Yield of temperate forage grassland species is either largely resistant or resilient to experimental summer drought

Abstract

Summary1. Due to climate change, an increasing frequency and severity of drought events are expected to impair grassland productivity, particularly of intensively managed temperate grasslands. 2. To assess drought impacts, a common field experiment to manipulate precipitation was set up at three sites (two Swiss and one Irish) using monocultures and mixtures with two and four key forage species. Species differed in their functional traits: a shallow-rooted nonlegume (Lolium perenne L.), a deep-rooted non-legume (Cichorium intybus L.), a shallowrooted legume (Trifolium repens L.) and a deep-rooted legume (Trifolium pratense L.). A 9-week summer drought was simulated, and soil water status, above-ground biomass yield and plant nitrogen (N) limitation were compared to a rainfed control. 3. Based on soil water measurements, the drought induced severe stress at both Swiss sites and extreme stress at the Irish site. Under severe stress, the legumes were more drought resistant and showed an average change in above-ground biomass (CAB, compared to rainfed control) of only À8% and À24% (for the two Swiss sites), while the non-legumes had an average CAB of À51% and À68%. The lower resistance of non-legumes coincided with an apparent limitation of plant N, which further increased under drought. Under extreme drought (Irish site), growth nearly ceased with an average CAB of À85%. 4. During a 6-week post-drought period with adequate water supply (Swiss sites), formerly drought-stressed species were highly resilient and either attained (legumes) or clearly outperformed (non-legumes) the yield level of the rainfed controls. This outperformance coincided with post-drought reductions in N limitation in formerly drought-stressed species. As a result, aggregated over the drought and the post-drought periods, a negative drought impact was found only for the shallow-rooted L. perenne at one of the severely stressed sites. 5. Significant overyielding by multispecies mixtures was evident under rainfed control conditions (+38% across all three sites, P < 0Á05) and was equally apparent under severe drought (+50%, P < 0Á05). This overyielding was greatest in mixtures with approximately equal species proportions and was sufficiently large that drought-stressed mixtures at least attained the same yield as the average of the rainfed monocultures. Under extreme drought, growth almost ceased in monocultures and mixtures. 6. Synthesis and applications. Yields of selected species of intensively managed temperate grasslands are either resistant to a single severe drought or are highly resilient as soon as soil moisture levels recover after the drought event. However, these forage species seem unable to 2016, 53, 1023-1034 doi: 10.1111/1365-2664.12694 cope with an extreme drought event. Combining species in mixtures can compensate for yield reductions caused by severe drought and it offers a practical management tool to adapt forage production to climate change. Journal of Applied Ecology

Severe water deficit restricts biomass production of Lolium perenne L. and Trifolium repens L. and causes foliar nitrogen but not carbohydrate limitation

“…This indicates severe tissue dehydration in T. repens (Lucero et al 1999). Thus, while T. repens might have an advantage over L. perenne under moderate drought events and predominant N limitation, it seems severely and at least equally impaired like the grass species under increasing water shortage and severe drought, a reaction that has recently been shown in a multi-site study (Hofer et al 2016). These results also indicate that the applied drought treatment resulted in an extreme stress for these fast growing, shallow-rooted forage species adapted to humid temperate climate.…”

“…This species had larger stomatal conductance and produced more living biomass than L. perenne under rainfed control conditions (Fig. 4 b) and also exhibited superior growth under moderate drought (Hofer et al 2016), most probably resulting from its benefit of symbiotic N 2 fixation (Hofer et al 2017). Thus, T. repens seems well adapted to moderate drought events (soil matric potentials below −1.5 MPa for less than 30 days, Hofer et al 2017) where N limitation plays a major role.…”

“…A ten-week summer drought event was induced from June 5th to August 14th with precipitation being excluded completely for the drought treatment using rainout shelters. Shelters were 3 m × 5.5 m and covered with a transparent and ultraviolet light-transmissible plastic foil (see Hofer et al 2016 for technical details). Sixty-four and 120 mm of precipitation were excluded during the first and the second half of the drought period, respectively, each half lasting for five weeks.…”

“…The effect of drought on soil water availability is usually most pronounced in the top soil layers (Hofer et al 2016), and the shallow-rooted species L. perenne and T. repens might be particularly affected by persisting drought. Both species have about 90% of their root mass down to 20 cm deep (Hofer et al 2017), and their water (and nitrogen) uptake seems to primarily occur in the top-most 10 cm of the soil (Hoekstra et al 2014;Hoekstra et al 2015;Prechsl et al 2015).…”

Forage legumes for future dry climates: Lower relative biomass losses of minor forage legumes compared to Trifolium repens under conditions of periodic drought stress

“…However, in future, legumes will benefit from the predicted increase of atmospheric CO 2 concentrations due to higher biological N 2 ‐fixation (Morison & Gifford, ; Soussana, Graux, & Tubiello, ). Recent studies provide evidence for improved elasticity towards drought stress by combining species in mixtures of differing rooting depth (Hofer et al., ). Thus, with the inclusion of minor forage species in mixtures, the resilience of grass‐based systems may be improved, especially in regions suffering from low water availability (Ergon et al., ; Frame et al., ).…”