Light limitation and litter of an invasive clonal plant, Wedelia trilobata, inhibit its seedling recruitment.

BACKGROUND AND AIMS: Invasive clonal plants have two reproduction patterns, namely sexual and vegetative propagation. However, seedling recruitment of invasive clonal plants can decline as the invasion process proceeds. For example, although the invasive clonal Wedelia trilobata (Asteraceae) produces numerous seeds, few seedlings emerge under its dense population canopy in the field. In this study it is hypothesized that light limitation and the presence of a thick layer of its own litter may be the primary factors causing the failure of seedling recruitment for this invasive weed in the field. METHODS: A field survey was conducted to determine the allocation of resources to sexual reproduction and seedling recruitment in W. trilobata. Seed germination was also determined in the field. Effects of light and W. trilobata leaf extracts on seed germination and seedling growth were tested in the laboratory. KEY RESULTS: Wedelia trilobata blooms profusely and produces copious viable seeds in the field. However, seedlings of W. trilobata were not detected under mother ramets and few emerged seedlings were found in the bare ground near to populations. In laboratory experiments, low light significantly inhibited seed germination. Leaf extracts also decreased seed germination and inhibited seedling growth, and significant interactions were found between low light and leaf extracts on seed germination. However, seeds were found to germinate in an invaded field after removal of the W. trilobata plant canopy. CONCLUSIONS: The results indicate that lack of light and the presence of its own litter might be two major factors responsible for the low numbers of W. trilobata seedlings found in the field. New populations will establish from seeds once the limiting factors are eliminated, and seeds can be the agents of long-distance dispersal; therefore, prevention of seed production remains an important component in controlling the spread of this invasive clonal plant.

Can differences in phosphorus uptake kinetics explain the distribution of cattail and sawgrass in the Florida Everglades?

BACKGROUND: Cattail (Typha domingensis) has been spreading in phosphorus (P) enriched areas of the oligotrophic Florida Everglades at the expense of sawgrass (Cladium mariscus spp. jamaicense). Abundant evidence in the literature explains how the opportunistic features of Typha might lead to a complete dominance in P-enriched areas. Less clear is how Typha can grow and acquire P at extremely low P levels, which prevail in the unimpacted areas of the Everglades. RESULTS: Apparent P uptake kinetics were measured for intact plants of Cladium and Typha acclimated to low and high P at two levels of oxygen in hydroponic culture. The saturated rate of P uptake was higher in Typha than in Cladium and higher in low-P acclimated plants than in high-P acclimated plants. The affinity for P uptake was two-fold higher in Typha than in Cladium, and two- to three-fold higher for low-P acclimated plants compared to high-P acclimated plants. As Cladium had a greater proportion of its biomass allocated to roots, the overall uptake capacity of the two species at high P did not differ. At low P availability, Typha increased biomass allocation to roots more than Cladium. Both species also adjusted their P uptake kinetics, but Typha more so than Cladium. The adjustment of the P uptake system and increased biomass allocation to roots resulted in a five-fold higher uptake per plant for Cladium and a ten-fold higher uptake for Typha. CONCLUSIONS: Both Cladium and Typha adjust P uptake kinetics in relation to plant demand when P availability is high. When P concentrations are low, however, Typha adjusts P uptake kinetics and also increases allocation to roots more so than Cladium, thereby improving both efficiency and capacity of P uptake. Cladium has less need to adjust P uptake kinetics because it is already efficient at acquiring P from peat soils (e.g., through secretion of phosphatases, symbiosis with arbuscular mycorrhizal fungi, nutrient conservation growth traits). Thus, although Cladium and Typha have qualitatively similar strategies to improve P-uptake efficiency and capacity under low P-conditions, Typha shows a quantitatively greater response, possibly due to a lesser expression of these mechanisms than Cladium. This difference between the two species helps to explain why an opportunistic species such as Typha is able to grow side by side with Cladium in the P-deficient Everglades.

Nutrient and growth responses of cattail (Typha domingensis) to redox intensity and phosphate availability.

BACKGROUND AND AIMS: In the Florida Everglades, the expansion of cattail (Typha domingensis) into areas once dominated by sawgrass (Cladium jamaicense) has been attributed to altered hydrology and phosphorus (P) enrichment. The objective of this study was to quantify the interactive effects of P availability and soil redox potential (Eh) on the growth and nutrient responses of Typha, which may help to explain its expansion. METHODS: The study examined the growth and nutrient responses of Typha to the interactive effects of P availability (10, 80 and 500 microg P L(-1)) and Eh level (-150, +150 and +600 mV). Plants were grown hydroponically in a factorial experiment using titanium (Ti(3+)) citrate as a redox buffer. KEY RESULTS: Relative growth rate, elongation, root-supported tissue/root ratio, leaf length, lateral root length and biomass, as well as tissue nutrient concentrations, were all adversely affected by low Eh conditions. P availability compensated for the negative effect of low Eh for all these variables except that low P stimulated root length and nutrient use efficiency. The most growth-promoting treatment combination was 500 microg P L(-1)/ + 600 mV. CONCLUSIONS: These results, plus previous data on Cladium responses to P/Eh combinations, document that high P availability and low Eh should benefit Typha more than Cladium as the growth and tissue nutrients of the former species responded more to excess P, even under highly reduced conditions. Therefore, the interactive effects of P enrichment and Eh appear to be linked to the expansion of Typha in the Everglades Water Conservation Area 2A, where both low Eh and enhanced phosphate availability have co-occurred during recent decades.

Vegetation responses to extreme hydrological events: sequence matters.

Extreme hydrological events such as flood and drought drive vegetation dynamics and are projected to increase in frequency in association with climate change, which could result in sequences of extreme events. However, experimental studies of vegetation responses to climate have largely focused on responses to a trend in climate or to a single extreme event but have largely overlooked the potential for complex responses to specific sequences of extreme events. Here we document, on the basis of an experiment with seedlings of three types of subtropical wetland tree species, that mortality can be amplified and growth can even be stimulated, depending on event sequence. Our findings indicate that the impacts of multiple extreme events cannot be modeled by simply summing the projected effects of individual extreme events but, rather, that models should take into account event sequences.

The 7-decade degradation of a large freshwater lake in central Yangtze River, China.

Freshwater lakes store water for human use and agricultural irrigation and provide habitats for aquatic fauna and flora. However, a number of these lakes have been degraded by human activities at a rapid rate. Here, we used historical land cover information and remotely sensed data to explore a 7-decade (between 1930s and 1998) shrinkage and fragmentation of Dongting Lake, the second largest freshwater lake in China, located in the drainage basin of Central Yangtze River. The water surface area of Dongting Lake decreased by 49.2%, from 4955 km2 in the 1930s to 2518 km2 in 1998, with an average decrease rate of 38.1 km2/yr in the past 7 decades. The lake was also fragmented, as indicated by a decreasing mean patch size from 4.2 km2 in the 1930s to 1.7 km2 in 1998. The degradation of the lake is largely attributed to a rapidly growing human population in the lake region that led to extensive impoldering. The degradation of the lake has resulted in negative ecological consequences, such as frequent flooding, a decline of biodiversity, and extinction of some endemic species. Our results also suggest that lake restoration projects implemented in this region since the end of the 1990s will help to decrease the lake degradation.

Growth and nutrient responses of Eloecharis cellulosa (Cyperaceae) to phosphate level and redox intensity.

Phosphorus (P) availability limits plant growth in many ecosystems. The ability of plants to explore for soil P is often impaired by nonresource stressors. Understanding the effects of these stressors on P acquisition in oligotrophic environments is critical in predicting species dominance. Growth and nutrient responses of Eleocharis cellulosa to redox intensity and phosphate level were evaluated under three redox potentials (Eh) and three phosphate (PO(4)) levels (P). Although low Eh (-150 mV) decreased root length at low P, Eh did not affect shoot height, relative growth rate (RGR), shoot elongation, photosynthesis, or biomass of E. cellulosa. Low PO(4) (10 µg P ·â€ŠL(-1)) strongly inhibited growth. Shoot height, RGR, elongation, photosynthesis, and biomass were lower at 10 µg P ·â€ŠL(-1) than at 80 or 500 µg P ·â€ŠL(-1). None of the growth variables, except the ratio of root-supported biomass to root biomass, significantly differed between the 80 and 500 µg P ·â€ŠL(-1) treatments. At low P, plants allocated relatively more biomass to roots than to shoots, compared to the medium and high P levels. Eleocharis cellulosa is well adapted to flooded conditions that lower soil Eh, and elevated PO(4) levels further promote its growth potential.

Interactive effects of redox intensity and phosphate availability on growth and nutrient relations of Cladium jamaicense (Cyperaceae).

Expansion of Typha domingensis into areas previously dominated by Cladium jamaicense in the Florida Everglades has been linked to anthropogenic phosphorus (P) enrichment and increased hydroperiod. The principal stress factor for plants in flooded soils is biochemical reduction, the intensity of which is measured as redox potential (Eh). The objective of this study was to assess the growth response of C. jamaicense to Eh (-150, +150, and +600 mV) and P availability (10, 80, and 500 µg P/L). Plants were grown hydroponically in a factorial experiment using titanium (Ti(3+)) citrate as an Eh buffer. Treatment effects on growth, biomass partitioning, and tissue nutrients were recorded. Growth approximately doubled in response to a 50-fold increase in P availability. Low redox significantly reduced growth and tissue P concentration. While plant P concentrations increased 20-fold between the 10 and 500 µg P/L treatments, P concentrations were 50-100% higher at +600 mV than at -150 mV within each phosphate level. At high Eh, C. jamaicense appears well adapted to low nutrient environments because of its low P requirement and high retention of acquired P. However, at low Eh the ability to acquire or conserve acquired P decreases and as a consequence, higher phosphate levels are required to sustain growth. Findings of this study indicate that young C. jamaicense exhibits low tolerance to strongly reducing conditions when phosphate is scarce.