1 INTRODUCTION

Droughts are projected to increase in frequency and severity globally with climate change (Dai, 2013). Ecosystem carbon (C) and nitrogen (N) cycles are affected by drought in a number of ways, including through effects on vegetation (Wu et al., 2011). Studies of plant traits have contributed to a better understanding of both the response of vegetation to environmental variation and the effect of vegetation on ecosystem functions (Funk et al., 2017; Lavorel & Garnier, 2002). Specifically, traits related to the ‘resource economic spectrum’ can be used to characterize trade-offs between resource-acquisitive and resource-conservative growth strategies (Reich, 2014).

Drought can alter plant community traits by affecting community composition and structure with some species being more susceptible to drought than others (Fry et al., 2013). Additionally, drought can induce trait changes within species due to phenotypic plasticity, the process by which a single genotype presents different forms, phenologies and physiologies depending on the environmental conditions (Sultan, 2000). Phenotypic plasticity can, in turn, have effects on plant–soil C and N cycling (de Vries et al., 2016). Plants have evolved traits that allow them to cope with drought through avoidance and/or tolerance strategies (Lambers et al., 2008) with phenotypic plasticity contributing to their responses (Lozano et al., 2020). Plants with an avoidance strategy have traits that increase water uptake and/or reduce losses, for example: (i) Resource-conservative root and shoot tissues with high leaf/root dry matter content (LDMC/RDMC), high lignin content and low specific leaf area (SLA), specific root length (SRL) and tissue N content can reduce water loss (Fort et al., 2013). (ii) Resource-acquisitive fine roots with a high SRL can improve water uptake (Fort et al., 2013; Padilla et al., 2013). (iii) An accumulation of non-structural carbohydrates (NSCs) in shoots and roots can reduce water loss by lowering plant tissue osmotic potential and also affect morphological traits, for example, increase LDMC and RDMC (Zwicke et al., 2015). (iv) A high root to shoot ratio can increase water uptake (Poorter et al., 2012). Plants with a tolerance strategy have the capacity to re-grow after drought, for example, by accumulating osmoprotectant NSC in tissues, protecting the plant against cell damage and facilitating re-growth (Zwicke et al., 2015). Intense water stress may, however, inhibit plant growth to such a degree that plant trait plasticity is mostly related to reduced growth, rather than being an adaptive response to drought. In this case, drought may lead to higher SLA, lower LDMC/RDMC and higher tissue N content (de Vries et al., 2016). In summary, drought can induce phenotypic trait plasticity in different directions, depending on the plant's drought strategy and on the severity of the drought stress.

Plant trait plasticity can potentially affect ecosystem processes, including litter decomposition. Litter decomposition is an important component of ecosystem C and N cycling, as it influences nutrient availability for plants and microbes and affects microbial community composition and C storage (De Deyn et al., 2008). Rates of litter decomposition depend on several interacting factors, including litter quality, climate, soil conditions and decomposer communities. However, results from a meta-analysis of decomposition experiments across biomes on six continents suggest that the influence of litter quality is larger than the influence of climatic variation (Cornwell et al., 2008). Even though chemical traits of litter differ from those of fresh plant material due to senescence and nutrient resorption (Orwin et al., 2010; Quested et al., 2003), a considerable part of the variation in root and shoot litter decomposability between species can often be linked to easily measurable traits of fresh plant material, such as LDMC and leaf nitrogen content (Bumb et al., 2018; Cornwell et al., 2008; Fortunel et al., 2009; Kazakou et al., 2009). Additionally, studies have shown a variety of chemical compounds to be important in explaining differences in litter decomposability between species, such as cellulose, lignin and a range of NSCs (Gunnarsson et al., 2008) and plant secondary metabolites (Chomel et al., 2016). Root decomposability is also likely linked to traits, even though less research has been conducted and the results are somewhat conflicting. For example, in Mediterranean herbaceous species, fine root decomposability was related to root chemical traits (phosphorus, NSC and hemicellulose), but not to morphological traits (Birouste et al., 2012). In contrast, temperate tree root decomposition was correlated with root diameter, root hemicellulose and NSC, but not with root lignin (Hobbie et al., 2010).

Drought can induce phenotypic plasticity in the traits that are correlated with differences in decomposition rates, such as LDMC/RDMC, and content of N, lignin, cellulose and NSC. However, to our knowledge, the consequences of drought-induced trait plasticity on litter decomposability have so far only been experimentally tested in one study on the roots of four tree species (Carrillo et al., 2022). The aim of our study was to investigate whether drought-induced plasticity of root and shoot traits alters their decomposability in three common European temperate grassland species, as grasslands cover more than a third of the global land surface (Suttie et al., 2005) and hold large C stocks (Read et al., 2001). We grew a grass Lolium perenne, a forb Plantago lanceolata and a legume Trifolium repens in the greenhouse and subjected them to a 5-week experimental drought. At the end of the drought, a range of root and shoot traits were measured and a decomposition assay of senesced root and shoot material was conducted. The following hypotheses were tested: (1) Shoot traits of all three species will shift to a more resource conservative strategy in response to drought. (2) Root traits will either shift to a more resource conservative strategy or to a more resource acquisitive strategy in response to drought, depending on plant species. (3) A shift to more conservative root or shoot traits will lead to slower litter decomposition while a shift to more acquisitive traits will lead to faster litter decomposition.