Neighbour effects on plant biomass and its allocation for forbs growing in heterogeneous soils

Abstract

摘要

INTRODUCTION

Focal plant species are affected by neighbouring plants either positively or negatively (Bertness and Callaway 1994; Brooker et al. 2008), e.g. via competition for resources such as space, light or nutrients (Butterfield and Callaway 2013), or through positive interactions such as hydraulic lift or microbial enhancement (Brooker et al. 2008). Such neighbour effects are crucial in shaping the structure and functioning of plant communities (Kaarlejarvi and Olofsson 2014; Navas and Fayolle 2012). Our previous study focussing on three grass species (i.e. Festuca elata, Bromus inermis and Elymus breviaristatus) demonstrated that soil heterogeneity and species composition affected plant biomass and its allocation (Liu et al. 2021). However, plant species from different functional groups may respond differently (Bertness and Callaway 1994; Brooker et al. 2008; Kraft et al. 2015), especially when growing in heterogeneous soils (Liu et al. 2021). Thus, in this study, we explored neighbour effects on plant biomass and its allocation in forbs growing in heterogeneous soils.

Soil heterogeneity is an important trait of soils (Fitter et al. 2000; Huber-Sannwald and Jackson 2001; Hutchings et al. 2003), reflecting the non-uniform distribution of soil resources (e.g. nutrients). Soil heterogeneity in natural ecosystems varies both horizontally and vertically (Stewart et al. 2000), and it significantly affects plant dynamics such as plant biomass (Baer et al. 2003; Liu et al. 2017b; Maestre and Reynolds 2007), species richness (Liu et al. 2019) and root distribution (Liu et al. 2017a). Previous studies found that soil heterogeneity could increase plant biomass via different ways, such as increasing nutrient capture (Mommer et al. 2012) or improving resource absorption from nearby patches (Liu et al. 2017a). Furthermore, the intermediate soil heterogeneity supported higher species richness (Liu et al. 2019), and root distribution varied more among different patches in relatively higher soil heterogeneity (Liu et al. 2017a). However, the neighbour effects of plants growing in heterogeneous soils on plant biomass and biomass allocation between plant shoots and roots need further elucidation.

To explore these effects, a controlled experiment was performed with a similar setup than in our previous experiment on grasses (Liu et al. 2021). It featured three forb species (i.e. Spartina anglica, Limonium bicolor and Suaeda glauca) growing together either in mixtures or in monocultures on soils with three levels of soil heterogeneity (i.e. with small, medium or large patch size). In line with our previous studies (Liu et al. 2017b, 2019, 2021), such heterogeneity was created by alternatively filling resource-rich and resource-poor substrates both horizontally and vertically. Specifically, we addressed the following questions: (i) what are the effects of soil heterogeneity on plant biomass and its allocation for forbs? (ii) Did these effects differ among communities that differed in species richness (i.e. species number in monocultures or mixtures) and species composition?

MATERIALS AND METHODS

To explore the neighbour effects of forbs growing in heterogeneous soils on plant biomass and its allocation between shoots and roots, a controlled experiment was performed at the Linze Grassland Agriculture Station of Lanzhou University (100°3ʹ25″ E, 39°14ʹ30″ N) from 15 May to 10 August 2022. This station is located in the middle of Hexi Corridor with an average elevation of 1400 m (Hou and Shen 1999). The local air temperature varies from −28 to 38°C, with a mean annual temperature of 9.3°C. The mean annual precipitation is 112.9 mm, with more than 60% of the precipitation occurring in summer and autumn. The low rainfall and high evaporation resulted in local soils being saline (Zhu et al. 1997).

This experiment included three forbs (i.e. S. anglica, L. bicolor and S. glauca) and three levels of soil heterogeneity (Fig. 1a). As for the forbs, S. anglica is a perennial herb, which is resistant to salt and high-pH soil (Xie et al. 2020), L. bicolor is a perennial herb and a salt-tolerant species (Yu 2009) and S. glauca is an annual halophytic herb (Qu et al. 2019). These forbs (i.e. S. anglica, L. bicolor and S. glauca, named as SA, LB and SG, respectively) were grown either in mixtures or in monocultures, resulting in seven species compositions (i.e. SA, LB, SG, SA + LB, SA + SG, LB + SG, SA + LB + SG). Soil heterogeneity was developed by alternatively filling resource-rich and resource-poor substrates in pots both in the vertical and horizontal direction according to the general methodology in our previous studies (Liu et al. 2017b, 2019, 2021). Here, resource-rich and resource-poor substrates were created by mixing local soil and sand in a ratio of 8:2 and 2:8, respectively. Traits of these substrates are shown in Supplementary Table S1, and these substrates were named as ‘resource-rich’ and ‘resource-poor’ substrates for simplification. Three levels of soil heterogeneity were created by varying the patch sizes (i.e. small, medium and large) in the pot (Fig. 1b, side view). In total there thus were 21 treatments (i.e. 3 soil heterogeneity levels × 7 species compositions), and each treatment had 5 replicates, resulting in 105 pots. Soil heterogeneity is considered to increase with decreasing patch size (Li and Reynolds 1995; Maestre et al. 2006; Maestre and Reynolds 2006).

Figure 1:

Setup of the experiment, where seven species compositions (i.e. Spartina anglica (SA), Limonium bicolor (LB) and Suaeda glauca (SG), SA + LB, SA + SG, LB + SG, SA + LB + SG, a) are grown in pots with three levels of patch sizes (i.e. small, medium and large, b). All pots are randomly distributed in the greenhouse.

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Pots with 28.5 cm top diameter, 20.0 cm bottom diameter and 22.0 cm height were applied in this experiment. Note that pots with such size were similar to patch sizes revealed in some natural conditions (10 cm, Kleb and Wilson 1997; or 20 cm, Farley and Fitter 1999). Seedling transplanting instead of sowing seeds was adopted in this experiment to reduce the impacts of soil heterogeneity on seed germination of plants (Liu and Hou 2021), instead of focussing on competition at later growth stages. Plant individuals with similar sizes were transplanted into each pot at the start of the experiment. Specifically, each pot contained 12 individuals, i.e. 12 individuals of S. anglica, L. bicolor or S. glauca in monocultures, or 6 individuals of each species in the two-species mixtures, or 4 individuals of each species in the three-species mixture. Each treatment had the same distribution pattern of plant individuals among the replicates. During the experiment, each pot received the same amount of water once per week to avoid drought stress and to reduce the impact of water amount on plant biomass (Liu and Li 2020). Ten holes with 10 mm diameters were drilled in the bottom of each pot to ensure the drainage of water and avoid water logging. All pots were randomly distributed in the greenhouse. A few individuals died during the experiment as would be expected under natural conditions. Any non-random mortality was considered to be a result of the treatments (i.e. stemming from competition).

At the end of the experiment, shoots and roots of forbs in each pot were harvested, and shoots and roots were separated by substrate and species identity. They were weighed after being oven-dried at 65°C to constant weight. Total biomass in each pot = shoot biomass + root biomass, and the root:shoot ratio (i.e. R:S) in each pot = root biomass:shoot biomass.

Statistical analyses

Similar analyses as in our previous study with grasses were made to facilitate comparison, i.e. we considered the pot, substrate and community scales. At the pot scale (i.e. combining all plants in each pot), an analysis of variance (ANOVA) was performed to explore the effects of patch size, species composition and their interaction on shoot biomass, root biomass, total biomass and root:shoot ratio of plants growing in all the three patch sizes. At the substrate scale (i.e. combining all plants growing in the same substrate in each pot), an ANOVA was performed to investigate the effects of patch size, substrate, species composition and their interactions on shoot biomass, root biomass, total biomass and root:shoot ratio of plants growing in pots with the medium and large patch, but not in the small patch as substrates are too difficult to separate. At the community scale (i.e. plant individuals were separately measured by substrate and species identity), a mixed model approach was used to test the effect of species number (i.e. species richness), patch size, substrate, species identity and their interactions on biomass and its allocation, with species composition treated as a random factor, except in the smallest patch size as substrates were too difficult to separate. Post hoc analyses (pairwise comparisons with Bonferroni) were used to compare the differences among variables. All statistics were done with SPSS 21.0 (IBM, Corp. 2012).

RESULTS

At the pot scale, patch size, species composition and patch size × species composition significantly affected shoot biomass, root biomass and total biomass. R:S ratio was significantly affected by species composition, but not by patch size and patch size × species composition (Table 1). Specifically, with increasing patch size, shoot biomass, root biomass and total biomass increased (Fig. 2). In general, mixtures including S. anglica had relatively higher shoot biomass, root biomass, total biomass and R:S ratio (Table 1).

Figure 2:

At the pot scale, boxplot of shoot biomass (a), root biomass (b), total biomass (c) and root:shoot ratio (R:S, d) of forbs along patch sizes (i.e. small, medium and large).

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At the substrate scale, patch size, species composition and patch size × species composition significantly affected shoot biomass, root biomass and total biomass, while R:S ratio was only affected by species composition (Table 2). Interestingly, no significant differences in shoot biomass, root biomass, total biomass and R:S ratio were found between resource-rich and resource-poor substrates both in the medium and large patch sizes (Fig. 3). However, more shoot biomass and total biomass were found in the large patch size than in the medium patch size, both in the resource-rich and the resource-poor substrate (Fig. 3).

Figure 3:

At the substrate scale, boxplot of shoot biomass (a), root biomass (b), total biomass (c) and root:shoot ratio (R:S, d) of forbs along substrate type (i.e. resource-rich and resource-poor substrate), separated by patch sizes (i.e. medium and large patch).

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At the community scale, patch size, species number, species identity, patch size × species number, species number × species identity significantly affected shoot biomass, root biomass and total biomass. Moreover, R:S ratio was significantly affected by substrate, species number, species identity, patch size × species number and species number × species identity. Specifically, monocultures had more shoot biomass, root biomass, total biomass and R:S ratio on average than those in the two- or the three-species mixture (Fig. 4; Table 3). Furthermore, a significant effect of patch size on both shoot biomass and total biomass was found with increasing species richness (Fig. 4; Table 3), while a significant effect of patch size on root biomass and R:S ratio was only found in the two-species mixtures (Fig. 4; Table 3).

Figure 4:

At the community scale, boxplot of shoot biomass (a), root biomass (b), total biomass (c) and root:shoot ratio (R:S, d) of forbs along species compositions (i.e. Spartina anglica (SA), Limonium bicolor (LB) and Suaeda glauca (SG), SA + LB, SA + SG, LB + SG, SA + LB + SG) in pots with medium and large patch sizes.

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DISCUSSION

In this study, we found that soil heterogeneity and species composition jointly affected plant biomass and its allocation for forbs, while their effects were different from those reported earlier for grasses (Liu et al. 2021). Specially, with increasing patch size, the shoot biomass of grasses decreased (Liu et al. 2021), while the opposite pattern was found for the forbs in the current study. Moreover, more shoot biomass and total biomass in resource-rich substrates than in resource-poor substrates found for grasses (Liu et al. 2021) were not confirmed in this experiment for forbs. Furthermore, monocultures had more shoot biomass, root biomass and total biomass than two- or three-species mixtures, a pattern also found in shoot biomass in our previous study on grasses (Liu et al. 2021).

Our first question was: what are the effects of soil heterogeneity on plant biomass of forbs? We found that with increasing patch size, shoot biomass, root biomass and total biomass of forbs increased (Fig. 1), in partial contrast to the result from Liu et al. (2021). Moreover, a significant interaction between soil heterogeneity and species composition was found in the current study, and not in Liu et al. (2021). The differences between both studies may be related to differing competitive strategies and traits of forbs and grasses, including rooting patterns, nutrient acquisition and aboveground complementarity. Specifically, Levang-Brilz and Biondino (2003) demonstrated significant differences in root lateral spread, root length and root surface area between grasses and forbs. Chen et al. (2023) found contrasts in nutrient acquisition between grasses and forbs. Other causes for differing results may be related to differences in plant species, the way of treating seeds and the growing conditions in these two experiments. The experiment of Liu et al. (2021) was carried out in natural conditions, where three grass species (i.e. F. elata, B. inermis and E. breviaristatus) were applied, and seeds of these species were sowed into each treatment at the beginning of the experiment, while the current manipulation experiment was conducted in a greenhouse, where three forb species (i.e. S. anglica, L. bicolor and S. glauca) were used, and seedlings of these species were transplanted at the start of the experiment. Indeed, previous studies found that species identity affected the responses of focal plant species to their neighbours (Brooker et al. 2008), and soil heterogeneity affected seed germination of grasses (Liu and Hou 2021).

Our second question was: did these effects differ among communities that differed in species richness and species composition? We found that the effects of patch size on shoot biomass, root biomass and total biomass of forbs were modified by species richness and species composition. Specifically, at the community scale, monocultures had more shoot biomass, root biomass and total biomass on average than the two-species mixtures or the three-species mixture. This finding is consistent with the result from Liu et al. (2021), while it does not support the biodiversity–ecosystem functioning theory, which states that through complementarity and selection effects (Bai et al. 2004; Hector 1998; Loreau and Hector 2001), a community with more species is more likely to support higher biomass (Bai et al. 2004; Fridley 2001; Loreau and Hector 2001; Xi et al. 2017). What caused a higher biomass to be found in monocultures than mixtures is not clear. The potential reasons are discussed in Liu et al. (2021), which could be related to the stronger competition in mixtures than in monocultures, and/or diversity effects could be modified by soil heterogeneity since plants could vary their responses to soil heterogeneity via strategies such as plasticity (Dudley and Schmitt 1996; Dudley and File 2007; Kingsolver 1995; Van Buskirk et al. 1997). Moreover, at the pot scale, the effects of patch size on shoot biomass, root biomass and total biomass were modified by species composition, specifically by the presence of S. anglica, i.e. a species with strong clonal growth, and tolerance to salt and high soil pH (Xie et al. 2020).

This work should be extrapolated and interpreted with caution due to the following limitations: (i) this study explored the neighbour effects of plants growing in heterogeneous soils at the early growing stage of forbs, and whether such effects work in the later growing stage of plants merits further research; (ii) species functional group plays an important role in driving plant biomass (Bergamini et al. 2001), while the current experiment only included three forb species, and more functional species (e.g. including both forbs and grasses) should be considered in the future research; (iii) biomass is a general indicator of plants in responses to heterogeneous soils. However, plant dynamic can be indicated by plant traits such as plant height as it can be directly and non-destructively measured during the experiment (Chen et al. 2014). Thus, whether other plant traits such as plant height respond similar to plant biomass should be further studied (Liu et al. 2022).

In conclusion, soil heterogeneity and species composition interactively impacted plant biomass and biomass allocation of forbs at different scales. These results differ from earlier findings on the responses of grasses. To further elucidate the effects of soil heterogeneity on the interactions between neighbour plants, we advise to conduct longer-term experiments featuring a variety of functional groups.

Supplementary Material

Supplementary material is available at Journal of Plant Ecology online.

Table S1: Mean ± SE of pH, electrical conductivity (EC), soil organic carbon (SOC), available phosphorus (AP) and total nitrogen of the resource-poor and resource-rich substrates tested at the start of the experiment.

Funding

The project supported by the Open Fund of Key Laboratory of Biodiversity and Environment on the Qinghai-Tibet Plateau, Ministry of Education (KLBE2024002). Yongjie Liu holds a start-up fund from Lanzhou University (508000-561119213).

Authors’ Contributions

Y.L. designed the study. C.M., M.W., G.L. and Y.L. conducted the experiment, collected the data and did the analyses. Y.L. wrote the draft of the manuscript. All authors contributed substantially to this work.

Acknowledgements

We acknowledge the assistance of Cheng Zhang, Wanhe Zhu and Shengwei Xu during the experiments.

Conflict of interest statement. The authors declare that they have no conflict of interest.

REFERENCES