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Litter decomposition is a key process of nutrient and carbon cycling in terrestrial ecosystems. The decomposition process will likely be altered under ongoing climate change, both through direct effects on decomposer activity and through indirect effects caused by changes in litter quality. We studied how hydrological change indirectly affects decomposition via plant functional community restructuring caused by changes in plant species’ relative abundances (community‐weighted mean (CWM) traits and functional diversity). We further assessed how those indirect litter quality effects compare to direct effects. We set up a mesocosm experiment, in which sown grassland communities and natural turf pieces were subjected to different hydrological conditions (dryness and waterlogging) for two growing seasons. Species‐level mean traits were obtained from trait databases and combined with species’ relative abundances to assess functional community restructuring. We studied decomposition of mixed litter from these communities in a common “litterbed.” These indirect effects were compared to effects of different hydrological conditions on soil respiration and on decomposition of standard litter (direct effects). Dryness reduced biomass production in sown communities and natural turf pieces, while waterlogging only reduced biomass in sown communities. Hydrological stress caused profound shifts in species’ abundances and consequently in plant functional community composition. Hydrologically stressed communities had higher CMW leaf dry matter content, lower CMW leaf nitrogen content, and lower functional diversity. Lower CWM leaf N content and functional diversity were strongly related to slower decomposition. These indirect effects paralleled direct effects, but were larger and longer‐lasting. Species mean traits from trait databases had therefore considerable predictive power for decomposition. Our results show that stressful soil moisture conditions, that are likely to occur more frequently in the future, quickly shift species’ abundances. The resulting functional community restructuring will decelerate decomposition under hydrological stress., We show for the first time not only that community‐weighted mean functional traits and functional diversity rapidly change under hydrological stress (dryness and waterlogging), but also that these shifts strongly decelerate litter decomposition. These indirect effects on litter decomposition via changes in litter quality parallel direct effects on decomposer activity, but are even larger in magnitude and last longer. To our knowledge, this is the first study showing a link between effects of hydrological conditions on functional community structure with consequences for an ecosystem process, such as litter decomposition.

Hydrology is a major environmental factor determining plant fitness, and hydrological niche segregation (HNS) has been widely used to explain species coexistence. Nevertheless, the distribution of plant species along hydrological gradients does not only depend on their hydrological niches but also depend on their seed dispersal, with dispersal either weakening or reinforcing the effects of HNS on coexistence. However, it is poorly understood how seed dispersal responds to hydrological conditions. To close this gap, we conducted a common‐garden experiment exposing five wind‐dispersed plant species (Bellis perennis, Chenopodium album, Crepis sancta, Hypochaeris glabra, and Hypochaeris radicata) to different hydrological conditions. We quantified the effects of hydrological conditions on seed production and dispersal traits, and simulated seed dispersal distances with a mechanistic dispersal model. We found species‐specific responses of seed production, seed dispersal traits, and predicted dispersal distances to hydrological conditions. Despite these species‐specific responses, there was a general positive relationship between seed production and dispersal distance: Plants growing in favorable hydrological conditions not only produce more seeds but also disperse them over longer distances. This arises mostly because plants growing in favorable environments grow taller and thus disperse their seeds over longer distances. We postulate that the positive relationship between seed production and dispersal may reduce the concentration of each species to the environments favorable for it, thus counteracting species coexistence. Moreover, the resulting asymmetrical gene flow from favorable to stressful habitats may slow down the microevolution of hydrological niches, causing evolutionary niche conservatism. Accounting for context‐dependent seed dispersal should thus improve ecological and evolutionary models for the spatial dynamics of plant populations and communities., To test how seed dispersal by wind responds to abiotic contexts, we estimate fecundity and seed dispersal of five plant species under various hydrological conditions by combining common‐garden experiments and mechanistic modeling, and quantified the relationship between seed production and seed dispersal. There was a general positive relationship between seed production and dispersal distance. The positive relationship between seed production and dispersal may reduce the concentration of each species to the environments favorable for it, thus counteracting species coexistence.

Successful alien plant invasion is influenced by both climate change and plant-plant interactions. We estimate the single and interactive effects of competition and extreme weather events on the performance of the global legume invader Lupinus polyphyllus (Lindl.). In three experimental studies we assessed (i) the stress tolerance of seedling and adult L. polyphyllus plants against extreme weather events (drought, fluctuating precipitation, late frost), (ii) the competitive effects of L. polyphyllus on native grassland species and vice versa, and (iii) the interactive effects of extreme weather events and competition on the performance of L. polyphyllus. Drought reduced growth and led to early senescence of L. polyphyllus but did not reduce adult survival. Fluctuating precipitation events and late frost reduced the length of inflorescences. Under control conditions, interspecific competition reduced photosynthetic activity and growth of L. polyphyllus. When subjected to competition during drought, L. polyphyllus conserved water while simultaneously maintaining high assimilation rates, demonstrating increased water use efficiency. Meanwhile, native species had reduced performance under drought. In summary, the invader gained an advantage under drought conditions through a smaller reduction in performance relative to its native competitors but was competitively inferior under control conditions. This provides evidence for a possible invasion window for this species. While regions of high elevation or latitude with regular severe late frost events might remain inaccessible for L. polyphyllus, further spread across Europe seems probable as the predicted increase in drought events may favour this non-native legume over native species.

Climate change is altering precipitation patterns, with higher frequency and magnitude of extreme events. Specifically, longer and more pronounced waterlogged conditions are predicted after rain spells as well as more frequent droughts, especially in Central Europe. Such hydrological changes can severely affect species performance and alter the function of ecosystems, as well as favor plant invasions. Competition with native communities may change depending on water stress. Bunias orientalis is an invasive plant that may benefit from disturbance or precipitation changes. We conducted a 3-year mesocosm experiment in a common garden to investigate how invasion success of B. orientalis in native German grassland communities is affected by varying hydrological conditions (from very dry to waterlogged). We measured the establishment and growth of B. orientalis in varying water table depths in bare soil (simulating disturbance) vs. in the community. Establishment and biomass of B. orientalis was generally highest under non-stress conditions. The species was also highly tolerant to dry conditions, but only when growing in bare soil. However, performance of B. orientalis was generally low, whereby interspecific competition in communities greatly limited invasion success. This might be due to the low competitive ability of the species in conditions of hydrological stress and the near-natural grassland communities with an extensive mowing regime used in our experiment. Our results suggest that invasion success of B. orientalis in grasslands will not increase if precipitation patterns change toward more extreme events. However, disturbance that creates bare soil patches might favor B. orientalis under drought conditions.

Sopotlieva, Desislava/0000-0002-9281-7039; Guler, Behlul/0000-0003-2638-4340; Velev, Nikolay/0000-0001-6812-3670; De Boeck, Hans/0000-0003-2180-8837; Kreyling, Juergen/0000-0001-8489-7289; Bahn, Michael/0000-0001-7482-9776; Dengler, Jurgen/0000-0003-3221-660X; Nijs, Ivan/0000-0003-3111-680X; Wellstein, Camilla/0000-0001-6994-274X; Apostolova, Iva/0000-0002-2701-175X; Stampfli, Andreas/0000-0002-5517-1363; Picon-Cochard, Catherine/0000-0001-7728-8936; Bloor, Juliette M.G./0000-0002-8668-1323; Arfin Khan, Mohammed Abu Sayed/0000-0001-6275-7023; Campetella, Giandiego/0000-0001-6126-522X WOS:000413145900006 PubMed ID: 28941071 Biodiversity can buffer ecosystem functioning against extreme climatic events, but few experiments have explicitly tested this. Here, we present the first multisite biodiversityxdrought manipulation experiment to examine drought resistance and recovery at five temperate and Mediterranean grassland sites. Aboveground biomass production declined by 30% due to experimental drought (standardised local extremity by rainfall exclusion for 72-98 consecutive days). Species richness did not affect resistance but promoted recovery. Recovery was only positively affected by species richness in low-productive communities, with most diverse communities even showing overcompensation. This positive diversity effect could be linked to asynchrony of species responses. Our results suggest that a more context-dependent view considering the nature of the climatic disturbance as well as the productivity of the studied system will help identify under which circumstances biodiversity promotes drought resistance or recovery. Stability of biomass production can generally be expected to decrease with biodiversity loss and climate change. ERA-Net BiodivERsA [01LC1201]; Belgian Federal Science Policy Office (BELSPO)Belgian Federal Science Policy Office; German Federal Ministry of Education and Research (BMBF)Federal Ministry of Education & Research (BMBF); Bulgarian Science Found; Ministere de l'Ecologie, du Developpement durable et de l'Energie (France) This work is part of the project SIGNAL which is funded by the ERA-Net BiodivERsA (http://www.biodiversa.org; Funding ID 01LC1201), with the national funders Belgian Federal Science Policy Office (BELSPO), German Federal Ministry of Education and Research (BMBF), Bulgarian Science Found and Ministere de l'Ecologie, du Developpement durable et de l'Energie (France) as part of the 2011-2012 BiodivERsA call for research proposals. For site FR, we thank P. Pichon, A. Salcedo, J. Pottier, M.E. Lefranc, M. Michaux, A. Vernay and G. Zannin for help during the course of the experiment. We thank three anonymous referees for insightful and constructive comments on an earlier version of this manuscript.

Extreme drought events challenge ecosystem functioning. Ecological response to drought is studied worldwide in a growing number of field experiments by rain-out shelters. Yet, few meta-analyses face severe challenges in the comparability of studies. This is partly because build-up of drought stress in rain-out shelters is modified by ambient weather conditions. Rain-out shelters can further create confounding effects (radiation, temperature), which may influence plant responses. Yet, a quantification of ecophysiological effects within rain-out shelters under opposing ambient weather conditions and of microclimatological artifacts is missing. Here, we examined phytometers—standardized potted individuals of Plantago lanceolata—under rain-out shelter, rain-out shelter artifact control, and ambient control during opposing outside microclimatological conditions. Furthermore, we tested for artifacts of rain-out shelters on plant responses in a long-term semi-natural grassland experiment. Phytometer plants below the rain-out shelters showed lower stomatal conductance, maximum quantum efficiency, and leaf water potential during warm ambient conditions with high evaporative demand than during cold conditions with low evaporative demand. Plant performance was highly correlated with ambient temperature and vapor pressure deficit (VPD). Rain-out shelter artifacts on plant responses were nonsignificant. Rain-out shelters remain a viable tool for studying ecosystem responses to drought. However, drought manipulations using rain-out shelters are strongly modified by ambient weather conditions. Attributing the results from rain-out shelter studies to drought effects and comparability among studies and study years therefore requires the quantification of the realized drought stress, for example, by relating ecosystem responses to measured microclimatological parameters such as air temperature and VPD.

Summary Climate change is predicted to increase the frequency and magnitude of extreme climatic events. These changes will directly affect plant individuals and populations and thus modify plant community composition. Little is known, however, about transgenerational effects (i.e. the influence of the parental environment on offspring phenotype and performance beyond the effects of transmitted genes) of climate extremes and community composition. Perennial plants have been particularly neglected. This impedes projections on species adaptations and population dynamics under climate change. Maternal plants of two widespread dwarf-shrub species (Genista tinctoria and Calluna vulgaris) recurrently experienced extreme weather event manipulations each year (drought and heavy rain). To test for transgenerational effects of community composition, C. vulgaris maternal plants were grown in communities differing in the number of neighbouring species. After 6 years, seeds of maternal plants were collected at least 2 months after the final weather treatments. We assessed transgenerational effects of the extreme events and of altered community composition on germination and monitored the development of offspring over 2 years. We show that extreme events experienced by maternal plants influence offspring germination and growth beyond the seedling stage. Seeds produced by maternal plants experiencing stress, indicated by increased tissue dieback, germinated earlier in both observed species. We observed differences in leaf stoichiometry and growth rates for G. tinctoria offspring throughout the first year: Offspring from heavy rain-treated mothers showed reduced leaf C:N ratio and higher growth rates. Results further indicate that not only community density, as investigated in prior studies, but also community composition trigger transgenerational effects. Synthesis. Our findings show that variation in the maternal environment not only affects the number, but also the performance of offspring. Extreme climatic events, terminated before seed set, induce transgenerational effects. Species richness of mother communities can affect the stress level of target species and thereby germination regardless of community density. In contrast to prior studies, which revealed direct effects of chronic stress on plant individuals, this study emphasizes the importance of addressing transgenerational effects of extreme weather events when projecting future ecological responses and adaptation to climate change.

Drought stress is one of the major stressors for plants under climate change, depressing growth through inhibition of photosynthesis and causing oxidative cell damage. It has been shown that plants can form a drought stress memory, improving their performance under recurring drought stress after they have been primed by drought before. Mechanisms underlying such a drought stress memory and possible cross-stress tolerance (improved performance to drought after plants have been stressed by other stressors) are currently unclear. We aimed to test whether previous waterlogging stress and drought stress leads to improved performance and changes in morphological, photosynthetic and anti-oxidative parameters under recurring drought. Therefore, we repeatedly subjected Alopecurus pratensis grass plants to waterlogging and drought over two years. In the third year, plants were well-watered for three weeks to recover and then subjected to drought stress for two weeks. Plants primed with drought before showed less tissue damage and higher levels of Rubisco content, anti-oxidative enzymes (POX, SOD) and chlorophyll b after the recovery and the drought period. We show for the first time a long-term drought stress memory in a grass species, lasting over several weeks. Our results indicate that drought priming enhances the activity of anti-oxidative enzymes, which is key for depressing oxidative damage and for improving tolerance to subsequent drought stress in A. pratensis.

Under climate change, both wetter and drier conditions, as well as an increase in extreme events like floods or droughts are projected for many areas. So far, studies only investigate the impact of drier or wetter conditions at a single stress severity level but do not consider how different intensities and types of changes affect insect herbivores feeding on stressed plants. Further, how effects of acute stress pulses differ from milder, chronic soil moisture stress is unclear. We investigated how changing soil moisture conditions affect a generalist insect herbivore feeding on grassland plants. We transplanted multi-species sections of grassland into pots and subjected them to different intensities and durations of flooding and drying stress. We compared effects of short, extreme drought and flooding pulses against the effects of milder, but chronic stress. Constantly drier conditions decreased plant and herbivore performance at all levels of stress severity. Severe permanent wetness did not affect plant growth, but decreased pupal weight (− 23%) and survival of larvae (− 34%). Extreme pulsed drought exacerbated negative effects of chronic drying, as most larvae died before they could benefit from rewetting plants after the drought (94% mortality). Pulsed flooding did not affect plants or larval development more than chronic severe wetness. Our findings imply that plant stress negatively affects generalist chewing herbivores, even with mixed diets. Both drier and severely wet, but not mildly wetter conditions, will reduce survival of some species. Especially, extreme droughts appear to have strong negative effects on generalist grassland herbivores.

Winter air temperature variability is projected to increase in the temperate zone whereas snow cover is projected to decrease, leading to more variable soil temperatures. In a field experiment winter warming pulses were applied and aboveground biomass and root length of four plant species were quantified over two subsequent growing seasons in monocultures and mixtures of two species. The experiment was replicated at two sites, a colder upland site with more snow and a warmer, dryer lowland site. Aboveground biomass of Holcus lanatus declined (−29 %) in the growing season after the warming pulse treatment. Its competitor in the grassland mixture, Plantago lanceolata, profited from this decline by increased biomass production (+18 %). These effects disappeared in the second year. There was a strong decline in biomass for P. lanceolata at the lowland site in the second year. These two species also showed a decline in leaf carbohydrate content during the manipulation. Aboveground productivity and carbohydrate content of the heathland species was not affected by the treatment. The aboveground effects of the treatment did not differ significantly between the two sites, thereby implying some generality for different temperate ecosystems with little and significant amount of snowfall. Root length increased directly after the treatment for H. lanatus and for Calluna vulgaris with a peak at the end of the first growing season. The observed species-specific effects emphasize the ecological importance of winter temperature variability in the temperate zone and appear important for potential shifts in community composition and ecosystem productivity.

The frequency and magnitude of extreme weather events such as drought are expected to increase in the future. At present, plant responses to recurrent extreme events have been sparsely examined and the role of stress history on subsequent stress response has been widely neglected. In a long-term field experiment, we investigated the response of grassland and heath communities to a very severe drought event, which exceeded the duration of projected drought scenarios. During the preceding 6 years, the plant communities experienced scenarios of varying water supply, including annually recurring drought, heavy rain, regular watering, and natural drought periods. Single species and plant communities that were regularly watered in the preceding years revealed the highest tissue die-back under a very severe drought when compared to plants that experienced mild or severe drought stress before. Contrary to expectations, the root to shoot ratio did not increase due to previous recurrent drought occurrences. Furthermore, pre-exposure effects on Vaccinium myrtillus and Plantago lanceolata tissue die-back and reproductive biomass (P. lanceolata) were altered by community composition. Recurrent mild drought stress seems to improve drought resistance of plant communities and species. Potential reasons could be epigenetic changes or soil biotic legacies. Morphological legacies such as altered root to shoot ratio did not play a role in our study. Imprinting events which trigger this ecological stress memory do not have to be extreme themselves. Thresholds, longevity of effects, and the role of biodiversity shown by the importance of community composition require further attention.

Under climate change, not only the magnitude, but also the frequency of extreme weather events is predicted to increase. Such repeated climate stress events may cause fundamental shifts in species compositions or ecosystem functioning. Yet, few studies document such shifts. One reason for higher stability of ecosystems than previously expected may be ecological stress memory at the single plant level. Ecological stress memory is defined here as any response of a single plant after a stress experience that modifies the response of the plant towards future stress events including the mode of interaction with other ecological units. Ecological stress memory is assessed on a whole plant level in ecological relevant parameters. It is therefore one important aspect of the broader concept of ecological memory that refers to whole communities and ecosystems. Here, we present studies which indicate the existence of ecological stress memory within single plants after drought, frost or heat stress. Possible mechanisms underlying an ecological stress memory are the accumulation of proteins, transcription factors or protective metabolites, as well as epigenetic modifications or morphological changes. In order to evaluate the importance of stress memory for stabilizing whole ecosystems and communities in times of climate change, cooperation between ecologists and molecular biologists is urgently needed, as well as more studies investigating stress memory on a single plant level. Only then the potential of plant stress memory for stabilizing ecosystems in times of intensifying climatic extremes can be evaluated and taken into account for measures of mitigation and adaptation to climate change.

Water availability and plant community composition alter plant nutrient availability and the accumulation of plant defence compounds therefore having an impact on herbivore performance. Combined effects of drought stress and plant community composition on leaf chemicals and herbivore performance are largely unexplored. The objective of our study was, therefore, to find out the impact of extreme drought and of plant community composition on plant–herbivore interactions. Larvae of the generalist butterfly Spodoptera littoralis were reared on leaves of the grass Holcus lanatus which was grown in experimental communities, differing in species- and functional group richness. These communities were either subjected to extreme drought or remained under ambient climatic conditions. Drought decreased relative water content, soluble protein content, nitrogen and total phenol content and increased the content of carbohydrates in the grass. As a consequence, the larvae feeding on drought-exposed plants revealed a longer larval stage, increased pupal weight and higher adult eclosion rates. Plant community composition mainly caused changes to the defensive compounds of the grass, but also marginally affected protein and carbohydrate content. Larvae feeding on species-richest communities without legumes showed the highest mortality. Our findings imply that climate change that is projected to increase the frequency of severe droughts, as well as alter plant community compositions, is likely to affect arthropod–plant interactions through an alteration of leaf chemicals.

The frequency of extreme drought events is projected to increase under global climate change, causing damage to plants and crop yield despite potential acclimation. We investigated whether grasses remain acclimated to drought even after a harvest and remember early summer drought exposure over a whole vegetation period. For this, we compared the response of Arrhenatherum elatius plants under a second, late, drought (they were pre-exposed to an early drought before), to plants exposed to a single, only late, extreme drought. Surprisingly, the percentage of living biomass after a late drought increased for plants that were exposed to drought earlier in the growing season compared to single-stressed plants, even after harvest and resprouting after the first drought. Relative leaf water content did not differ between the two treatments. Net photosynthesis was non-significantly reduced by 25% in recurrent drought treatment. Maximum quantum efficiency ( F v / F m ) and maximum fluorescence ( F m ) were reduced in plants that were exposed to recurrent drought. These findings indicated improved photoprotection in double-stressed plants. Our results provide first hints towards a “drought memory” over an entire vegetation period, even after harvest and resprouting. However, the advantage of improved photoprotection might also cause reductions in photosynthesis that could have adverse effects on crop yield under more severe or longer droughts.

Little is known about direct and indirect effects of extreme weather events on arbuscular mycorrhizal fungi (AMF) under field conditions. In a field experiment, we investigated the response of mycorrhization to drought and heavy rain in grassland communities. We quantified AMF biomass in soil, mycorrhization of roots of the grass Holcus lanatus and the forb Plantago lanceolata, as well as plant performance. Plants were grown in four-species communities with or without a legume. We hypothesised that drought increases and heavy rain decreases mycorrhization, and that higher mycorrhization will be linked to improved stress resistance and higher biomass production. Soil AMF biomass increased under both weather extremes. Heavy rain generally benefitted plants and increased arbuscules in P. lanceolata. Drought neither reduced plant performance nor root mycorrhization. Arbuscules increased in H. lanatus several weeks after drought, and in P. lanceolata several weeks after heavy rain spells. These long-lasting effects of weather events on mycorrhization highlight the indirect influence of climate on AMF via their host plant. Legume presence increased plant community biomass, but had only minor effects on mycorrhization. Arbuscule colonisation was negatively correlated with senescence during the dry summer. Mycorrhization and biomass production in P. lanceolata were positively related. However, increased mycorrhization was related to less biomass in the grass. AMF mycelium in soil might generally increase under extreme events, root colonisation, however, is host species specific. This might amplify community shifts in grassland under climate change by further increasing stress resistance of species that already benefit from changed precipitation.

Adaptation to the adverse effects of climate change is being investigated more and more through the introduction of species from warmer and drier climates, such as the (sub-) mediterranean Pinus nigra to dry sites in temperate Central Europe. Winter survival, however, may pose a serious threat to this strategy as cold extremes, which naturally determine the poleward range limits of forest trees, are not expected to follow the general warming trend in the near future. Here, juveniles of P. nigra from eight provenances throughout Europe were exposed to different climate change scenarios (factorial combinations of 42 days of drought and warming by 1.6 °C) in a common garden experiment in Bayreuth, Germany. Cold hardiness (LT50) was determined by the Relative Electrolyte Leakage method (REL) in two consecutive winters. Cold hardiness of foliage differed by 10 °C between the provenances studied and a local adaptation to minimum temperature was found. Cold hardiness was further affected by extreme summer drought, increasing cold hardiness by 3.9 °C on average in the subsequent winter, and by summer warming, increasing cold hardiness by 3.4 °C. Year-round warming had no significant effect on cold hardiness. Cold hardiness was related to the content of soluble carbohydrates and to the composition of fatty acids and alkanes in the needles. Juveniles of P. nigra exhibited a comparable cold hardiness as juveniles of species native to Central Europe ( Pinus sylvestris , Picea abies , Fagus sylvatica and Quercus petraea ) under the same climatic conditions. Cold hardiness of the fine roots of P. nigra averaged −16.5 °C compared to −23.8 °C on average for needles. Our results imply that the cold hardiness of the foliage is adaptive to both long-term growing conditions at the seed origin (genetic heritage) and short-term alterations of these conditions (individual plasticity), while first hints suggest that cold hardiness of the roots is high and might not be adaptive. For P. nigra , below- and above-ground cold hardiness of selected provenances in mid-winter appear suitable for cultivation in temperate regions.