The study presents first results of the long-term field experiments on PriOrity Effect Mechanisms (POEM), initiated in 2020 and designed to test how priority and year effects modulate the structure and functioning of dry acidic grassland plant communities over time, both aboveground and belowground.

Info: Priority effects
“Priority effects occur when species that arrive and establish before others can significantly influence the establishment and success of species that arrive later, thus also sometimes influencing ecosystem functioning. Priority effects can lead to alternative vegetation states and thus may play a key role in conservation.”

The POEM set-up

At a former agricultural field in Niederhaverbeck, Northern Germany, the research team established the POEM field experiment in 2020. It uses a replicated, multi-year design with different plant functional group (PFG) sowing sequences (grasses, forbs, legumes) and compares plots established in different years to capture weather effects. Using transparent tubes called minirhizotrons and a camera system, they tracked root growth over three years – without ever digging up a plant.

Findings that dig deep

The first results of the long-term experiment are already exciting. First, the scientists found that the time since establishment was the strongest driver of plant community composition, more than the plant functional group (PFG) order of arrival or the year of initiation. Species richness and diversity were influenced by PFG order of arrival, evolving over time and interacting with the year of sowing. Plots where grasses were sown first had lower diversity, due to their dominance (e.g. Bromus hordeaceus).

While root productivity (overall root biomass/density) did not differ significantly between treatments, the vertical distribution of roots was strongly affected: Communities where forbs or legumes were sown first rooted deeper than those where grasses were planted first. These findings, if generally found in other grasslands, could be a useful way to help create plant communities with deeper roots that are more adapted to droughts. Further research in POEM’s third experiment (being set up in 2025) will test this question. Here, minirhizotrons are installed in the soil and will allow the researchers to test whether there is a repeatable effect of the order of arrival on root distribution. Similar findings in a controlled experiment (Alonso-Crospo et al. 2022 Oikos) are promising, but the researchers need to do the hard work of setting up the same experiment again in the field to know to what extent this finding is generalizable.

Another finding of this study was that aboveground productivity was primarily influenced by year of initiation, with 2020 plots being more productive than those from 2021 — possibly due to more favorable weather in the first year. Thus, the year of sowing, reflecting different weather conditions, greatly impacted early community dynamics.

Implications that go beyond the plot

As restoration becomes an increasingly vital tool for biodiversity and climate issues, nuanced insights and recommendations are critical. The POEM experiment will continue to unravel how ecological history, priority effects, functional traits, and weather variation interact in shaping restoration outcomes.

For practitioners, the experiment has clear takeaways. Among other things, it shows that weather conditions in the establishment year strongly influence productivity. This suggests that restoration projects should time sowing to favorable environmental windows when possible. Deeper rooting from forbs- or legumes-first sowing may promote soil stabilization, water access, and long-term persistence in degraded soils. In the face of increasing drought frequency, these communities will likely be more stable and persistent. Furthermore, deeper roots can contribute more to long-term soil carbon pools, which are more stable than surface biomass, promoting natural climate solutions (NCS).

Ecological experiments like this, which may appear solely scientific at first glance, have a strong practical relevance – especially in our times of overlapping ecological crises.

Want to know more about the study? You can find the paper here: https://onlinelibrary.wiley.com/doi/10.1111/jvs.70026

Are you interested in learning more about priority effects? Then read our article on: Breaking Down Barriers: A Call for Cohesion in Priority Effect Studies – Ecologically speaking – Blog on Ecological Research at Leuphana University Lueneburg

The bigger picture

From its definition alone, the term sounds as if it’s quite connected with community ecology. Priority effects are actually studied in this discipline but are also investigated and relevant in many other research areas. However, there are a variety of terms that describe this effect in different contexts. For example, related terms are “alternative stable states” in restoration and community ecology, “induced resistance” in immunology and disease ecology or “monopolization” in evolutionary ecology. The scientists around Stroud et al. (2024), including Vicky Temperton from the Institute of Ecology at Leuphana University, uncovered the lack of conceptual cohesion and differences in language that currently hinder the understanding of priority effects as a broader biological phenomenon. Based on that, they reviewed the variety of biological fields that address priority effects in their research to develop a framework for better understanding the conditions and mechanisms of priority effects, and their consequences across spatial and temporal scales.

Drawing a network

For this review and to build up such a framework, Stroud and colleagues conducted a co-citation network analysis of studies related to priority effects. A co-citation analysis examines how often two studies are referenced together in published works. A network algorithm is used to identify clusters of studies that share more citations within a cluster than between clusters. The results of the analysis clearly show inconsistent cohesion among studies of priority effects and related topics across various biological subfields. In more detail, the analysis revealed four primary clusters of studies that are more frequently co-cited together: (1) general ecology, (2) animal ecology, (3) plant ecology, and (4) evolutionary ecology. Called the “super-cluster”, this well-connected set of disciplines shares a common interest in priority effects and thus stands in contrast to two further, barely connected clusters: (5) parasitology, and (6) polar biology.

Call for cross-cutting communication

The co-citation network analysis underscores the importance of fostering interdisciplinary dialogue and collaboration to advance research on priority effects. As a first step towards overcoming the semantic barriers between the different subfields, Stroud and colleagues have compiled a list of all terms related to priority effects, providing researchers studying priority effects with keywords to work with in the future. This forms the basis for a unified framework promoting a comprehensive understanding of priority effects which then can enhance ecological predictions, facilitate cross-disciplinary research collaboration, improve ecosystem management strategies, and provide practical applications in biomedical and agricultural research, as well as insights into community reassembly under climate change. Stroud and colleagues further aim to support researchers investigating priority effects by encouraging the adoption of findings, research methodologies and conclusions from different disciplines. Additionally, they aim to spark interest among biologists who have not yet come in touch with this concept, encouraging them to explore the potential influence of priority effects within their own studies. Collaborating and communicating insights across disciplines not only enriches our understanding of biological and ecological dynamics but also underscores the interconnectedness of different processes across scales. And: Cross-disciplinary communication is even gaining more and more importance for tackling the complex challenges of our time.

If you are interested in diving deeper into this review and in discovering the subfields and terms connected to priority effects, you can find the paper of Stroud and colleagues here:
Stroud, J. T., Delory, B.M., Barnes, E.M. et al. Priority effects transcend scales and disciplines in biology. Trends in Ecology & Evolution. (2024). https://doi.org/10.1016/j.tree.2024.02.004