At the beginning, there was a question

Although we have learned much about the relationship between climate and ecosystems, the interactions between biodiversity and climate are still very much in the dark (1). These interactions are essential not only for predicting how and how fast ecosystems and the climate interact but also for understanding how ecological systems can be bolstered and made more sustainable in a world of constant change. Plant diversity's positive role in mitigating climate change impacts by maintaining plant productivity and ecological stability has been extensively observed. However, exactly how to explain these phenomena is still debatable. Since it's well-established that soil temperature and its consistency are critical factors governing fundamental ecosystem processes such as nutrient and water intake, we hypothesized that the effects of plant diversity might manifest in the soil temperature dynamics. This hypothesis consecutively could explain and unify much of the observed improvements in productivity and stability. 

Surprisingly, we found that there were no experiments yet that have investigated this relationship. Instead, only a few studies have looked at the effects of plant diversity on surface air temperature. We took this as an opportunity to roll up our sleeves.

But how exactly could we test this relationship?

Figure 1 | Depiction of the Jena Experiment field site, our unique data source in Central Germany.

The Jena Experiment setup - a collaborative effort 

The Jena Experiment (2), a large grassland experiment in the heart of Germany, has helped us to investigate our hypothesis. Now running for 20 years (Happy Birthday!), numerous experiments on the effects of biodiversity have been conducted over the years. Tied to these experiments, many measurements have been taken, including plant biomass, soil properties, etc. The field site, roughly 14 soccer fields, consists of many experimental plots with a gradient of plant species richness, varying from monocultures to communities with up to 60 species. To our luck, the founders of this experiment were quite far-sighted and set up soil temperature measurement stations right from the beginning of the experiment. This gave us access to a previously unprecedented 18-year high-resolution soil temperature dataset. This dataset allowed us to investigate our hypothesis comprehensively. 

Together with the many covariates’ measurements on the field site, we were also able to find the mechanisms responsible for explaining the relationship between plant diversity and soil temperature.

Unique combination of ecology and computer science

When data is collected automatically over such a long time with 1-minute resolution, data cleaning and processing becomes a major task. After all, Windows XP was still relatively new when the experiment was launched (2002) - just to provide one example of the rapid developments across two decades of measuring soil temperature. Many issues had to be addressed, from measurement units that had problems during a few years over floods that allowed fishing in the field to winter-time shifts in Germany and hardware changes. In total, we had over 1.5 Billion individual measurements to be processed. As a computer scientist, handling such a complex data source was mainly my job. Although it probably does not sound particularly exciting to process this dataset, such a process can be quite entertaining. Learning something about the history of the field while watching how this history imprinted itself into the temperature data made the whole process more entertaining than many other endeavors in data science. We also want to emphasize that these interdisciplinary projects can be highly fruitful and should be conducted more frequently. Complex data sources such as ours often end up in the drawer, even though they could help investigate many exciting hypotheses.

Once the data was processed and ready for analysis, we gained some fascinating insights.

Figure 2 | The effects of plant diversity on the soil temperature stability. Daily resolution data were derived by averaging the annual variations, leaving variations of 80 plots and 366 days (n = 29,280). Data with soil temperature at 5 cm depth is shown. A depicts the temperature buffer effect of plant diversity throughout the year. B depicts how the diversity effect (Temperature differences between high and low diversity) is related to the air temperature.

What we found out

To make it as short as possible, we could confirm our hypothesis. We found a strong relationship between plant diversity and soil temperature. Our findings suggest that plant diversity is a natural buffer against soil temperature fluctuations. It helps to prevent soil from overheating during hot spells and maintains heat during colder periods. Importantly, this biodiversity effect persists throughout the year. Further, it becomes more pronounced as the experimental plant communities mature and is particularly stronger under extreme climate conditions, such as scorching hot days. To further understand these findings, we tested possible mechanisms behind this effect. While we found that plant diversity generally increases plant leaf cover, which improves shading, we also found that plant diversity increases the amount of soil organic carbon. The soil organic carbon, in turn, decreases thermal diffusivity. Thermal diffusivity indicates the rate at which a temperature change is transmitted through the soil. Since this causes temperature changes to propagate slower through the soil, the soil temperature in higher plant diversity communities becomes more stable throughout the year.

A multitude of biological factors (such as micro- and macro-organism activities, plant root growth), chemical factors (including cation exchange capacity, soil carbon content, available nutrients, and soil pH), and physical factors (like soil structure, aggregate stability, and soil moisture) are significantly influenced by soil temperature. We estimate that many of plant diversity's previously reported stabilizing effects can be partly explained through its impact on soil temperature.

With these insights, we underscore the potential of utilizing plant diversity as a natural approach to combat climate change. Maintaining a more stable soil environment through plant diversity can play an important role in mitigating potential climate feedback effects and help to ensure the continuous provision of many essential ecosystem functions.

References:

1. MAHECHA, Miguel D., et al. Biodiversity loss and climate extremes—study the feedbacks. Nature, 2022, 612. Jg., Nr. 7938, S. 30-32.
2. WEISSER, Wolfgang W., et al. Biodiversity effects on ecosystem functioning in a 15-year grassland experiment: Patterns, mechanisms, and open questions. Basic and applied ecology, 2017, 23. Jg., S. 1-73.