1 Introduction
Climate change, such as extreme drought and rainfall, varies spatially and temporally (IPCC 2007; Benestad et al., 2012). Such changes significantly affect plants. For example, global warming caused substantial damage on plants due to the drought and heat stress (Lipiec et al., 2013; Elst et al., 2017). Rainfall variation impacted terrestrial ecosystems since water limited plant growth, reproduction and productivity (De Boeck et al., 2017). Resource allocation is a vital strategy for plants to respond to the environmental variation (Harper et al., 1970; Mokany et al., 2006). For instance, the energy originated from photosynthesis of a plant can be allocated between aboveground and belowground organs such as leaves and roots. The former enables plants to do photosynthesis, while the latter could store resources for plants to grow. Biomass allocation between aboveground and belowground organs is known as root: shoot ratio (i.e. R: S, Guo et al., 2007). Several studies explored the effects of climate change on plants (Bai et al., 2008; Gonzalez-Dugo et al., 2010; Knapp et al., 2008, 2017). However, contrasting results were found in plant growth and productivity, which merits further research.
Previous studies found that abiotic factors such as temperature and rainfall impacted the R: S (Fay et al., 2003; Yang et al. 2018). For example, rainfall affected the biomass allocation and the belowground ecological processes of plants (Fay et al., 2003), where plants allocated more biomass to roots in order to explore soil water when they growing on dry conditions (Bray 1963), while they allocated more biomass to shoots in order to seize the light when plants growing on wet conditions (Villar et al., 1998). Moreover, higher levels of soil heterogeneity increased R: S (Michael et al., 2004; Wu et al., 2014; Liu et al., 2017a), where plants growing on patches with low quality grew more roots into their neighboring patches with high quality (Liu et al., 2017b, Liu et al., 2019). However, effects of biotic factors on biomass allocation were complicated. Some studies found that R: S was not affected by the aboveground competition (Zhang et al., 2014), and belowground competition did not increase the biomass allocation to roots. However, some other studies found that R: S was influenced by the type of grasslands (Coupland 1980) and plant growth (Gedroc et al., 1996; James et al., 2003). Thus, further studies are needed.
Climate changes such as extreme drought are predicted to be more frequency (Benestad et al., 2012; Felton et al., 2019). Thus, it is crucial to explore the effects of such changes on plant productivity and their allocation strategies (Bai et al., 1997; Cai et al., 2005 Lv et al., 2016). Here an experiment was conducted to explore the effects of water availability on biomass allocation of plants at the population scale, where 4 plant species (Leymus chinensis , Stipa grandis , Artemisia frigida , Potentila acaulis ) that dominant at degraded grasslands in Inner Mongolia steppe were treated with 8 levels of water additions, which was set to simulate the rainfall scenarios in the face of climate change. Note that the degradation of grasslands in Inner Mongolia are caused by activities such as grazing (Liu et al., 2006, 2007), where L. chinensis and S. grandis are the two dominant species in the lightly degraded grasslands (Li et al., 2005), while A. frigida and P. acaulis are the dominant species in the heavily degraded grasslands. We expect that species at the lightly degraded grasslands will be more sensitive to the rainfall variation since they grow fast and tend to have relatively larger plant sizes, while species at the heavily degraded grasslands could withstand the water additions as they tend to grow slowly and to have relatively smaller plant sizes (Ma 2015). Specially, the biomass allocation between the aboveground and belowground organs of L. chinensis and S. grandis are expected to vary with water availability; while water availability is expected to not impact the biomass allocation of A. frigida and P. acaulis . This study can improve our understanding of grassland management and the restoration of degraded grasslands, especially in the face of climate change.