Introduction

Seed dispersal and germination, and seedling emergence, are defining points in the successful establishment of a plant and thus determine the initial stages of plant distribution and community composition. For this reason, many studies have focused on how seed traits (e.g., mass, shape, and external morphology such as appendages) interact with microsite characteristics (e.g., climate, soil moisture, existing plants, and physical soil properties) (Grubb 1977; Baskin and Baskin 1998) to affect these processes. External seed morphology is believed to influence germination via effects on water absorption (Baskin and Baskin 1998; Fenner and Thompson 2005) and to affect dispersal through either enhancing seed movement (e.g., to escape competition or predation) or inhibiting it (e.g., retention in a favorable environment) (Boeken and Shachak 1994; Fenner and Thompson 2005; Gros et al. 2006). In all desert ecosystems, scant and highly variable precipitation also exerts strong selective pressures on all aspects of plant development, including seeds (Noy-Meir 1973).

Biological soil crusts (biocrusts) typically consist of a mixture of mosses, lichens, algae, cyanobacteria, bacteria, fungi, and microfauna. Owing to their extraordinary abilities to survive extreme conditions such as complete desiccation, high temperatures (up to 70 °C), solar radiation, and high soil pH and salinity, biocrusts occur in deserts worldwide and can constitute as much as 70 % of the living ground cover (Belnap 2003). As documented by numerous studies, biocrusts play significant ecological roles in desert ecosystems (Eldridge and Greene 1994; Belnap 2003). These communities can benefit soil structure (Zhang et al. 2006; Chen et al. 2009), stability, and fertility (Kidron 2007; Belnap et al. 2008; Li et al. 2008; Zhang et al. 2009); affect surface hydrology (Belnap 2006); increase carbon (Li et al. 2012) and nitrogen (N) in soils (Evans and Ehleringer 1993; Housman et al. 2006; Wu et al. 2009), as well as enrich the diversity and abundance of soil fauna (Lalley et al. 2006; Li et al. 2006).

Biocrusts can affect germination and establishment of vascular plants, although their influence can be positive, negative, or neutral, depending on the species of plant and the external morphology and composition of the biocrust community (Zaady et al. 1997; Prasse and Bornkamm 2000; Gutterman 2003; Hawkes 2004; Li et al. 2005; Serpe et al. 2006; Deines et al. 2007; Escudero et al. 2007). Studies show biocrusts can have a positive influence on the colonization of vascular plants in the field in many settings, including polar (Anderson and Bliss 1998; Bliss and Gold 1999; Cooper et al. 2004; Breen and Levesque 2006), cool (Harper and Belnap 2001), and hot arid areas (DeFalco et al. 2001; Scott and Morgan 2012).

Experiments under laboratory conditions have shown that biocrusts can facilitate seed germination (St Clair et al. 1984; Rivera-Aguilar et al. 2005) and increase plant height and biomass (Belnap 2003). Studies have also shown negative correlations between vascular plant success and biocrusts across various biocrust types, including lichen-dominated (Deines et al. 2007; Serpe et al. 2008), moss-dominated (Serpe et al. 2006), and cyanobacterial ones (Zaady et al. 1997; Prasse and Bornkamm 2000). Other researchers have reported that the occurrence of biocrusts has negligible effects on germination and seedling growth compared with bare soil (Li et al. 2005; Rivera-Aguilar et al. 2005; Serpe et al. 2006; Deines et al. 2007). Studies have also shown the presence of biocrusts can alter nutrient concentrations in plant tissue, especially N (DeFalco et al. 2001; Pendleton et al. 2003; Langhans et al. 2009, 2010). In this study, a series of greenhouse experiments were used to test the hypothesis that biocrusts affect vascular plants differentially at various life stages, depending on seed morphology, life form, and species.

Materials and methods

Study site description

The Gurbantunggut Desert is situated in center of the Junggur Basin in the Xinjiang Uygur Autonomous Region of northwestern China. It is the second largest desert in China with an area of 48.8 × 103 km2. Mean annual precipitation is approximately 79 mm, falling predominantly during spring. Mean annual evaporation is 2606 mm. Average temperature is 7.3 °C. Wind speeds are greatest during late spring, being 11.2 m s−1, and are predominantly in WNW, NW and N directions. Natural vegetation is dominated by the shrubs Haloxylon ammodendron and H. persicum, with a total vegetative cover of less than 30 %. The area is covered by large and dense semi-fixed sand dunes. Abundant biocrusts cover all soil surfaces except the top half of sand dunes (Zhang et al. 2007).

Plant selection and seed collection

We conducted an initial field survey to determine the frequency and abundance of vascular plant species in our study area. Using this data, we chose the 15 most common plant species. At least 100 seeds were collected from at least 20 individuals of these species at the end of June and August of 2011. The collected seeds were then tested for germinability on filter paper. For each species, we placed 20 seeds in five replicate 90 mm Petri dishes containing filter papers continuously moistened with distilled water. Seeds were incubated in a growth chamber with a constant temperature of 20 °C. Of the 15 species tested, we then selected H. persicum, Ephedra distachya, Ceratocarpus arenarius, Malcolmia africana and Lappula semiglabra for further study, as they all had consistently high (>80 %) germination and best met our desired range of plant life form (herbaceous and woody plants) and seed traits (size, shape, and weight). We then characterized a subset of the seeds from these species for their size, shape, weight and the presence or absence of seed appendages. The rest of the seeds were then stored under dry conditions at 5 °C until used for the greenhouse experiment.

Biocrust and bare soil sample collection for greenhouse experiments

Lichen-dominated biocrusts are the prevalent biocrust type in the Gurbantunggut Desert (Zhang et al. 2006, 2007). In October, 2012, we selected a typical interdune area with an approximate size of 30 m x100 m in southern part of this desert, where the most dominant lichen species is Collema tenax, and low growing forms of other lichen species such as Caloplaca and Placidium, and the moss Syntrichia caninervis occur. Within this sampling area, we also randomly selected a 10 × 10 m sub-site where biocrusts and the top 5 cm of underlying soil were removed to create bare soil for future sample collecting; the rest of the interdune area was not disturbed. After seven months’ treatment (April 2013), we returned and excavated biocrust and soil samples for the greenhouse experiment in open interspaces between perennial vegetation. Using cylindrical collecting augers (15 cm in diameter, 17 cm in height), we randomly collected 25 intact biocrusted samples and 25 bare soil samples where the biocrust had been previously removed.

Greenhouse experiments

The experiment was carried out in a shade-net greenhouse in the Fukang Desert Station of Chinese Academy of Sciences, which is located in the southern fringe of the Gurbantunggut Desert. The biocrusted and crust-removed samples were taken to the greenhouse. In order to exhaust the seed bank in the samples, we watered the dishes daily and removed all seedlings that appeared. After seven consecutive days with no seedlings, we considered the soil samples seedless and used them for our seed germination experiment. For each of the five plant species, we used five replicates of biocrusted and five replicates of biocrust-removed pots for a total of 10 pots per species and 50 pots for the experiment. Twenty seeds of each species were spread randomly on the surface of each pot, without inserting them into the soil, to imitate the first stages of natural dispersal. Every second day 150 ml of water was applied as a fine mist to each pot. Radicle emergence from the seed was recorded daily and used to calculate percent germination. We also measured seedling height of five marked seedlings in each pot every 5 days. The greenhouse experiment lasted for 2 months. At the end of the experiment, we harvested leaves and stems for above ground biomass and roots for below ground biomass separately for each species. Materials were then oven-dried at 70 °C and weighed.

Measurement of element concentrations in plant tissues

Plant tissue element analyses were performed by the Key Laboratory of Biogeography and Bioresources, Chinese Academy of Sciences. Plant tissue samples were prepared and analyzed using the traditional methods as described in Harper and Pendleton (1993).

Data analysis

For each species, the effects of both biocrusted soil and crust-removed soil on percent emergence, seedling growth rate, shoot height, ending biomass accumulation and root:shoot ratios were analyzed using one-way ANOVA to determine whether there were significant differences among treatments. Differences among treatments were detected with Tukey’s HSD test. All the statistical analyses were performed using SPSS, version 9.0 (SPSS, 1998).

Results

Seed traits

The main seed traits and seed morphology of five tested species are shown in Fig. 1. Three of the tested species had appendages. Seeds of H. persicum have membranous winged appendage around the seeds, whereas seeds of L. semiglabra have structures on the opposite side of the germination pore, with several short thorns on the center of this side. Seeds of C. arenarius are wrapped in a cuniform utricle that is covered by stellate hairs. In addition, there are needle-like thorns in two corners. However, unlike H. persicum or L. semiglabra, seeds of C. arenarius are flat. In contrast, the seeds of M. africana and E. distachya have no appendages and their surfaces are smooth. Therefore, we had woody and herbaceous seeds with and without appendages.

Fig. 1
figure 1

Seed morphology of five plant species. a Haloxylon persicum; b Ephedra distachya; c Lappula semiglabra; d Ceratocarpus arenarius; e Malcolmia africana. Scale bar 1 mm. Three of them have seed appendages and two of them have smooth surface

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Seed germination

Biocrusts affected percent germination differently among the five tested plant species (Fig. 2). Percent germination was the same on biocrusted and biocrust-removed soils for the two plants whose seeds lack appendages: the woody E. distachya and the herbaceous M. africana (P > 0.05). In contrast, the emergence of H. persicum, C. arenarius and L. semiglabra were all significantly inhibited by biocrusts (P < 0.05) and all three have relatively large seed appendages.

Fig. 2
figure 2

The influence of biocrusts on seed germination of five desert plants. (An asterisk (*) indicate statistical significances of P < 0.05). Germinations of seeds with appendages were significantly inhabited by existence of biocrusts, regardless of shrub or herbaceous species. (Hp: Haloxylon persicum; Ed: Ephedra distachya; Ca: Ceratocarpus arenarius; Ma: Malcolmia africana; Ls: Lappula semiglabra)

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For E. distachys and M. africana, both of which have smooth seeds but are of greatly disparate life form, seed size, and seed weight, there was no significant difference in germination between seeds planted in biocrusted and biocrust-removed soil. In contrast, biocrusted soil inhibited germination in H. persicum, C. arenarius, and L. semiglabra, all of which have appendaged seeds (P < 0.05), whereas there was no effect when grown without biocrusts. Seed weight and size varied widely among these species, but neither trait showed any correlation with the ability to germinate on biocrusted soils.

Seedling growth

Shoots of the shrub E. distachys were taller on biocrusted soils when compared to biocrust-removed soils until day 20 (Fig. 3, P < 0.05). Two (M. africana, L. semiglabrata) of the three herbaceous plants were taller at all measurement times on the biocrusted than biocrust-removed soil. There were no significant differences at any time for the shrub H. persicum or the herbaceous C. arenarius.

Fig. 3
figure 3

Variations in shoot height for the five plant species at different cultivation times. An asterisk (*) indicate statistical significances of P < 0.05. The growth of herbaceous species were significantly enhanced when grew on biocrust-coverd soils. a Haloxylon persicum; b Ephedra distachya; c Ceratocarpus arenarius; d Malcolmia africana; e Lappula semiglabra

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Ending seedling biomass

The effects of biocrusts on biomass accumulation varied among plant species at the end of the experiment (Fig. 4). Above-ground, below-ground, and total biomass of the herbaceous M. africana and L. semiglabra were significantly greater when growing in biocrusted than biocrust-removed soils (P < 0.05). In contrast, there was no difference observed for H. persicum, E. distachya, or C. arenarius for any biomass category.

Fig. 4
figure 4

Effect of biocrusts on ending biomass of the five plant species. Asterisks (* and **) indicates significant differences P = 0.05 and P = 0.01, respectively. The existence of biocrusts could significantly enhance ending biomass accumulations of herbaceous species, and no differences were observed among shrub species. a Above ground biomass; b below ground biomass; c above + below ground biomass

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No significant differences in root:shoot ratios were found between biocrusted and biocrust-removed soils within five tested plat species (Fig. 5). The species E. distachya showed the highest root-shoot ratio among all the species, while the lowest value was found in M. africana.

Fig. 5
figure 5

Effects of biocrusts on root:shoot ratios of the five plant species. No significant differences were observed among all tested plants. (Hp: Haloxylon persicum; Ed: Ephedra distachya; Ca: Ceratocarpus arenarius; Ma: Malcolmia africana; Ls: Lappula semiglabra)

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Element concentrations

All five species growing in the biocrusted soils had significantly greater N concentrations in their tissue than plants growing in biocrust-removed soils (Table 1). In contrast, biocrusts had no influence on tissue concentrations of phosphorus (P), potassium (K), or magnesium (Mg) in any of the tested species. Effects of biocrusts on the other nutrients was species specific: calcium (Ca) increased in the shrub H. persicum and the herbaceous C. arenarius, but decreased in E. distachya and L. semiglabra, whereas biocrusts reduced sodium (Na) absorption in H. persicum, E. distachya and L. semiglabra, while increasing Na in C. arenarius.

Table 1 Influence of biocrusts on tissue concentrations of nutrients in five desert plants (μg g−1)
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Biocrusts only affected copper (Cu) uptake in M. africana, while strongly inhibiting uptake of maganese (Mn) in M. africana and L. semiglabra. Absorption of iron (Fe) was slightly decreased by biocrusts in E. distachya, while being greatly increased in C. arenarius, M. africana, and L. semiglabra. Biocrusts also reduced Zn uptake in H. persicum while increasing it in E. distachya, M. africana and C. arenarius.

Discussion

Plant germination and growth

Germination is the first bottleneck plants must pass through for successful establishment. Therefore, any factors that influence this step can be critical in determining plant community composition. Because biocrusts can influence many soil factors such as soil surface roughness, porosity, structure, moisture, pH, nutrient availability, and temperature, there are multiple ways biocrusts could influence the germination of seeds and seedling growth. This range of responses may be attributed to other effects of biocrusts on the soil such as moisture, which influence the availability of cations and other nutrients (Concostrina-Zubiri et al. 2013; Wu et al. 2013; Zhao et al. 2014) and influence the availability of water and nutrients reaching nearby plants (Boeken and Orenstein 2001). In semiarid Negev ecosystems, seedling establishment in annuals benefits from biocrust disturbance and removal because these disturbance increases seed capture and water infiltration rates (Eldridge et al. 2000).

Biocrust composition was held constant in this study, and thus the differential response we saw in seed germination is most likely attributable to traits of the seeds we used. Our results show that the size, shape, or weight of seeds did not determine whether a seed would more successfully germinate on biocusted soils when compared to soils where the biocrusts had been removed. However, when appendages such as those found on H. persicum, L. semiglabra, and C. arenarius were present, germination was negatively affected.

There are several ways in which appendages can negatively affect seed germination under greenhouse and field condition. First, appendages can reduce or eliminate contact between the seed and the soil surface, reducing absorption of water and nutrients (Eldridge and Greene 1994). During our pre-testing, all seeds were grown on filter paper and kept wet constantly by misting. Under these conditions, we obtained high and consistent germination. However, when seeds with appendages were placed on biocrusted soils, they had only minimal contact with the soil surface. As a likely consequence, we observed seeds did not stay wet as long as seeds on bare soils or those in greater contact with the soil surface. Second, under field settings, it is likely that because appendages make the propagule larger, they inhibit seeds from slipping into the small cracks commonly found on biocrusted surfaces. In deserts, seed burial can be important, as seeds remaining on the soil surface have access to less moisture and nutrients than buried seeds and are exposed to higher predation rates (Boeken and Shachak 1994). Third, appendages can facilitate (e.g., adhering to animal fur) or inhibit (e.g., anchoring to soil surface) seed dispersal. Depending on the specific situation, this can have positive or negative effects on seed germination and subsequent plant establishment. For example, in cool deserts such as the Gurbnatunggut Desert and the Colorado Plateau Desert in the US, biocrusts roughen the soil surface and thus can increase seed retention in plant interspaces which are considered safe sites for seeds (Eckert et al. 1986). In contrast, the smooth hard biocrusts found in hot deserts can facilitate seed movement to the nearest obstruction and thus decrease germination in the interspace (Prasse and Bornkamm 2000).

Plant shoot height and ending biomass

In the current study, we found that plant life form did not predict response to the presence of biocrusts, as the woody plant E. distachya and the herbaceous plants (M. africana and L. semiglabra) were taller in biocrusted soils, whereas the other woody shrub H. persicum or the other herbaceous species did not respond at any point.

Most studies from many different climate zones show biocrusts either a positive or neutral effect (Li et al. 2005; Deines et al. 2007; Serpe et al. 2008). For instance, in a cool desert, Harper and Pendleton (1993) found plants growing in biocrusts had four times greater biomass than those in uncrusted soils. Another study showed that the biomass of Festuca octoflora was two times higher in biologically crusted soil than in soil without biocrusts (Belnap and Harper 1995).

Only a few studies have addressed the effect of biocrusts on the root:shoot ratio of plants. Bliss and Gold (1999) and Pendleton et al. (2003) reported lower root:shoot ratios of plants associated with biocrusts compared to those growing without crusts. Langhans et al. (2009) found that root:shoot ratios decreased with biocrust age, indicating less allocation to roots with improved fertility. Furthermore, Thiet et al. (2014) showed that seedlings in biocrusts had shorter roots than bare ground controls. In this present study, however, we found no significant difference in root:shoot ratios in any of the five tested species when biocrusted and biocrust-removed soils were compared.

Elemental content

The biocrust on the fixed sand increased the organic matter and total N content of the soil, but only on the upper 5 cm (Gao et al. 2010). This unequal effect of the biocrust on N content may lead to differences in N uptake among plant species. Plants with shallow roots usually benefit more from the biocrust presence (DeFalco et al. 2001; Yan 2009).

Higher P availability beneath the biocrust is associated with higher concentrations of P in plant tissues. For example, in southeastern Utah foliar levels of P in Festuca octoflora were 78 % higher in plants growing in biocrust covered soil than uncrusted soil (Belnap, 2011). Positive effects of biocrusts on plant P uptake were also observed in the Kubuqi deserts, although the extent of this effect varied among plant species; overall, the enhancement of P uptake was greater for herbaceous than shrub species (Yan 2009). We found all plant species had an increase in N plant tissue concentration when growing in biocrusted soil compared to biocrust-removed soil. This is a common finding among studies: biocrusts fix N and much of the N they fix is leaked to underlying soils. Therefore, more N is likely available for plant uptake (Belnap et al. 2003; Langhans et al. 2009, 2010). Stable isotopes have shown that biocrusts can be the dominant source of N in desert soils (Evans and Ehleringer 1993). In addition, it has been found that cyanobacterial polysaccharides of biocrusts can improve nitrate reductase activities and root vigor, and thus stimulate the root growth and uptake of nitrate–N and ions for plants, although very high levels of polysaccharides could suppress these responses (Xu et al. 2013). Some studies do not report enhanced N concentrations in plants growing in biocrusted soil (DeFalco et al. 2001). Biocrusted soils generally support greater shoot biomass and density of plants than bare soils. Therefore, if a study does not take this into account, nutrient concentrations can be diluted in an individual plant.

The effects of biocrusts on plant uptake of cations have ranged from negative to positive (Harper and Belnap 2001; Pendleton et al. 2003). In addition, competition for nutrients with microbes in biocrusts might cause lower cation concentrations. Although conflicting results are found in the literature, the uptake of some cations including Cu, K, Mg, and Zn tends to increase in the presence of biocrusts (Harper and Belnap 2001; Pendleton et al. 2003). In contrast, plant Fe uptake appears to be lower in soil covered with biocrusts. For other cations including Ca, Mn, and Na positive, minimal, and negative effects of biocrusts on plant uptake of these cations have been reported (Harper and Belnap 2001; Pendleton et al. 2003). Like for P and N, differences in cation uptake among plant species reflect in part differences in root architecture.

In this study, we did not see any effect on P and K concentrations in any species. Levels of the micronutrients Fe and Zn and the macronutrient Ca was affected in four of the five species, although not always in the same direction. The micronutrients Cu and Mn and the macronutrients Mg and Na were only affected in 1–2 of the species, but again not always in the same direction. The shrub H. persicum, with concentrations of three of the ten nutrients affected, was the least responsive to the presence of biocrusts. The shrub E. distachya had five of the ten nutrients affected. Zinc and Ca also responded in both species, but the response was opposite in sign. Interestingly, the herbaceous C. arenarius had an almost identical response pattern to the shrub E. distachya. The herbaceous M. africana showed the greatest response of the five species, with eight of the ten nutrient levels different in biocrusted soils. There were also similarities between L. semiglabra and M. africana, although several nutrients that were different in M. africana were not significantly so in L. semiglabra, despite tending in the same direction.

Given the complexity of response we observed in cations and micronutrients in this study, explaining the data is difficult. In addition, only a few studies have been done to provide background for our results. Belnap et al. (2003) summarized these studies, reporting that 50 % showed biocrusts enhanced Ca, Mn and Na, and 70 % showed an enhancement of Cu, K, Mg and Zn with a concomitant reduction of P and Fe.

There are many reasons to believe that biocrusts differentially affect absorption of elements by plants in multiple ways. Biocrusts could fix C, increasing levels in soils (Belnap 2003). Increased organic matter stimulates soil fauna and thus decomposition rates, thereby increasing soil fertility. Biocrusts alter pH and secrete phosphatase, likely affecting the bioavailability of micronutrients. Indeed, several studies have shown micronutrient concentrations, especially Fe, in plants growing in biocrusts are often higher than those in uncrusted soils (Harper and Pendleton 1993; Harper and Belnap 2001). Biocrusts also alter hydrologic relations and thus nutrient availability. Lastly, plants in biologically crusted soils have a greater rate of mycorrhizal fungal infection compared with plants growing in adjacent soil without biocrusts (Pendleton et al. 2003).

However, very few of the manipulative experiments needed to directly test the mechanisms of the relationship between biocrusts and plant tissue concentrations have been done, making it impossible to elucidate any general patterns and or make predictions about this relationship at this time. Of all aspects of biocrust ecology, this is one of the most critical areas for future investigations.

Conclusion

Our results show that the influence of biocrusts on seed germination can be species-specific. Biocrusts negatively affected the germination of seeds with appendages. Once seeds had germinated, the effect of biocrusts on growth rate and ending biomass (aboveground, belowground, and total) depended on plant life form rather than seed traits. Two of the three herbaceous plants were stimulated by the presence of biocrusts, whereas neither woody species responded (although E. distachys did respond initially). The presence of biocrusts increased concentrations of N and did not affect P or K in all tested species, whereas the uptake of other tested nutrients was species-specific. Our study showed that biocrusts can favor germination of certain species over others and also can influence growth and elemental uptake. Therefore, they may be an important trigger for determining desert plant diversity and community composition in deserts.