case study from southern Brazil coming to similar results.

A question of perspective

But why do we and policymakers then typically think of forest restoration when it comes to biodiversity conservation and restoration? Staude and colleagues assume and discuss two main reasons here.

Firstly, the prevailing conception of forests as the natural vegetation of central Europe leads people to favour this state of perceived wilderness over ecosystems like grasslands. Following this, grasslands would be degraded ecosystems. However, this theory is subject to a long-standing debate, with current evidence (Pearce et al. 2023) indicating that the historic landscape may rather had more diverse, mosaic-like vegetation structures with large components very open (somewhat like in wood pastures), as Staude and colleagues explain.

Secondly, biodiversity conservation and climate change mitigation are typically equated with carbon sequestration and as forests sequester large amounts of carbon aboveground they can be a very effective natural climate solution in this regard. Belowground they do not necessarily store more C than other habitats, however. Integrated measurement of belowground carbon stores and sequestration across habitats is rare however, and urgently needs addressing in integrated scientific studies to enable evidence-based recommendations for biodiversity and climate change actions on the ground. There is evidence that grasslands can store as much C belowground as forests, plus through their albedo effects and high resilience to extreme weather events, which will be both examined in more detail in the next section, they form a powerful tool to address the many pressures raining down on us within the climate change and biodiversity polycrises, as Staude et al. point out.

Both of these prevailing perceptions about forests result in much higher focus and investment in restoration efforts for these, according to the authors, while traditionally managed high nature-value grassland (with their vast biodiversity) have almost silently disappeared from our cultural landscapes, and with them an army of pollinators.

Grasslands against climate change?

In addition to this, Staude and colleagues argue that there are also good reasons to pay more attention to grasslands for climate mitigation and adaptation reasons, as indicated above. While grasslands cannot sequester as much carbon as forest aboveground, they also can store large amounts of carbon belowground. These carbon stocks are overall more resilient to extreme weather events, like fires and droughts, which are predicted to increase due to climate change. Hence, the sequestered carbon would be stored longer in such grassy ecosystems. Following Staude and colleagues, this is due to their long history and coevolution with high disturbance regimes such as frequent and regular fire, drought, and grazing. Forests, on the other hand, in some regions already face the risk of turning into carbon sources soon due to the extreme heat and drought stress trees are facing, leading to increased tree mortality. At the same time, the capability of grasslands to store carbon could be even increased by applying (and researching) the optimum grassland management to foster biodiversity as well as C sequestration and storage.

Nevertheless, more carbon sequestration is not immediately equal to more cooling. Therefore, Staude and colleagues also draw attention to other factors affecting global warming, such as the albedo of grasslands and forests. The albedo describes the capacity of a surface to reflect the sunrays shining on it. A bright surface reflects a lot of sunrays and stays cool, while a dark surface absorbs more sunrays and heats up (you can feel the difference when walking barefoot on a meadow and on a street on a sunny summer day, see diagram below). Consequently, a forest with its rather dark tree canopy absorbs more solar radiation than a grassland which is lighter, thus a forest area warms the local climate more than a grassland. Therefore, Staude and colleagues argue that grasslands could be powerful complementary players next to forests as natural climate solutions and deserve a lot more attention. At the same time, the paper makes abundantly clear that if we want to bend the biodiversity curve then we should focus strongly on restoring species-rich grasslands across Germany and many other temperate regions (maybe also outside of the temperate biomes).

All at once – Grassland restoration for biodiversity conservation, climate change mitigation and adaptation

Working towards biodiversity protection and climate change mitigation and adaptation at the same time, Staude and colleagues conclude that grasslands ought to be considered much more in restoration policies and have the potential to become linchpin solutions to both the biodiversity and the climate crises. Additionally, afforestation should not happen at the expense of valuable grasslands that harbour much biodiversity but also may be our best bet for keeping the C in the ground as the climate warms further. Also, they stress that they do not want to dismiss the value of forests when it comes to restoration: forests should be restored where they and their species are declining, however, they should not be in the focus of restoration efforts, when they are not. Additionally, a recent study by Mo et al. (2023) emphasises that we will be able to store more C by preserving existing forests (and allowing them to grow to maturity) than by planting a trillion trees in areas where forests are currently not found.

With all this in mind, perhaps the next time hearing the term restoration we may also think of a beautiful grassland with humming bees and butterflies visiting the great variety of its flowers.

If you want to dive deeper into this topic, you can find the paper of Staude and colleagues here: Staude, I. R., Segar, J., Temperton, V. M., Andrade, B. O., de Sá Dechoum, M., Weidlich, E. W., & Overbeck, G. E. (2023). Prioritize grassland restoration to bend the curve of biodiversity loss. Restoration Ecology, e13931. https://doi.org/10.1111/rec.13931

References

Hoekstra, J. M., Boucher, T. M., Ricketts, T. H., & Roberts, C. (2004). Confronting a biome crisis: global disparities of habitat loss and protection. Ecology letters, 8(1), 23-29. https://doi.org/10.1111/j.1461-0248.2004.00686.x

Jandt, U., Bruelheide, H., Jansen, F., Bonn, A., Grescho, V., Klenke, R. A., … & Wulf, M. (2022). More losses than gains during one century of plant biodiversity change in Germany. Nature, 611(7936), 512-518. https://doi.org/10.1038/s41586-022-05320-w

Mo, L., Zohner, C. M., Reich, P. B., Liang, J., De Miguel, S., Nabuurs, G. J., … & Ortiz-Malavasi, E. (2023). Integrated global assessment of the natural forest carbon potential. Nature, 1-10. https://doi.org/10.1038/s41586-023-06723-z

Pearce, E. A., Mazier, F., Normand, S., Fyfe, R., Andrieu, V., Bakels, C., … & Svenning, J. C. (2023). Substantial light woodland and open vegetation characterized the temperate forest biome before Homo sapiens. Science advances, 9(45), eadi9135. https://doi.org/10.1126/sciadv.adi9135

Staude, I. R., Segar, J., Temperton, V. M., Andrade, B. O., de Sá Dechoum, M., Weidlich, E. W., & Overbeck, G. E. (2023). Prioritize grassland restoration to bend the curve of biodiversity loss. Restoration Ecology, e13931. https://doi.org/10.1111/rec.13931