Weathering the storm: plant community flood resilience in intensively managed grasslands and the role of the plant economic spectrum

As flooding events increase in frequency and severity, how will managed grasslands weather the storm? Can we use the traits or ‘strategies’ of the plants that make up these grasslands to predict their resilience? Natalie Oram and colleagues address this issue in their new Journal of Applied Ecology article. Here they discuss their work further.

Long story short: in flooded conditions, resource-conservative plant communities are more resilient than resource-acquisitive communities; better resisting and recovering from flooding. However, we could not predict community resistance or recovery based on the plant community’s resource acquisition strategy in non-flooded conditions.

Our article has a sister paper: Can flood-induced greenhouse gas emissions be mitigated by trait-based plant species choice? These two articles have different focal points but report the same greenhouse experiment. If you would like to have a behind-the-scenes look at the greenhouse experiment reported in these two papers, check out this video:

Do grassland plant communities vary in their flood resilience and is this related to their resource-acquisition strategy?

We were curious and wanted some answers, so we headed to the greenhouse. But first made a quick stop in the field. About 18 months prior to this spark of curiosity, Diego Abalos and colleagues at the Soil Biology Group at Wageningen University had set up a field experiment to test how different intensively managed grassland communities mediated nitrogen cycling. They chose four plant species (Lolium perenne, Festuca arundinacea, Poa trivialis, and Trifolium repens) and grew them in monocultures and all combinations of two and four species. We thought it would be great to take intact monoliths from this field experiment, and thanks to the knowledge and expertise of the Wageningen Unifarm (as well as their impressive backhoe machine), we were able to!

Authors in field
John approaches with the beast, Julia and Evert-Jan taking monoliths, Dina and I getting the monolith into its frame and placed

We collected two monoliths from each of Diego’s field plots (110 monoliths in total) and took them to the greenhouse for a four-month greenhouse experiment. We exposed one monolith from each field plot to a repeated flooding treatment. The paired monolith from the same field plot was maintained at 60% water holding capacity throughout the experiment to serve as a control/baseline. We considered two components of resilience: resistance – how a plant community withstands flooding and recovery – how a plant community returns to the baseline (the sister control monolith, in our case) once the flood ended.

Conceptual figure of plant community response to flooding
Conceptual figure of plant community response to flooding, adapted from Bahn and Ingrisch, 2018:

We considered above-ground biomass (the dry weight of the leaves and shoots) as our indicator of resistance and recovery. Physiologically, this is a good measure as it is an integrative measure of plant stress and nutrient uptake. The downside is that it is partially destructive – limiting how many time-points we can measure. Therefore, in this study, we quantified resistance directly after the flood events and recovery after five weeks. Non-destructive measures to understand when a community returns to baseline (i.e. its recovery rate) will give a more complete picture of resilience. Something on our to-do list for future research!

Monoliths in greenhouse
Monoliths set up in the greenhouse, Monocultures of Trifolium repens and Lolium perenne deal with the flood

We measured leaf and root traits and used these to characterise the plant community’s resource-acquisition strategy, based on the leaf and root economic spectrum frameworks. These suites of traits co-vary along a spectrum from resource-acquisitive (i.e. the ‘live fast and die young’ mantra: grow fast, produce thin, nitrogen-rich tissues) or resource-conservative (i.e. the ‘slow and steady wins the race’ school of thought: slower growing and construct more durable, nitrogen-poor tissues). We then related each community’s resource acquisition strategy to its resistance and recovery to repeated flooding.

Authors in lab
Lisette samples leaves to measure their traits, Veronica and Sigrid measuring leaf saturated weight and leaf area, Diego and Veronica washing roots for root trait measurements.

We found that, in flooded conditions, conservative plant communities were more resistant and recovered better from both flood events than acquisitive communities. However, a community’s ‘inherent’ strategy (measured in the ‘sister’ control/baseline monolith maintained at ambient rainfall conditions) was not related to flood resistance or recovery. This means that either species turnover (a shift in the relative abundance of species in a community) or intraspecific variation (a shift in species’ traits) is causing traits to change in flooded conditions. We found that shifts in community resource-strategy due to species turnover were not related to resistance or recovery, but that shifts due to intraspecific variation were related to resistance and recovery to the second flood.

Thus, a plant community’s strategy can be related to its flood resistance and recovery, but this cannot be predicted based on its strategy in ambient conditions. We show, like studies before us on hydrological or elevation gradients, that intraspecific trait variation plays an important role in explaining community-level trait shifts. These shifts can help explain a plant community’s response to flooding.

Read the full, open access article, Plant community flood resilience in intensively managed grasslands and the role of the plant economic spectrum, in Journal of Applied Ecology.

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