Shiromi Karunaratne

Dynamic modeling of the growth of Phragmites australis: model description

A dynamic model was developed to simulate the growth dynamics of a monospecific stand of Phragmites australis in freshwater ecosystems. Five state variables (biomass of shoots, inflorescence, roots, old rhizomes and new rhizomes) were selected to illustrate the growth of P. australis. Growth was described using mathematical relationships. The net growth of the plant stand was the integral effect of photosynthesis, respiration, mortality and assimilate translocation between shoots and below-ground plant organs. Below-ground biomass (i.e. rhizome and root biomass) before the growth commencement, daily total global radiation and daily mean air temperature were input data. The model is capable of simulating the seasonal variation of above-ground biomass (shoots, stems, leaves and panicles), leaf area index, rhizome, new rhizome, root biomass and shoot height with correlation coefficients close to 1.0 for most of the parameters. The model estimated the conversion efficiency of photosynthetically active radiation varying from 3.76 to 7.19% from northern temperate regions to warmer southern temperate regions. The carbon budget was constructed using the modelled predictions. Analysis of annual net production and fluxes showed that irrespective of the varying climatic conditions, the percentage of annual fluxes of an event, as a proportion of the total photosynthetic production remained almost same. The respiration of shoots, as well as rhizomes and roots, was shown to consume a considerable amount of photosynthetic production: 25% by shoot respiration and 40% by rhizome and root respiration.

Seasonal fluctuations in live and dead biomass of Phragmites australis as described by a growth and decomposition model: implications of duration of aerobic conditions for litter mineralization and sedimentation

Abstract We developed a model of Phragmites australis growth and decomposition to evaluate the material budget and nutrient cycles of a reed stand in Neusiedlersee, Austria. The model describes the growth of each organ of P. australis, the collapse of standing dead shoots, the decomposition of leaves and stalks, and nutrient uptake and release during these processes. The model was calibrated using growth and decomposition data from the literature, and subsequently applied to predict the effects of P. australis stands on a marsh ecosystem. From the start of its decomposition in water, the litter was assumed to stay in the aerobic water layer for 6, 12 or 24 months before entering the anaerobic sediment layer. Because decomposition increases with increasing oxygen and temperature, the aerobic decomposition rate (before the litter was transferred to the anaerobic substrate) increased markedly, especially from spring to autumn. The model predicted that between 33 (6 months aerated) and 48% (24 months aerated) of the annual aboveground production would decompose within 1 year, while the rest would remain in the anaerobic substrate. Rates of nitrogen and phosphorus release were 1.4 times higher between late spring and the end of summer than during autumn and winter. A higher proportion of phosphorus than nitrogen was expected to remain trapped in the anaerobic layer. The uptake of nitrogen and phosphorus during the growing season exceeded release during decomposition 4–6 and 5–7-fold, respectively. The model is useful for quantifying the nutrient cycles of reed-dominant marshes.

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.

Verification of a mathematical growth model of Phragmites australis using field data from two Scottish lochs.

A growth model ofPhragmites australis was verified using two independent sets of published field data. The model simulates the growth pattern of a well-established, monospecific stand ofP. australis in the absence of genetic diversity and environmental stresses of mainly nutrient and water deficiency. The model formulated using first order differential equations was combined with plant phenology and comprises five subroutines in which photosynthetically active radiation, shoot, root, rhizome and new rhizome biomass are calculated. Using the model, experimental results were reproduced within reasonable limits having concordance correlation coefficients of more than 0.75 for 70% of the output parameters, which was the main objective of the study. The modelled efficiencies of PAR were 7.15% and 3.09%, as opposed to 7.7% and 2.53% in experimental estimations, for Loch of Forfar and Loch of Balgavies, respectively. Production and seasonal fluxes of dry matter ofP. australis in Scottish lochs were estimated using the modelled quantities for the 1975 growing season in g m−2. They showed that 31% and 37% of total net photosynthate translocated to rhizomes before shoot senescence began in Loch of Forfar and Loch of Balgavies, respectively. Also in both lochs approximately 45% of total downward translocation came from accumulated shoot dry matter during senescence, while the rest came from photosynthesis before the shoots started to senesce.

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.

Colour-based estimation of rhizome age in Phragmites australis

The colour of different age groups of Phragmites australis (Cav.) Trin. ex Steudel rhizomes was studied from April through October 2000 at approximately one-month intervals to propose a more efficient method to identify the rhizome age based on the Munsell colour-order system. Seven rhizome age-classes were recognized, from <1 to 6 years old, based on descriptions published in the scientific literature. During April and May sampling, spectral reflectance between 400 and 700 nm of different rhizome ages was measured at 10 nm intervals, using a spectral colorimeter. Rhizomes of different ages were assigned colours by selecting one/two shortest Euclidian distances between the mean spectral reflectance of each rhizome age category and the Munsell colours on the four-dimentional subspace, made by Principal Component Analysis of the spectral reflectance data of 1289 Munsell colours. The Munsell colour for new to six-year-old rhizomes changed from yellow to yellow-red, and the value decreased from new to six-year-old rhizomes, indicating a darkening with ageing. The age of rhizomes collected from April through October was estimated using the colour key, in addition to the age attribution based on branching hierarchy. Between 87% and 100% of the rhizomes attributed to a certain age class based on branching hierarchy were assigned to the same age class using colours during all sampling dates. There was a strong correlation (r = +0.96) between rhizome age estimated by branching hierarchy and colour. At each sampling, bulk density, an indicator of rhizome storage levels, measured as a verification of age identification, varied among the age categories indicating distinct differences in storage levels. These results confirmed that rhizomes of a specific age category could be assigned a distinct colour, which remains more or less unchanged throughout the growing season. Thus, colour can be used as a primary criterion in the estimation of the age of P. australis rhizomes.