Kentaro Yutani

Growth performance of Phragmites australis in Japan: influence of geographic gradient

Abstract Most of the research on Phragmites australis is restricted to sites on the European continent even though P. australis occurs abundantly in many regions in the Asian and other continents under different climatic and habitat conditions. The effect of latitude on the growth and phenological characteristics of P. australis is of importance when translating results from one geographic site to another to effectively manage and conserve reed stands. Therefore, the effects of seasonal variations of above- and below-ground biomass, stand morphology and production, and radiation conditions on growth performance of a P. australis stand in Akigase Park in Saitama Prefecture, Japan, were investigated to examine the hypotheses that: (a) the overall light extinction coefficient of P. australis at a given growth stage may be modified by the sun elevation; and (b) phenological and growth/production traits of P. australis may be correlated with the latitude, by comparing the present study with published field studies from Europe and Australia. The P. australis stand was moderately productive, having a net aerial and below-ground production of 1980 and 1240 g m−2, respectively, and a maximum shoot density of 120±9 shoots per m2. We found that the overall light extinction coefficient, κ, at the different growth stages of P. australis depends on sun elevation, θ, displaying a quadratic distribution (κ=−7.58+0.28θ−0.002θ2). Therefore, in detailed production studies, κ should always be presented with its respective θ values to estimate light attenuation characteristics. The comparison of the growth performance of P. australis across the geographic gradient revealed differences in phenological and growth/production traits. Shoot growth and panicle formation started earlier in northern latitudes (on the European continent) and later in southern latitudes (on the Australian continent) than in Japan (on the Asian continent). Strong correlations were observed between the °C-day-based growth parameters and the latitudes illustrating the dependence of the phenological and growth/production traits on temperature in the different geographic regions. These results are discussed with respect to possible effects on adaptation of P. australis to colder climates.

Effect of broken dead culms of Phragmites australis on radial oxygen loss in relation to radiation and temperature

The amount of oxygen released from the roots of Phragmites australis was quantified to examine the effects of airflow through dead culms, radiation, and temperature on radial oxygen loss (ROL). To investigate the effect of dead culms on ROL quantitatively, the ROL of individual plants with open dead culms was compared to that of plants with sealed dead culms as a function of light intensity and temperature. The relationship between ROL and plant morphology (aboveground biomass, shoot diameter, shoot height) was investigated. When exposed to 300, 600, and 900 μmol m−2 s−1 light, the ROL was 15.6, 22.5, and 30.9 μmol O2 g−1 dry root day−1, respectively, from plants with open dead culms and 11.0, 16.4, and 23.3 μmol O2 g−1 dry root day−1, respectively, from plants with sealed dead culms. The ROL from plants with open dead culms was obviously higher than that from plants with sealed dead culms in every condition. The ROL from plants with open culms was 37% and 30% higher than that from plants with sealed culms at 20°C and 30°C, respectively. The effects of plant-specific parameters such as leaf area and shoot diameter on radial oxygen loss were evident. From the point of view of rhizosphere oxidation during the growing season, the existence of open dead culms should be taken into consideration for optimal plant management in constructed wetlands. This study provides a theoretical understanding of the effects of open dead culms, light conditions, and temperature on radial oxygen loss.

Age-specific seasonal storage dynamics ofPhragmites australis rhizomes: a preliminary study

Age-specific seasonal rhizome storage dynamics of a wetland stand of Phragmites australis (Cav.) Trin. ex Steud. in Japan, were investigated from April to October 2000. For each sampling date, above- and below-ground biomass and age-specific rhizome bulk density, ?rhiz were measured. Seven rhizome age classes were recognized, from <1 year to six years old, based on their position within the branching hierarchy as main criteria and rhizome color, condition of nodal sheaths and condition of the shoots attached to vertical rhizomes as secondary criteria. P. australis stand was moderately productive, having a net aerial and below-ground production of 1980 and 1240 g m−2, respectively, and a maximum mean shoot height of 2.33 ± 0.12 m. In spring, shoot growth started at the expense of rhizome reserves, decreasing the rhizome biomass as well as ?rhiz. Both parameters reached the seasonal minimum in May followed by a subsequent increase, indicating a translocation of reserves to rhizomes from shoots after they become self supporting. For each sampling date, ?rhiz increased with rhizome age. Given that the quantity of reserves remobilized by the rhizomes for spring shoot growth, as assessed by the drop in bulk density from April to May, were positively correlated (r = 0.97, P < 0.05) with rhizome age, it is proposed that for spring shoot formation older rhizomes remobilize stored reserves more actively than younger ones. Given that the accumulation of rhizome reserves (rise in bulk density) from May to August, May to September or May to November was negatively correlated (r = 0.97, 0.92 and 0.87, respectively, P < 0.05) with rhizome age, it seemed possible that younger rhizomes were ‘recharged’ at a higher rate than older ones. These resource allocation mechanisms pertaining seasonal rhizome storage dynamics are of paramount importance in formulating management and conservation strategies of wetlands and aquatic habitats. Our results indicate that a harvest of above-ground biomass from May to June would be more effective in reducing the growth than a harvest in July to August or later, when rhizome reserves have already been replenished. However, the latter may remove a larger shoot bound nutrient stock, still preserving a healthy stand for the subsequent years.