Population Growth of the Floating Weed Salvinia molesta: Field Observation and a Global Model Based on Temperature and Nitrogen

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Abstract
(1) Salvinia molesta Mitchell causes serious problems in many tropical countries by forming thick mats on the surface of freshwaters. Some of these problems have been solved by biological control using the beetle Cyrtobagous salviniae Calder & Sands (Coleoptera: Curculionidae). (2) Intrinsic rates of growth by S. molesta were measured every week over periods of from 16 weeks to 2 years at seven sites located near the equator to 33⚬S. The rates varied between zero, at the most southerly site in winter, up to 0.17 leaves per leaf per day (0.16 g g-1 d-1). (3) Self-crowding when the water surface was just completely covered, 50% shading, and one adult C. salviniae per five ramets of S. molesta, reduced intrinsic growth rates by 0.04, 0.03 and 0.005 g g-1 d-1 respectively. (4) Among the sites, intrinsic rates of growth and net assimilation rates were generally significantly correlated with air temperature and NPK content of the plant. In contrast, leaf weight ratio, top/root ratio and ramet weight were generally negatively correlated with the same variables. (5) Intrinsic growth rates, when adjusted for temperature effects using a relationship found in earlier controlled-environment studies, had a relationship with the nitrogen content of the plant very similar to that found in controlled-environment studies of the effects of nitrogen. (6) A model, based on the above relationship with N-content and controlled environment studies of the effects of temperature, explained between 40% and 80% of the variance in intrinsic growth rates observed at the different sites. There was little effect on the amount of variance explained if the model was driven by water temperatures calculated from observed air temperatures or by water temperatures calculated from latitude and date, and by mean temperatures or hourly temperatures interpolated from daily maxima and minima. Adding sunshine and P-content of the plant did not increase the fit of predicted to observed rates of growth. (7) At one site, the intrinsic growth rate was shown to be limited by the seasonal progression of temperature to a smooth annual cycle onto which were superimposed shorter frequency peaks and troughs resulting from the sequence: rainfall, runoff, elevated concentration of N in lakewater, elevated concentration of N in tissues of S. molesta, increased rate of growth by the plant. (8) A model was built to predict rates of growth for any latitude, altitude, time of year and nitrogen content of the plant. Predicted mean annual growth rates are presented for latitudes between zero and 50 ⚬C and four different nitrogen contents.