Compound Drought-Heatwaves During Establishment Push Perennial Ryegrass Toward Total Dominance in Grassland Mixtures

Managed grasslands cover some 3.37 billion hectares worldwide, roughly 70 percent of all agricultural land and about half of Europe’s farmland. For decades, the trend in these systems was simplification: dense stands of a single grass species, bred for uniform growth and maximum digestibility. In recent years, however, volatile fertiliser prices and rising sustainability demands have pushed farmers and researchers back toward diverse mixtures of grasses, legumes and forbs, which can deliver high yields at lower nitrogen inputs, improve forage quality and fix nitrogen biologically. A growing body of evidence also suggests that diverse swards buffer drought better than monocultures. But a new study raises a troubling caveat: that buffering may hinge on a narrow and vulnerable window, the establishment phase, and compound drought-heatwave events striking during this window can quietly dismantle the very diversity that makes mixtures valuable.

The research, published in the journal Plant and Soil, was led by Sophia Moracchioli Philadelphi of the University of Hohenheim in Germany, together with colleagues at Hohenheim and Aarhus University in Denmark. The team set out to answer a question that most drought experiments have sidestepped: what happens when a compound drought-heatwave, or CDHW, hits a young grassland community that is still assembling itself? Mature swards have been studied extensively, but during establishment the competitive hierarchies, root systems and canopies that underpin complementarity are still forming. Stress at this stage can leave lasting legacies for productivity, species composition and the ecosystem services a sward will deliver for years afterward.

To simulate a realistic western European summer extreme, the researchers ran a 112-day controlled growth-chamber experiment at Hohenheim. They grew mesocosms containing three functionally contrasting forage species: perennial ryegrass (Lolium perenne), a fast-growing grass; white clover (Trifolium repens), a nitrogen-fixing legume; and chicory (Cichorium intybus), a deep-rooted non-leguminous forb. Each species was grown alone, and three mixture types were created: a binary grass-forb mixture and two three-species mixtures with either 33 or 67 percent legume. Climate profiles followed weekly averages recorded at Stuttgart Airport between 1991 and 2005, mimicking a typical regional growing season. For the first 90 days after sowing, all mesocosms were kept identical and well watered at 65 percent of field capacity.

Then came the stress. From day 91, a nine-day CDHW event was imposed in three of the chambers: irrigation was withheld to create an edaphic drought while air temperatures were raised by up to 8 degrees Celsius at the daily maximum and 3.4 degrees at the minimum, with relative humidity cut by 17 percent. The result was a doubling of the vapour pressure deficit, from about 1.1 to 2.21 kilopascals, a measure of how hard the atmosphere pulls water out of leaves. Dry soils suppress evapotranspiration and cloud formation, amplifying surface heating, which is precisely why drought and heat so often arrive together. The team tracked the consequences daily, measuring gross primary production, evapotranspiration, gravimetric soil water content and leaf water potential, then harvested all biomass at the end.

The physiological responses were strikingly species-specific. White clover proved the most resistant: its photosynthetic rate per unit of canopy area barely declined during stress and showed only weak coupling to soil moisture, evapotranspiration or leaf water status. Chicory took the opposite path, losing 24 percent of its normalised gross primary production during the event, the highest hydraulic sensitivity, but then staging the most dramatic recovery, rebounding to an average of 204 percent of control levels once water returned. Perennial ryegrass was the least resistant, its photosynthesis tracking soil drying closely, with reductions of roughly 37 to 47 percent in the grass monoculture and all mixtures. Leaf water potential, a direct indicator of plant hydraulic strain, diverged from control values within just two to four days of stress onset in all three species.

Yet here lay the study’s central paradox: this rich physiological diversity never scaled up to the community level. In mixtures, the photosynthetic trajectories closely followed those of the dominant ryegrass, regardless of whether the more resistant clover or the faster-recovering chicory were present. The authors point to the mass-ratio hypothesis, which holds that ecosystem functioning is governed primarily by the traits of the most abundant species. Because ryegrass already accounted for the bulk of the biomass, the physiological strengths of the subordinate species were simply underrepresented at the canopy scale, and no community-level physiological buffering could be detected.

The biomass data told a subtler story. Across all communities, the CDHW event cut aboveground shoot dry weight by about 20 percent, an estimated mean loss of 147 grams per square metre. All three monocultures ended with harvest ratios significantly below one relative to their controls, with chicory suffering the steepest decline at 41 percent, followed by clover at 28 percent and ryegrass at 20 percent. The mixtures, by contrast, all showed ratios that did not differ significantly from one, indicating greater biomass stability, exactly as the team’s first hypothesis predicted. Net diversity effects, the difference between observed mixture biomass and the sown-proportion-weighted expectation from monocultures, were positive in both climates and grew substantially under stress, rising on average by nearly 88 grams per square metre under CDHW.

But that stability came at a hidden cost. The diversity effects were non-transgressive: mixtures matched, but never exceeded, the best-performing monoculture, which was ryegrass itself. And beneath the stable totals, the community composition was being rewritten. Under control conditions, ryegrass already dominated the mixtures, contributing roughly 69 to 75 percent of shoot dry weight. Under CDHW, its share surged to 94 to 97 percent, a statistically significant reinforcement of grass dominance, while chicory and clover were squeezed down to minor fractions of 1 to 5 percent, though neither species was completely eliminated. Crucially, both subordinates experienced more negative leaf water potentials in mixtures than in their own monocultures during the stress, direct evidence that the dominant grass intensified their hydraulic stress through competition for belowground water.

The mechanism, the authors argue, is structural rather than physiological. Ryegrass entered the stress period with greater canopy development and finished with by far the largest root biomass, about 2,300 grams per square metre, significantly more than any other community. That early structural pre-emption, combined with rapid regrowth capacity, allowed it to convert the disturbance into a competitive legacy. Clover, despite its physiological resistance, lacked the structural reserves to capitalise on it, while chicory lost too much tissue during the event to recover its position. Rapid photosynthetic recovery after rewatering, the study shows, was simply not enough to restore biomass within the experiment’s timeframe, because shallow roots and limited carbon reserves constrained compensatory growth in these young swards.

The implications reach well beyond the growth chamber. A grassland mixture can hold its yield steady through a compound extreme and still lose the functional diversity that underpins biological nitrogen fixation, forage quality and long-term multifunctionality. If CDHW events, which are becoming a defining feature of western European summers, increasingly strike during establishment, farmers may end up with swards that look diverse on the seed label but function as near-monocultures in the field. The authors suggest that management strategies promoting balanced early establishment, such as optimised sowing rates and sowing timing, may be essential if mixtures are to deliver their promised benefits under climate change. Short-term biomass stability, this study makes clear, is no guarantee of compositional stability, and the most important battle for diversity in a grassland may be fought in its first few months of life.

Subject of Research: Effects of compound drought-heatwave events on species interactions and community assembly during grassland establishment

Article Title: Compound drought-heatwave events during establishment reinforce perennial ryegrass dominance in grassland mixtures

Article References: Philadelphi, S. M., Berauer, B. J., Eriksen, J., Schweiger, A., Högy, P., & Malisch, C. S. (2026). Compound drought-heatwave events during establishment reinforce perennial ryegrass dominance in grassland mixtures. Plant and Soil. https://doi.org/10.1007/s11104-026-09168-1

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09168-1

Keywords: grassland ecology, compound drought-heatwave, perennial ryegrass, white clover, chicory, plant diversity, drought resistance, gross primary production, species dominance, establishment phase, climate extremes, ecosystem services