Hong- Li

Effects of soil nutrient heterogeneity on intraspecific competition in the invasive, clonal plant Alternanthera philoxeroides.

BACKGROUND AND AIMSnFine-scale, spatial heterogeneity in soil nutrient availability can increase the growth of individual plants, the productivity of plant communities and interspecific competition. If this is due to the ability of plants to concentrate their roots where nutrient levels are high, then nutrient heterogeneity should have little effect on intraspecific competition, especially when there are no genotypic differences between individuals in root plasticity. We tested this hypothesis in a widespread, clonal species in which individual plants are known to respond to nutrient heterogeneity.nnnMETHODSnPlants derived from a single clone of Alternanthera philoxeroides were grown in the greenhouse at low or high density (four or 16 plants per 27·5 × 27·5-cm container) with homogeneous or heterogeneous availability of soil nutrients, keeping total nutrient availability per container constant. After 9 weeks, measurements of size, dry mass and morphology were taken.nnnKEY RESULTSnPlants grew more in the heterogeneous than in the homogeneous treatment, showing that heterogeneity promoted performance; they grew less in the high- than in the low-density treatment, showing that plants competed. There was no interactive effect of nutrient heterogeneity and plant density, supporting the hypothesis that heterogeneity does not affect intraspecific competition in the absence of genotypic differences in plasticity. Treatments did not affect morphological characteristics such as specific leaf area or root/shoot ratio.nnnCONCLUSIONSnResults indicate that fine-scale, spatial heterogeneity in the availability of soil nutrients does not increase competition when plants are genetically identical, consistent with the suggestion that effects of heterogeneity on competition depend upon differences in plasticity between individuals. Heterogeneity is only likely to increase the spread of monoclonal, invasive populations such as that of A. philoxeroides in China.

Burial Depth and Stolon Internode Length Independently Affect Survival of Small Clonal Fragments

Disturbance can fragment plant clones into different sizes and unstabilize soils to different degrees, so that clonal fragments of different sizes can be buried in soils at different depths. As a short-term storage organ, solon internode may help fragmented clones of stoloniferous plants to withstand deeper burial in soils. We address (1) whether burial in soils decreases survival and growth of small clonal fragments, and (2) whether increasing internode length increases survival and growth of small fragments under burial. We conducted an experiment with the stoloniferous, invasive herb Alternanthera philoxeroides, in which single-node fragments with stolon internode of 0, 2, 4 and 8 cm were buried in soils at 0, 2, 4 and 8 cm depth, respectively. Increasing burial depth significantly reduced survival of the A. philoxeroides plants and increased root to shoot ratio and total stolon length, but did not change growth measures. Increasing internode length significantly increased survival and growth measures, but there was no interaction effect with burial depth on any traits measured. These results indicate that reserves stored in stolon internodes can contribute to the fitness of the A. philoxeroides plants subject to disturbance. Although burial reduced the regeneration capacity of the A. philoxeroides plants, the species may maintain the fitness by changing biomass allocation and stolon length once it survived the burial. Such responses may play an important role for A. philoxeroides in establishment and invasiveness in frequently disturbed habitats.

Vegetative Propagule Pressure and Water Depth Affect Biomass and Evenness of Submerged Macrophyte Communities

Vegetative propagule pressure may affect the establishment and structure of aquatic plant communities that are commonly dominated by plants capable of clonal growth. We experimentally constructed aquatic communities consisting of four submerged macrophytes (Hydrilla verticillata, Ceratophyllum demersum, Elodea nuttallii and Myriophyllum spicatum) with three levels of vegetative propagule pressure (4, 8 and 16 shoot fragments for communities in each pot) and two levels of water depth (30 cm and 70 cm). Increasing vegetative propagule pressure and decreasing water level significantly increased the growth of the submerged macrophyte communities, suggesting that propagule pressure and water depth should be considered when utilizing vegetative propagules to re-establish submerged macrophyte communities in degraded aquatic ecosystems. However, increasing vegetative propagule pressure and decreasing water level significantly decreased evenness of the submerged macrophyte communities because they markedly increased the dominance of H. verticillata and E. nuttallii, but had little impact on that of C. demersum and M. spicatum. Thus, effects of vegetative propagule pressure and water depth are species-specific and increasing vegetative propagule pressure under lower water level can facilitate the establishment success of submerged macrophyte communities.

Do Amplitudes of Water Level Fluctuations Affect the Growth and Community Structure of Submerged Macrophytes

Submerged macrophytes are subjected to potential mechanical stresses associated with fluctuating water levels in natural conditions. However, few experimental studies have been conducted to further understand the effects of water level fluctuating amplitude on submerged macrophyte species and their assemblages or communities. We designed a controlled experiment to investigate the responses of three submerged macrophyte species (Hydrilla verticillata, Ceratophyllum demersum and Elodea nuttallii) and their combinations in communities to three amplitudes (static, ± 30 cm, ± 60 cm) of water level fluctuations. Results showed that water level fluctuating amplitude had little effects on the community performance and the three tested species responded differently. H. verticillata exhibited more growth in static water and it was negatively affected by either of the water level fluctuations amplitude, however, growth parameters of H. verticillata in two fluctuating water level treatments (i.e., ± 30 cm, ± 60 cm) were not significantly different. On the other hand, the growth of C. demersum was not significantly correlated with different amplitude treatments. However, it became more abundant when water levels fluctuated. E. nuttallii was inhibited by the two fluctuating water level treatments, and was less in growth parameters compared to the other species especially in water level fluctuating conditions. The inherent differences in the adaptive capabilities of the tested species indicate that C. demersum or other species with similar responses may be dominant species to restore submerged macrophyte communities with great fluctuating water levels. Otherwise, H. verticillata, E. nuttallii or other species with similar responses could be considered for constructing the community in static water conditions.

Nitrogen addition increases intraspecific competition in the invasive wetland plant Alternanthera philoxeroides, but not in its native congener Alternanthera sessilis

Nitrogen is often released in pulses with different frequencies, and N supply pulses may affect growth, reproduction, and biomass allocation of plants. However, few studies have examined how N supply pulses affect intraspecific competition of clonal plants and whether such an effect depends on the N supply amount. We grew one (no competition) or 12 ramets (with intraspecific competition) of both an invasive clonal plant Alternanthera philoxeroides and its native congener Alternantherau2009sessilis in five different N treatments: control (no N addition), low/high amount with low/high frequencies (pulses). Nitrogen addition significantly increased the growth of both species, while intraspecific competition decreased it. Nitrogen addition significantly increased intraspecific competitive intensity of A.u2009philoxeroides as measured by the log response ratio of growth traits, but did not affect that of A.u2009sessilis. Despite the N supply amount, N pulses had little effect on the growth and thus intraspecific competition of the two species. Therefore, increasing N deposition may change population structure and dynamics and the invasion succession of A.u2009philoxeroides, but changes in N pulses may not.

Sediment Type Affects Competition between a Native and an Exotic Species in Coastal China

Different types of sediments in salt marsh have different physical and chemical characters. Thus sediment type plays a role in plant competition and growth in salt marsh ecosystems. Spartina anglica populations have been increasingly confined to upper elevation gradients of clay, and the niche sediment has changed. Because the niches of S. anglica and the native species Scirpus triqueter overlap, we conducted a greenhouse experiment to test the hypothesis that plant competition has changed under different types of sediments. Biomass and asexual reproduction were analyzed, and inter- and intraspecific competition was measured by log response ratio for the two species in both monoculture and combination under three sediment types (sand, clay and mixture of sand and clay). For S. anglica, biomass, ramet number and rhizome length in combination declined significantly compared with those in monoculture, and the intensity of interspecific competition was significantly higher than that of intraspecific competition under all sediments. For S. triqueter, the intensities of intra- and interspecific competition were not significantly different. This indicates that S. triqueter exerts an asymmetric competitive advantage over S. anglica across all sediments, but especially clay. Thus the sediment type changes competition between S. anglica and S. triqueter.

Does clonal fragmentation of the floating plant Eichhornia crassipes affect the growth of submerged macrophyte communities

Few studies have tested the effect of clonal fragmentation at the community level. Floating plants and submerged macrophytes can coexist, and the presence of floating plants may greatly restrict the growth of submerged macrophyte communities. Clones of floating plants are frequently fragmented by disturbance, but little is known about how clonal fragmentation of floating plants affects the growth of submerged macrophyte communities and whether such effect depends on the intensity (frequency) of fragmentation. We assembled experimental aquatic communities consisting of four submerged macrophytes (Hydrilla verticillata, Ceratophyllum demersum, Chara fragilis and Myriophyllum spicatum), and grew them without the floating plant Eichhornia crassipes, with an intact clone of E. crassipes, or with a clone fragmented every 5, 10 or 15 days. Clonal fragmentation, irrespective of the frequency, did not significantly affect the growth of the floating plant. Consequently, it did not significantly affect the growth of the submerged macrophyte community or that of each of the four submerge macrophytes. However, compared to the treatment without E. crassipes, the presence of E. crassipes significantly decreased the biomass of the macrophyte community because it greatly decreased the biomass of the most abundant species H. verticillata. Therefore, the presence of floating plants can influence the establishment of submerged macrophyte communities, but clonal fragmentation of floating plants may not. To restore submerged macrophyte communities, measures should be taken to restrict the spread of floating plants, and breaking clones of floating plants may not be an effective measure.

Impacts of sediment type on the performance and composition of submerged macrophyte communities

To restore deteriorated lake ecosystems, it is important to identify environmental factors that influence submerged macrophyte communities. While sediment is a critical environmental factor for submerged macrophytes and many studies have examined effects of sediment type on the growth of individual submerged macrophytes, very few have tested how sediment type affects the growth and species composition of submerged macrophyte communities. We constructed submerged macrophyte communities containing four co-occurring submerged macrophytes (Hydrilla verticillata, Myriophyllum spicatum, Ceratophyllum demersum and Chara fragilis) and subjected them to three sediment treatments, i.e., clay, a mixture of clay and quartz sand at a volume ratio of 1:1 and a mixture at a volume ratio of 1:4. Compared to the clay, the 1:1 mixture treatment greatly increased overall biomass, number of shoot nodes and shoot length of the community, but decreased its diversity. This was because it substantially promoted the growth of H. verticillata within the community, making it the most abundant species in the mixture sediment, but decreased that of M. spicatum and C. demersum. The sediment type had no significant effects on the growth of C. fragilis. As a primary nutrient source for plant growth, sediment type can have differential effects on various submerged macrophyte species and 1:1 mixture treatment could enhance the performance of the communities, increasing the overall biomass, number of shoot nodes and shoot length by 39.03%, 150.13% and 9.94%, respectively, compared to the clay treatment. Thus, measures should be taken to mediate the sediment condition to restore submerged macrophyte communities with different dominant species.

Responses to simulated nitrogen deposition in invasive and native or non-invasive clonal plants in China

Previous studies suggest that a disproportionately high number of invasive plant species are clonal, and that an increase in N availability often promotes the spread of introduced plants. We tested the hypothesis that greater ability to increase performance in response to the increase in N availability is associated with greater invasiveness in clonal plant species on a regional basis in China, where the potentials for new introductions and for N deposition are high. We compared growth, allocation of mass, morphology, and N use efficiency in six pairs of closely related, widespread species of clonal plants with and without N addition designed to simulate future N deposition of 15xa0gxa0Nxa0m−2xa0year−1. Within each pair, one species was introduced and invasive in China, and the other was native or in one case introduced but not invasive. Added N increased the final dry mass of species by 10–120xa0% and the final number of ramets by up to 300xa0%. However, responses to N did not differ consistently (Pxa0>xa00.05) between invasive and native or non-invasive species; increase in total mass with added N ranged from being 6 times greater in the invasive species to 3 times greater in the native species in a pair. Results suggest that increased N availability due to deposition in China will favor the spread of some but not all introduced, clonal plant species in China.

Sediment type and nitrogen deposition affect the relationship between Alternanthera philoxeroides and experimental wetland plant communities

Wetlands have been demonstrated to be susceptible to invasions. Nutrient availability of wetland sediment is strongly affected by both sediment type and nitrogen deposition. We performed a greenhouse experiment to investigate the main effects and interactions between the presence of Alternanthera philoxeroides, sediment type and nitrogen deposition on biomass and evenness of experimental wetland plant communities. We established two types of plant communities, specifically wetland plant communities without and with A. philoxeroides, in two different sediment types crossed with two nitrogen deposition treatments. Experimental wetland plant communities consisted of four native or naturalised wetland species. Sediment type and nitrogen deposition significantly promoted A. philoxeroides growth. At the community level, the presence of A. philoxeroides decreased the total biomass of wetland plant species and increased community evenness, whereas sediment type significantly decreased evenness. At the species level, the presence of A. philoxeroides significantly decreased total biomass of Iris wilsonii and increased total biomass of Pontederia cordata. However, the interaction between invasion and nitrogen deposition significantly increased total biomass of Butomus umbellatus. These findings suggest that both sediment type and nitrogen deposition promote A. philoxeroides growth and exacerbate A. philoxeroides invasion into wetland plant communities. However, the presence of A. philoxeroides can increase the evenness of the wetland plant communities at a small scale by suppressing dominant species. The findings of the present study provide insights into the management of A. philoxeroides in wetlands.