Yonghong Xie

Trade-off between Tolerance to Drought and Tolerance to Flooding in Three Wetland Plants

We tested whether a trade-off exists between tolerance to flooding and tolerance to drought in wetland plants by assessing biomass accumulation, relative growth rate (RGR), survival rate, and physiological response of three wetland plants growing in drought or flooded environments. In wetlands of China’s Sanjiang Plain, Carex lasiocarpa typically occurs at low elevations (10–50 cm water depths), Carex limosa at medial elevation (10–30 cm depths), and Deyeuxia angustifolia at high elevation (0–10 cm depths). Plants of three species were subjected to flooding and drought treatments (25 days) in a greenhouse experiment. In the flooding treatments, biomass accumulation (range 0.007–0.031 g per plant) and survival rate (11%) were lowest in D. angustifolia. Relative growth rate (RGR) was highest in C. lasiocarpa (-0.006 d−1), intermediate in C. limosa (-0.051 d−1), and lowest in D. angustifolia (-0.118 d−1) at the end of the flooding experiment. Alcohol dehydrogenase (ADH) activity in C. lasiocarpa and C. limosa increased with flooding time, whereas ADH in D. angustifolia did not vary over the experimental period. These results indicated that tolerance to flooding from highest to lowest among the three species was: C. lasiocarpa > C. limosa > D. angustifolia. In the drought experiment, RGR was lower in C. lasiocarpa, but higher in C. limosa and D. angustifolia. At this experiment’s end, only D. angustifolia plants still survived. Under drought conditions, production of malondialdehyde (MDA, an indicator for assessing a plant’s ability to tolerate drought) showed the same pattern as ADH production under flooded conditions for all species. These results indicated that tolerance to drought from highest to lowest among the three species was: D. angustifolia > C. limosa > C. lasiocarpa. Our experiments indicate that a trade-off exists between tolerance to flooding and tolerance to drought in the three marsh plants.

The Risk Assessment of Sediment Heavy Metal Pollution in the East Dongting Lake Wetland

Total concentrations of cadmium (Cd), lead (Pb), chrome (Cr), and nickel (Ni) in surface sediment (0–10 cm) from the East Dongting Lake wetlands were determined and the spatial distribution of heavy metals was mapped. The results showed that the single risk indices () of heavy metals were ranked in the order of Cd > Pb > Ni > Cr. The content of Cd and Pb was gradually reduced from the east (Xiangjiang River) to the west, while the Cr and Ni content had a patchy distribution pattern in the East Dongting Lake wetlands. Cd and Pb contents were correlated with soil pH significantly, while Cr and Ni contents were correlated with soil organic carbon (SOC) and total nitrogen (TN). The origination of heavy metal (Cd, Cr, Ni, and Pb) could be divided into two groups: Cd and Pb from anthropogenic source and Cr and Ni from parent material weathering. Our results indicated that Cd posed a high risk to local ecosystem. The relatively lower pH and higher soil organic carbon (SOC) and total nitrogen (TN) in sediment may inhibit the fixation of heavy metals, which in turn increased the concentration of heavy metal in sediment.

Plasticity of the Clonal Growth in the Wetland Sedge Carex brevicuspis Along a Small-Scale Elevation Gradient in Dongting Lake Wetlands, China

To determine the growth strategies of the clonal wetland sedge Carex brevicuspis under different conditions, we measured the ramet density and biomass; numbers of total rhizomes (TR), long rhizomes (LR), and short rhizomes (SR); and environmental factors along a small-scale elevation gradient (23–26 m) in Dongting Lake wetlands, China. Ramet density and biomass were higher at low elevations as compared with those at intermediate and high elevations. There was no significant difference in the number of SR per ramet among the three elevations. However, the number of LR per ramet was higher at high and intermediate elevations as compared with that at low elevation. These results indicate that C. brevicuspis can adapt to increasing environmental stress (i.e., decreasing soil moisture contents) along an elevation gradient by producing more long rhizomes. Plasticity of the C. brevicuspis clonal growth through the modification of its meristem allocation patterns may allow for wide distribution of the species.

Trade-off between allocation to reproductive ramets and rhizome buds in Carex brevicuspis populations along a small-scale elevational gradient

The trade-off between allocation to sexual and clonal reproduction in clonal plants is influenced by a variety of environmental factors; however, it has rarely been examined under field conditions. In this study, we investigated the trade-off between two modes of reproduction in Carex brevicuspis C. B. Clarke across a small-scale elevational gradient (21–27 m a.s.l.) at the Dongting Lake wetlands, China. The proportion of biomass allocated to and the density of reproductive ramets were higher at low than at intermediate and high elevations. In contrast, the proportion of biomass allocated to and the density of rhizome buds were lower at low than at intermediate and high elevations. Redundancy analysis showed that sexual reproduction was positively correlated with soil moisture content, soil organic matter, total phosphorus, and pH, and negatively correlated with elevation and ramet density. Our findings suggested that allocation to sexual reproduction is favored in disturbed habitats with fertile soils, whereas allocation to vegetative propagation is favored in stable and competitive habitats. Trade-off between allocation to sexual reproduction and vegetative propagation along an elevational gradient might be a reproductive strategy of C. brevicuspis to adapt to the water level fluctuations in wetland habitats.

Effects of young poplar plantations on understory plant diversity in the Dongting Lake wetlands, China

This study evaluated the effects of young poplar plantations on understory plant diversity in the Dongting Lake wetlands, China. Poplar plantations resulted in a higher species number and Shannons diversity. Species compositions were different between areas with poplar and reed populations: a lower ratio of hygrophytes but a higher ratio of mesophytes, and a higher ratio of heliophytes but a lower ratio of neutrophilous or shade plants in poplar areas compared to reed areas. Poplar plantations supported a higher ratio of ligneous plants in the entire Dongting Lake area, but there was no difference in the monitored plots. Unlike reedy areas, poplar plantations had higher light availability but lower soil water content during the growing seasons. These data suggest that young poplar plantations generally increased species richness and plant diversity, but significantly changed species composition due to the reduced soil water and increased light availability.

Colonization by fragments of the submerged macrophyte Myriophyllum spicatum under different sediment type and density conditions

In this paper, the effect of plant density, sediment type, and macrophyte fragment size on the fragment colonization ability of Myriophyllum spicatum was evaluated in an outdoor experiment. The relative growth rate (RGR) was higher in the mud and low-density treatments than in the sand and high-density treatments. The relative elongation rate (RER) decreased with increasing density and fragment size, with RER values being much higher in the mud than the sand treatments. Both branching number and shoot diameter increased with decreasing density and increasing fragment size, and were significantly higher in the mud than the sand treatments. The shoot : root ratio was higher in the mud treatments than in the sand treatments. Total N content in both the shoot and root was significantly higher in the mud and low-density treatments than in the sand and high-density treatments. Shoot P content only decreased with increasing density, while root P content was higher in the mud and low-density treatments than in the sand and high-density treatments. These data indicate that fragment colonization by M. spicatum is improved by large fragments, low density, and nutrient-rich sediments, and that these conditions contribute to the rapid population expansion of this species.

Effects of sedimentation on soil physical and chemical properties and vegetation characteristics in sand dunes at the Southern Dongting Lake region, China.

Sedimentation is recognized as a major factor determining the ecosystem processes of lake beaches; however, the underlying mechanisms, especially in freshwater sand dunes, have been insufficiently studied. To this end, nine belt transects from nine freshwater sand dunes, classified into low (<23.7 m), medium (25.4–26.0 m), and high-elevation groups (>28.1 m) based on their elevations in 1972, were sampled to investigate differences in sedimentation rate and soil and vegetation characteristics in Southern Dongting Lake, China. Sedimentation rate, soil sand content, and soil pH increased, whereas soil clay, fine silt, moisture (MC), organic matter (OM), total N, and total K content, in addition to the growth and biodiversity of sand dune plants generally decreased with decreasing belt transect elevation. Regression analyses revealed that the negative effects of sedimentation on the ecosystem functions of sand dunes could be attributed to higher fine sand content in deposited sediments and stronger inhibition of plant growth. These results are consistent with previous studies performed in coastal sand dunes, which highlights the importance of sedimentation in determining ecological processes.

The role of seedling recruitment from juvenile populations of Carex brevicuspis (Cyperaceae) at the Dongting Lake wetlands, China

Seedlings and vegetative ramets may contribute differentially to the recruitment of clonal populations in different growth phases, but this has rarely been investigated. In this study, we quantified the number and survivorship of seedlings and vegetative ramets monthly in juvenile and mature populations of Carex brevicuspis. During the first growing season after flooding (from October to January), 9 seedlings m−2 (13% of all established shoots) were found in juvenile populations, while no seedlings were found in mature populations. During the second growing season before flooding (from February to May), no new seedling recruits were found either in juvenile or in mature populations. All shoots of seedlings were withered during the dormant season (January and February), but 62.5% seedlings could produce vegetative ramets in the following growing season. During the dormant season, all the early emerging ramets (sprouted in October) withered, but the later emerging ones (sprouted in November and December) survived in both mature and juvenile populations. These results indicated that seedling recruitment was only apparent in juvenile populations of C. brevicuspis. The genetic diversity in mature C. brevicuspis populations may be established in juvenile populations by seedling recruitment, and sustained in mature populations by vegetative reproduction.

Assessment of regeneration potential in the clonal macrophyte Miscanthus sacchariflorus (Poaceae) after burial disturbance based on bud bank size and sprouting capacity.

The demography of the bud bank and its sprouting capacity are important for understanding the population dynamics of clonal plants and their potential responses to disturbances. To this end, we investigated the size and composition of the bud bank of Miscanthus sacchariflorus (Maxim.) Hack. immediately after flooding (November), in winter (January), in spring (March), and before flooding (May) in the wetlands of Dongting Lake. We then examined the sprouting capacity of axillary buds after sediment burial at 0, 5, 10, 15, and 20 cm. Total bud density of M. sacchariflorus ranged from 2524 buds m-2 in November to 4293 buds m-2 in March. Rhizome segments with inactive axillary buds, which represented the majority of the bud population (88.7% in November, 93.3% in May), did not sprout during the 140 days of the experiment (n = 250). The sprouting ratio was the highest for active axillary buds buried at 0 cm (64%) and decreased when buried at 10–20 cm (34%–40%). Due to the large number of active axillary buds in the bud bank (211–277 buds m-2 from November to the following March), M. sacchariflorus could completely replace its aboveground shoot population, except in May (142 buds m-2). Increasing burial depth delayed bud emergence and reduced the growth period of shoots; however, burial depth did not affect the resulting plant height and only reduced the accumulated biomass at 20 cm. Therefore, the belowground bud bank and its strong sprouting capacity are important factors in the maintenance of local populations and colonization of new habitats for M. sacchariflorus after burial disturbances. The present methodology, which combined measurements of bud bank demography and sprouting capacity, may reflect the regeneration potential of clonal plants after burial disturbances.

Morphological and physiological responses to burial depth and sediment type in the wetland macrophyte Miscanthus sacchariflorus

The aim of this study is to elucidate how the wetland macrophyte Miscanthus sacchariflorus acclimatizes to sedimentation by investigating growth, morphology and nutrient content in a five month greenhouse experiment under five treatments: no burial, 6 cm sand, 6 cm clay, 12 cm sand and 12 cm clay (using one time burial). Compared to no burial treatment, biomass accumulation decreased only in the 12 cm sand burial treatment (9.5 versus 13.7 g plant). As depth of burial increased under both the sand and clay treatments, stem and leaf mass ratios, root diameter and P content in stems increased but root mass ratio and root length decreased. At 6 cm deep root mass ratio, mean root length and specific root length (SRL) were higher, while leaf and stem mass ratios, root diameter, P content in stems and N content in roots and leaves were lower in the sand than in the clay burial treatments. At 12 cm deep, only root mass ratio was higher, while stem mass ratio, N and P contents in roots were lower in the sand than in the clay burial treatments. This data indicates that the growth of M. sacchariflorus is not influenced by shallow burial through adjustments of biomass allocation and root morphology, and that clay facilitates M. sacchariflorus in acclimatizing to deep burial due to sufficient nutrient supply.