C. Mony a,b, *, T.J. Koschnick c, W.T. Haller c, S. Muller b

Transcription

1 Aquatic Botany 86 (2007) Competition between two invasive Hydrocharitaceae (Hydrilla verticillata (L.f.) (Royle) and Egeria densa (Planch)) as influenced by sediment fertility and season C. Mony a,b, *, T.J. Koschnick c, W.T. Haller c, S. Muller b a University of Rennes1, UMR Ecobio, Avenue du Général Leclerc, Rennes Cedex, France b University of Metz, LBFE, Avenue du Général Delestraint, Metz, France c University of Florida, Center for Aquatic and Invasive Plants, 7922 NW 71st St., Gainesville, FL 32653, USA Received 30 November 2005; received in revised form 19 October 2006; accepted 9 November 2006 Abstract Competition between two invasive plants of similar growth form, Hydrilla verticillata (L.f.) (Royle) and Egeria densa (Planch), was studied in response to season and sediment fertility. These two invasive species were grown in outdoor concrete tanks in monocultures and mixtures. Five fertilization rates were tested for monocultures and two for mixtures where six combinations of planting densities were used in two seasons (spring and fall). Monitoring of plant biomass was made at the end of each of these 2-month-experiments. In contrast to E. densa, clear seasonal patterns in biomass production and in reproductive allocations of H. verticillata were evident. Competitive pressure for both species was lower during the fall experiment. Biomass production increased with fertilization for H. verticillata in monocultures and changes either in allocative ratios or in tuber production patterns were shown in response to nutrient availability. However, E. densa growth was not affected by fertilization. In most cases, H. verticillata was a better competitor than E. densa except when sediment was pure sand. Competition occurred mainly for nutrient uptake rather than for light harvesting. These results suggest that despite the similar ecology, H. verticillata may outcompete E. densa in many situations, probably due to its higher plasticity. # 2006 Elsevier B.V. All rights reserved. Keywords: Ecological trade-off; Root:shoot ratio; Morphological plasticity; Nutrient stress 1. Introduction The understanding of the competitive strategies of exotic plant species has been of significant interest in the last 20 years (Rejmanek and Richardon, 1996; Lonsdale, 1999; Williamson, 1999). Competitive strategies are linked with the optimal resource use theory (Bloom et al., 1985; Tilman, 1988), predicting that plants allocate resources according to their proportional limitation for each of the resources. The ability to optimize resources requires therefore phenotypic plasticity of allocation to vegetative (root/shoot) versus reproductive (asexual/sexual) parts (Bradshaw, 1965; Grime et al., 1986; Sultan, 1987). Relatively few investigations * Corresponding author at: University of Rennes1, UMR Ecobio, Avenue du Général Leclerc, Rennes Cedex, France. Tel.: ; fax: address: cendrine.mony@univ-rennes1.fr (C. Mony). described interactions among exotic species (Simberloff and Von Holle, 1999; Richardson et al., 2000) and especially between plants of similar ecological strategies and growth form. However, this is where the most intense competition may be expected to occur (Gopal and Goel, 1993). The present work aimed at studying competitive outcomes between Hydrilla verticillata (L.f.) (Royle) and Egeria densa (Planch), with varying resource availabilities (as fertilizer rate) and investigating how the species manage the trade-offs in resource allocation. H. verticillata and E. densa are both exotic aquatic species to N. America, native, respectively, to Asia and eastern Australia (Cook and Lüond, 1982; Swarbrick et al., 1982) and south America (eastern Argentina, southern Uruguay, South-eastern Brazil) (Cook et al., 1974). They occur potentially in similar habitats and have a wide ecological range though they do not often occur together. Some field observations show that H. verticillata outcompeted successfully E. densa in nutrient-rich situations (Haller, Personal Communication). It is however /$ see front matter # 2006 Elsevier B.V. All rights reserved. doi: /j.aquabot

2 C. Mony et al. / Aquatic Botany 86 (2007) difficult to draw general patterns on the occurrence of both species in relation with nutrient level in the field. Both species are highly competitive in meso-eutrophic waters (Howard- Williams, 1993; Hoyer et al., 1996). However, at low nutrient availability, H. verticillata becomes a weaker competitor against some native North-American species, such as Vallisneria americana (L.) (Haller and Sutton, 1975; Van et al., 1999) or Potamogeton nodosus (Poir.) (Spencer and Ksander, 2000). We hypothesized that in nutrient-poor sites, E. densa may outcompete H. verticillata and that the reverse might be found in nutrient-rich sites. The present study aimed therefore to analyze the competitive interference between H. verticillata and E. densa at low and high fertilizer rates. Varying fertilizer rates were used to manipulate sediment fertility and nutrient availability. A first experiment was conducted at the beginning of spring equinox long day period (spring experiment) and a second experiment was made during fall equinox (short-days) at the end of the vegetative period (fall experiment). The following questions were addressed: (i) Does sediment fertility influence the growth of H. verticillata or E. densa in monoculture and in competition (mixtures)? (ii) Does the outcome of competition between the two species differ with the period of growth? (iii) Did both species display different resource allocation patterns? 2. Materials and methods The experiments were conducted in outdoor concrete tanks (220 cm 77 cm 65 cm) at the University of Florida in Gainesville, Florida. Cultures were grown from April 10 to June 5 of 2002 (spring experiment) and from October 10 to December 10 of 2003 (fall experiment). Air and water temperature were measured with an Optic StowAway- TELO (C) ONSET during the fall experiment, whereas only air temperature was measured via a local weather station during the spring experiment. Therefore, water temperatures for the spring experiment were deduced from the correlation between air and water temperature obtained from the fall experiment. Plants were collected from the Santa Fe River, Florida, where both species co-occur. The river is spring fed. Plants were collected 2 days before the experimental set up and stored at room temperature (20 8C). H. verticillata was of the dioecious type. Apical tips of 10 2 cm in length were planted in 3-L plastic pots (17.1 cm diameter 13.3 cm deep). Pots were filled with a mixture of sand and a controlled slow-release fertilizer (Osmocote 1 Plus; ). Tanks were filled with water with a ph of The fertilizer was formulated for a month release rate at 21 8C. The effects of nutrient availability in the sediment was tested as both species preferentially assimilate nutrients by roots (Barko and Smart, 1980). In each tank, a maximum of eight pots were placed randomly and allowed to grow under ambient conditions. Pots with the same fertilizer modality were grouped in two different tanks for 0, 1 and 4 g/kg sand/pot and in five tanks for 0.5 and 2 g/kg sand/pot. Enough space was left between pots to avoid interaction among plants of different pots. A 70% shade cloth was put over tanks to keep shallow tank water cooler. Plants were grown in monocultures and in mixed cultures in individual pots. Growth of each species was measured in monocultures with a density of nine individuals under five separate modalities of fertilization (0, 0.5, 1, 2, 4 g/kg sand/ pot). In mixed plantings, two rates of fertilizer, 0.5 g/kg sand Osmocote (low fertility) and 2 g/kg sand (high fertility) per pot were used, and the experimental design followed an addition series (Spitters, 1983) consisting of factorial combinations of different densities of the two species. The Hydrilla:Egeria planting densities were 3:0; 9:0; 3:3; 3:9; 9:3; 9:9; 0:3; 0:9; plants per container (Van et al., 1999). Five replicates for each treatment combination and fertilizer levels were made. Cultures were maintained for 2 months, and all plants were then harvested and measured. Five biological traits were measured: total dry weight, root:shoot ratio, proportion of dry weight allocated to tubers, number of tubers, mean dry weight of a tuber. The three last traits concerning tubers were only measured for H. verticillata as E. densa do not produce tubers. One-way anovas testing for each species and each collection time the effect of fertilizer on plant traits and for each combination of species, collection time and fertilizer level, the effect of planting densities on plant traits. True randomization was not possible and the grouping of treatment replicates, though statistically undesirable, was necessary because of the difficulty to separate each pot in individual experimental tanks. Limitations might therefore be created on analyses of the results. Tukey a-posteriori tests were used to identify specific differences between the species across collection times. Before all statistical analyses, data were log-transformed if required to ensure normality and homogeneity of variances (Sokal and Rohlf, 1995). The relative competitive abilities for the two species were analyzed using the reciprocal-yield model (Spitters, 1983). This model involves multiple linear regressions of the form: 1 W H ¼ a HO þ a HH N H þ a HE N E 1 W E ¼ a EO þ a EE N E þ a EH N H with W H and W E as mean dry weight per plant and N H and N E as respective densities for H. verticillata and E. densa, respectively. Planting densities were chosen following the experiment of Van et al. (1999) to allow potential comparisons. The intercepts (a HO and a EO ) estimate the reciprocal of the maximum weight of isolated plants. Intraspecific competition was estimated by the partial regression coefficients a HH and a EE, while interspecific competition was estimated by a HE and a EH.The significance of these coefficients was tested with F-tests. Competitive interactions were analysed for root or shoot weight in order to separate below-ground from above-ground competition.

3 238 C. Mony et al. / Aquatic Botany 86 (2007) Results Experiments (spring and fall) were characterized by similar water temperatures, except for the end of the fall experiment when temperatures dropped (Fig. 1, spring = C 2.5; fall = C 4.8). The maximum temperature during the spring experiment was C whereas it was C for the fall experiment (Fig. 1). E. densa and H. verticillata rapidly grew and occupied at harvest about 60 70% of the available space Monocultures H. verticillata had a higher dry weight/plant, except when no fertilizer was added (0 g/kg), and root:shoot ratio than E. densa in the spring experiment (Fig. 2). H. verticillata growth in response to fertilizer rates differed between the spring and fall experiments. In spring, H. verticillata shoot dry weight increased with fertilization and reached 2.1 g/plant for fertilization 4.0 g/kg. In fall, the dry weight was the highest for the concentrations 0.5, 1.0, and 2.0 g/kg fertilization, but lower for the fertilization concentrations 0 and 4 g/kg. In contrast, E. densa dry weight did not increase with fertilization. In fall, E. densa displayed a higher dry weight for concentration 0.5 g/kg than for the other concentrations. Variation of the root:shoot ratio was dependent on nutrient availability for both species during the spring experiment (Fig. 2). Both species had higher root:shoot ratios at low fertilizer concentration, with the exception of E. densa at 0 g/kg. H. verticillata had a high root:shoot ratio in pure sand (not amended with fertilizer), but this may reflect a strong limitation of overall growth as the ratio was similar for all other fertilization rates. No differences in root:shoot ratio were observed for either species in the fall experiment. A higher number and biomass were allocated to tubers produced during fall experiment compared to spring: a mean number of tubers/plant was counted in spring, whereas tubers/plant were produced in fall with contrasting dependence on nutrient availability. In spring, H. Fig. 1. Water temperature during spring and fall experiments. verticillata allocated more biomass to tubers in low fertilizer concentrations (0.5 g/kg) than in high (1, 2, and 4 g/kg). Fertilizer added at 4.0 g/kg seemed to be somewhat inhibitory to H. verticillata tuber production in the fall. In spring, tuber weight tended to be higher for concentration 0.5 g/kg. In fall, similar tuber weights were measured for all concentrations except 4.0 g/kg Mixed cultures Analyses following Spitters model show a significant competitive effect for H. verticillata at low nutrient concentration in spring. No competitive effect was found in spring for E. densa and in fall for either species (Table 1). As both above- and belowground competition interact to affect plant growth, the data were analyzed based on root or shoot biomass. The results indicate a significant belowground competition by E. densa at both fertilizer concentration in spring and only at low concentration in fall (Table 1). A significant belowground competition was found for H. verticillata at low concentration during both experiments. Correlation ratios were approximately two times greater for E. densa in spring. A significant above-ground competition was only found for H. verticillata in spring at low fertilizer rates. One-way ANOVA underlined a significant impact of planting densities on E. densa s dry weight at high fertility during fall experiment and H. verticillata s dry weight at low fertility during spring experiment (Table 2; Fig. 3). In both cases, dry weight was lowest at the highest plant density (9:9). At low fertility, root:shoot ratio in E. densa was greater for planting density 0:3 than for 3:3 and 9:9, which may suggest that the competition with H. verticillata caused a greater allocation to roots. With respect to reproductive traits, only the effect of plant density on tuber individual weight in spring at low fertility was found as significant (Table 2; Fig. 3). 4. Discussion 4.1. Seasonality in biomass allocation and plant competition Season strongly influenced results for H. verticillata whereas this seasonal effect was less marked for E. densa. H. verticillata achieved intense vegetative growth during spring experiment but less than 5% of biomass was allocated to tuber production whereas in fall, though less biomass is produced, around 14 20% was allocated to vegetative reproduction. This corresponds with phenological stages previously described by Haller et al. (1976). Inspringafew big tubers were produced but numerous small tubers resulted in fall. E. densa achieved the same growth in spring or fall with lower root/shoot ratio obtained during the latter experiment. Seasonal effects were also shown in competition. More significant and stronger interactions were found in spring (four significant interactions with three with r 2 > 0.4 in spring compared to two with r 2 < 0.3infall),whichmaybelinked with individual H. verticillata seasonal patterns of growth.

4 C. Mony et al. / Aquatic Botany 86 (2007) Fig. 2. H. verticillata and E. densa s biological traits (mean standard deviation) in function of sediment fertility. The letters a, b, c, represent significant different statistical groups calculated through post hoc tests after significant one-way anovas carried out for each combination (species, season). DW: dry weight. While in competition, H. verticillata showed higher growth capacity than E. densa in spring. These results may reflect interspecific differences either in relative growth rates (i.e. H. verticillata has a faster RGR than E. densa)oringrowthperiod (i.e. E. densa starts growing later in the season than H. verticillata). In either case, H. verticillata is likely to gain advantage over E. densa by pre-emptive acquisition of resources and occupation of space Did sediment fertility influence growth and resources allocations of H. verticillata and E. densa? Monoculture experiments highlighted different allocation patterns as a function of fertilizer rate. For H. verticillata, dry weight increased linearly with fertilizer concentrations in spring and following a parabolic variation in fall. Spring results are quite similar to experimental behavior of other macrophyte

5 240 C. Mony et al. / Aquatic Botany 86 (2007) Table 1 Synthesis of Spitter s models analyzing below-ground and above-ground competition for spring and fall experiments Total competition Below-ground Competition Above-ground Competition Spring experiment Egeria (low) Egeria (high) Hydrilla (low) Hydrilla (high) Fall experiment Egeria (low) Egeria (high) Hydrilla (low) Hydrilla (high) 1/W = N H N E (R 2 = 0.11; n.s.) 1/W = N H N E (R 2 = 0.24; <0.1) 1/W = N H N E (R 2 = 0.31; ***) 1/W = N H 0.001N E (R 2 = 0.05; n.s.) 1/W = N H N E (R 2 = 0.46; <0.1) 1/W = N H N E (R 2 = 0.20; <0.1) 1/W = N H N E (R 2 = 0.15; n.s.) 1/W = N H 0.003N E (R 2 = 0.08; n.s.) 1/W root = N H N E (R 2 = 0.57; ***) 1/W root = N H N E (R 2 = 0.46; **) 1/W root = N H N E (R 2 = 0.22; *) 1/W root = N H 0.05N E (R 2 = 0.13; n.s.) 1/W root = N H + 2.3N E (R 2 = 0.28; *) 1/W root = N H N E (R 2 = 0.04; n.s.) 1/W root = N H N E (R 2 = 0.23; *) 1/W root = N H 0.11N E (R 2 = 0.002; n.s.) In bold are highlighted significant regressions. W, dry weight; N H, Hydrilla density; N E, Egeria density. 1/W shoot = N H N E (R 2 = 0.06; n.s.) 1/W shoot = N H N E (R 2 = 0.19; n.s.) 1/W shoot = N H N E (R 2 = 0.47; ***) 1/W shoot = N H 0.002N E (R 2 = 0.04; n.s.) 1/W shoot = N H N E (R 2 = 0.13; n.s.) 1/W shoot = N H N E (R 2 = 0.18; n.s.) 1/W shoot = N H N E (R 2 = 0.14; n.s.) 1/W shoot = N H 0.017N E (R 2 = 0.04; n.s.) species (Best et al., 1996). Together with this increase of dry weight, we noted a simultaneous decrease in root/shoot ratio. This represents a classical adaptive plasticity with allocation to tissue responsible for nutrient harvesting in limiting nutrient conditions rather than to shoots responsible of light harvesting in non-limiting nutrient conditions (Barko and Smart, 1981, 1986; Idestam-Almquist and Kautsky, 1995; Greulich and Bornette, 1999). Allocation of biomass to tubers was also affected by nutrient fertilization in spring in contrast to fall when plants growing in pure sand produced less tubers but with a mean weight for each propagule similar to those found for higher fertilization rates. At 4 g/kg however, the low allocation of biomass to tubers was characterized either by an intermediate number of tubers but also by the lowest propagule weight. Rodriguez-Girones et al. (2003) highlighted this size/ number trade-off in aquatic plant propagule production and especially in tuber production. This trade-off conducts plants to produce an optimal plant propagule size and number in relation to resource availability. H. verticilla is therefore able to display Table 2 F-values and p-values for the effect of density on E. densa s and H. verticillata s biological traits for spring and fall experiments at low and high fertility Low fertility High fertility Spring Fall Spring Fall E. densa Dry weight 2.26 < n.s. 1.24n.s. 2.71* Root:shoot ratio 4.19** 3.53* 1.40n.s. 0.90n.s. H. verticillata Dry weight 6.98*** 0.96n.s. 1.03n.s. 1.89n.s. Root:shoot ratio 0.75n.s. 1.15n.s. 0.23n.s. 0.28n.s. Tubers allocation 1.12n.s. 1.81n.s. 0.78n.s < 0.1 Number of tubers 0.30n.s. 1.06n.s. 1.40n.s. 1.51n.s. Tuber individual weight 2.63* 1.35n.s < n.s. In bold are highlighted significant tests. rapid and strong morphogenetic responses in order to (i) grow rapidly and (ii) produce a high number of tubers that may be dispersed to colonize less dense sites. In contrast, E. densa showed no significant variation in dry weight nor root:shoot ratio based on fertilizer rate. These results may suggest that E. densa optimized growth under these experimental conditions and should uptake sufficient nutrients through passive diffusion from water when sediment is nutrient-poor and from sediment when it is nutrient-rich. This opportunist strategy has been reported for Myriophyllum spicatum (L.) (Carignan and Kalff, 1980) Did sediment fertility influence H. verticillata and E. densa competitive strategy? E. densa and H. verticillata, when grown in mixtures, competed more strongly for underground resources than for light. This may be explained by complementary occupations of space in the vertical water column. H. verticillata elongates rapidly to the water surface, branches profusely and places its canopy within the first 5 cm of the water surface (Haller and Sutton, 1975) whereas E. densa develops a more diffuse canopy that comes deeper in the water (Moon, 1994). In the spring experiment, a significant competitive effect on above-ground biomass was shown for H. verticillata at low fertilizer rates. H. verticillata s lack of competitiveness under low nutrient situations was demonstrated in previous work on competition between H. verticillata and several native American species of contrasting growth form (Haller and Sutton, 1975; Van et al., 1999; Spencer and Ksander, 2000). Such weak competitiveness may be the indirect effect of nutrient stress. Tuber production was insensitive to planting densities. This may be a short-term response to competitive pressure as longer experiments showed decreasing tuber production in H. verticillata when grown with high densities of E. canadensis, E. densa or L. major (Hofstra

6 C. Mony et al. / Aquatic Botany 86 (2007) Fig. 3. Planting density effect on E. densa and H. verticillata s biological traits. (Mean + standard deviation) is presented on graphs for 2 g/kg sand and (mean standard deviation) for 0.5 g/kg sand. X-axis represents Hydrilla:Egeria density combinations. F-values and p-values are synthesized in Table 2. et al., 1999). Therefore, the two species may co-occur for a short time (2 months) without apparent influence on tuber or above shoot production. At a longer term, this co-occurrence may not be stable. In some locations, quick replacements of E. densa by H. verticillata have been observed whereas this was not noticed in many Florida sites, where H. verticillata has been introduced recently (Haller, Personal Communication). The outcome of competition should also depend on other environmental factors, such as water temperature. E. densa reaches maximum biomass at a lower temperature range than H. verticillata (Cook and Urmi-König, 1984; Haramoto and Ikusima, 1988; Moon, 1994; Madsen and Owens, 2000). E. densa may also preferentially grow in flowing waters rather than stagnant (Moon, 1994). Finally, disturbance, such as alternative floods and droughts may explain co-occurence of both species in the Santa Fe River (Haller, Personal Communication). Acknowledgements We are sincerely grateful to Drs. R. K. Stocker and A.M. Fox for providing useful advice when elaborating this study and to J. Vermaat and two anonymous referees for their constructive comments on a previous version of this paper. We want to thank also L. Huey and M. Glenn for technical assistance during the experiments. References Barko, J.W., Smart, R.M., Mobilization of sediment phosphorus by submersed freshwater macrophytes. Freshwater Biol. 10, Barko, J.W., Smart, R.M., Comparative influences of light and temperature on the growth and metabolism of selected submersed freshwater macrophytes. Ecol. Monographs 51, Barko, J.W., Smart, R.M., Sediment-related mechanisms of growth limitation in submersed macrophytes. Ecology 67,

7 242 C. Mony et al. / Aquatic Botany 86 (2007) Best, E.P.H., Woltman, H., Jacobs, F.H.H., Sediment-related growth limitation of Elodea nuttallii as indicated by a fertilization experiment. Freshwater Biol. 36, Bloom, A.J., Chapin III, F.S., Mooney, H.A., Resource limitation in plants-an economic analogy. Ann. Rev. Ecol. Syst. 16, Bradshaw, A.D., Evolutionary significance of phenotypic plasticity in plants. Adv. Genet. 13, Carignan, R., Kalff, J., Phosphorus sources for aquatic weeds: water or sediments? Science 207, Cook, C.D.K., Lüond, R., A revision of the genus Hydrocharis (Hydrocharitaceae). Aquat. Bot. 14, Cook, C.D.K., Urmi-König, K., A revision of the Genus Egeria (Hydrocharitaceae). Aquat. Bot. 19, Cook, C.D.K., Gut, B.J., Rix, E.M., Schneller, J., Seitz, M., Water Plants of the World: a Manual for the Identification of the Genera of Freshwater Macrophytes. Junk Publishers, The Hague, Netherlands. Gopal, B., Goel, U., Competition and allelopathy in aquatic plant communities. Bot. Rev. 59, Greulich, S., Bornette, G., Competitive abilities and related strategies in four aquatic plant species from an intermediately disturbed habitat. Freshwater Biol. 41, Grime, J.P., Crick, J.C., Rincon, J.E., The ecological significance of plasticity. In: Jennings, D.H., Trewavas, A.J. (Eds.), Plasticity in Plants. Cambridge University Press, Cambridge, pp Haller, W.T., Sutton, D.L., Community structure and competition between hydrilla and vallisneria. Hyacinth Control J. 13, Haller, W.T., Miller, J.L., Garrard, L.A., Seasonal production and germination of hydrilla propagules. J. Aquat. Plant Manag. 14, Haramoto, T., Ikusima, I., Life cycle of E. densa Planch., an aquatic plant naturalized in Japan. Aquat. Bot. 30, Hofstra, D.E., Clayton, J., Green, J.D., Auger, M., Competitive performance of Hydrilla verticillata in New Zealand. Aquat. Bot. 63, Howard-Williams, C., Processes of aquatic weed invasion: the New Zealand example. J. Aquat. Plant Manag. 31, Hoyer, M., Canfield, D., Horsburgh, C., Brown, K., Florida Freshwater Plants. University of Florida, 263 pp. Idestam-Almquist, J., Kautsky, L., Plastic responses in morphology of Potamogeton pectinatus L. to sediment and above-sediment conditions at two sites in the northern Baltic proper. Aquat. Bot. 52, Lonsdale, W.M., Global patterns of plant invasions and the concept of invasibility. Ecology 80, Madsen, J.D., Owens, C.S., Factors contributing to the spread of hydrilla in lakes and reservoirs. In: Aquatic Plant Control Technical Notes Collection (ERDC TN-APCRP-EA-01), US Army Engineer Research and Development Center, Vicksburg, MS. Moon, M., The effect of temperature on the distribution physiology and competitive ability of Hydrilla verticillata and Egeria densa. MS Thesis, University of Florida, Gainesville, FL. Rejmanek, M., Richardon, D.M., What attributes make some plant species more invasive? Ecology 77, Richardson, D.M., Pysek, P., Rejmánek, M., Barbour, M.G., Panetta, F.D., West, C.J., Naturalization and invasion of alien plants: concepts and definitions. Divers. Dist. 6, Rodriguez-Girones, M.A., Sandsten, H., Santamaria, L., Asymmetric competition and the evolution of propagule size. J. Ecol. 91, Simberloff, D., Von Holle, B., Positive interactions of nonindigenous species: invasional meltdown? Biol. Invasions 1, Sokal, R., Rohlf, J., Biometry: The Principle and Practice of Statistics in Biological Research. Freeman and Company, New York. Spencer, D.F., Ksander, G.G., Interactions between American pondweed and monoecious Hydrilla grown in mixtures. J. Aquat. Plant Manag. 38, Spitters, C.J.T., An alternative approach to the analysis of mixed cropping experiments. 1. Estimation of competition effects. Neth. J. Agric. Sci. 31, Sultan, S.E., Evolutionary implications of phenotypic plasticity in plants. Evol. Biol. 21, Swarbrick, J. T., Finlayson, C.M., Cauldwell, A.J., The Biology and Control of Hydrilla verticillata (L.f. Royle). Biotrop. Special Publ. No. 16. Biotrop Bogor, Indonesia. 34 pp. Tilman, D., Plant Strategies and the Dynamics and Structure of Plant Communities. Princeton University Press, Princeton. Van, T.K., Wheeler, G.S., Center, T.D., Competition between Hydrilla verticillata and Vallisneria americana as influenced by soil fertility. Aquat. Bot. 62, Williamson, M., Invasions. Ecography 22, 5 12.