Community Structure And Competition Between Hydrilla And Vallisneria 1
\VlLLlAi\[ 'r. I-IALLER and D. 1,. SUTTON
Assi.l/olil P)'()/('S.I()),S, j\1ain Calfl/Jus ({lid II/(: Agricultllral Research Cellier at Ft. Lall(/err/alc, Ulli'twnity of Florida .. (;(/i1l('.IVi[[(:, Florida 32611
ABSTRACT The competitive ability o[ hydriUa probably results frolll seYeral combined factors. The purpose 01 these studies Structural relationships of the desirahle submersed was to C(»IlP;lIC: hydrilla and vallisllcria populations to aquatic plant, vallisneria (Vallislleria lIeolrojJicafis l\farie determille tile possible ellects of commullity structure on Viet.) and the trollblesome aq uatic weed, hydrilla (Hydrilla competition between these two species. '()erticiliata Royle) were compared to determine the eJTeels of plant structure on competition. j\lthough both species are members or the Hydrocharitaccae family, they differ METHODS AND MATERIALS considerahly in their g-rowth hahit. Hydrilla forms a dellse canopy at the water surface and limits light penetration by Plallt' popUlations lIse(l to compare the characteristics at least 95(;~) in Lhe first 0.3 m of the water column, whereas or hydl'ilLt :tllcl vallisneria were growing in 0.08 ha earthen vallisneria communities (Ire less dense and light penetration ponds in Orange COllnty, Florida. These ponds (1.5 m is similar to that of open water. The low ratio of root deep) were est:tblished in July 1971, and plant samples tissue to foliage produced by hydrilla indicate!, that the wcre collected ill the 'winter 01 1~)T)-7L Both hydrilla and majority of fixed carbon is lIsed ill the production of com vallisl1cria populations were well developed and appearc<.l petitive standing crop. Hydrilla also produces several to he typical 01 natural communities. million potentially reproductive propagules awl meristems The leaf arca index (LA 1) of hydrilla and vallisneria per ha, which make control methods difficult and competi was measured by harvesting four complete pLtnts of each tion from other species almost impossible. species. The plants were dissected and their leaves were placed 011 blueprint paper. The leaves were removed from the paper after exposure to ultraviolet light, and dried. INTRODUCTION The imprints 01' the leaves 011 the hlueprint paper were Hydrilla has continuecl to sprca<.l throughout the waters cut out and weighed. The ratio of area (obtained by of the United States since it was first introdnced into weighing a paper of known area) of traced leaves to the Florida 15 years ago. It has heen found in at least six states dry weight of the traced leaves made it po!->sihle to calcu and may occur in several others being incorrectly identified late the LA! hy lllltlti ply.ing the ratio by the weight of the as an elodea species (1). standing crop. The rapid rate with which hydrilla spreads and The depth distribution of plant biomass was obtained dominates the vascular flora o[ a waterway is cansing COll by removing the plants from the hydrosoil and cutting cern to many (Hluatic biologists. Sutton reported that 1 them into 10-011 sections from the hydrosoil to the water year al'ter planting' a pond to 25% hydrilla, it had in surface. The lO-cm sections were dried and their weights creased its coverage to over 90% of the pond alea, crowding expressed as a percent of the total plant material har out other submersed spccies. 2 Louisiana binlogists reported vested. that hydrilla in a reservoir spread from an area of 5 ha Dry weight of the stalHling crop was found by sampling to over 10 ha in just 6 weeks (6). Hydrilla was found in a 92~) Clll~ area of the lllat alter the ponds were drained 1 ha of Rodman Reservoir in 1971 and in 5 11a 1 year in January 1974.Al'ter the standing crop was harvested, later (4:). At the present time, hydrilla is estimated to the roots 'were removed from the hydrosoil, rinsed, and cover ill excess of 1000 ha in Rodman Reservoir and is dry ,,,'eights de term i lled. severely restricting water use.:l In order to estimate the number of tubers in the hyclrilla pond, 16 core samples were collected at ralHlom from the O.OH ha area. The lO.2-cm diameter core sampler 1 This study is a portion of a dissertation presented by tile senior author to the (;radllatc COllllcil ()f the lJniHTsily 01 Florida in was forced into the hydrosoil to a depth of approximately partial fullilllnent or the requirements for the degree of Doctor of 20 cm. Aller thc cort ,vas removed from the sampler it Philosophy. The l"Cscanh was supported by a Florida Departl11ent of Natural Resources (;, ant. Published as Jourllal Serics .:'\(). :-)N::!!l of 'was rillsed with 'water ;1I1d the propagules were counted. the Fla. Agr. Exp. Sta. Penctration of photosynthetically active radiation (400 :.!Sutton, D.L. 1m2. Control ()r aquatic pl'\l1t growth ill earthen to 700 nm) through scveral hydri lla antI v;tllisneria C0111- ponds by the "whitc amm. Ft. Lallderdale .\RC Res. Rcpt. FL 7::!el, 77 pp. JlHlllitics was mcasllred in October ] 974 ,It O.:J-lll depth :Qlcstalld, R.S. 1971. Pcrsonal COllllllunicaLioll. intervals ill Rodmall Rescrvo.ir with a Lamda Ll-185 light meter4 equipped with a submersible sensor. The sensor in the plant. The high ratio of non-photosynthetic to was modified to fit on a hook made of aluminum conduit. photosynthetic tissue in vallisneria negates any advantage This alteration helped maintain the sensor parallel to that its greater LAI would seemingly' give vallisneria the water surface and produced minimal disturbance of over hydrilla. The predominance of thin leaves and few plants and water while heing lowered into a plant com roots among other submersed aquatic plants also suggests munity. the advantage of having low non-photosynthetic to photo synthetic tissue ratios. RESULTS AND DISCUSSION The production of reproductive propagules and large numbers of axillary meristems by hydrilla favor its There have been several attempts to design equipment colonization and growth over most other aquatic plants. that would quantitatively sample submersed vegetation Hydrilla produced an estimated 257 tubers and 62 turions (2). The most accurate method is to remove the water m-2 in the pond during a 2.5-year period. Based on these from the sampling site. However, this is often impossible combined values, 1.0 ha would contain over 3 million and accounts in large measure for the lack of biomass data propagnles. Although the longevity of the hydrilla propa for submersed plants. gules is not known, a supply of reproductive structures of Data obtained from drained ponds is presented in this number would probably provide a source for rein Table I. The total biomass of vallisneria was greater than festation for many years. The significance of this number hydrilla; however, vallisneria produced much greater becomes apparent when one considers that a typical corn amounts of non-photosynthetic tissue (roots) than hydrilla. field contains a population of about 75,000 plants ha-1 • Over 87j~ of the hydrilla biomass was standing crop The horizontal distribution of hydriUa and vallisneria compared to 60% for vallisneria. is compared in Figure 1. The depths which encompassed The area per unit weig'ht of leaf tissue indicated that one-half the standing crop were 0.45 m for hydrilla and vallisneria leaves were thicker and heavier than hydrilla, 1.05 m for vallisneria. Hydrilla formed an extensive 2.2 and 8.3 dm2 g dry weight,-l respectively. The LA! of canopy on the water surface, having nearly 20% of its vallisneria was 8.7 compared to 4.8 for hydrilla. The LAI total biomass in the top 10 cm of water. The localization of hydrilla is comparable to that of corn (Zea mays L.) and of the meristcm is believed to be the factor governing that of vallisneria comparable to waterhyacinth (EichhoYnia the horizontal distribution of aquatic plants (5). The crassijJes (Mart.) 80lms). A LAI of at least 3.0 is necessary for 100% interception of radiant energy (3). The thick leaves and large amounts of roots produced 5 by vaHisneria contribute to its inability to compete suc cessfully with hydrilla, as non-photosynthetic tissue derives its respiratory energy from photosynthetic tissue elsewhere 25 4LAMBDA Instruments Corporation, Lincoln, Nebraska. Mention of a trademark name or proprietary product does not constitute a 35 guarantee or warranty of the product by the University of Florida and docs not imply its approval to the exclusion of other products • Vallisneria E 45 that also may be suitable. u ... Hydrilla I (I) 0,66 hydrilla biomass in the top le"v em of w;lter not only pro u ro hibits competition from other submersed plallts, hut takes "- "- advantage of the maximum levels 01 light for growth :::l (/) which OCClir at the water surface. The presence of millions E COll 0 of meristcm (I) REFERENCES ...... 1.33 ro • Open wa ter 3: 1. Blackhllrn, R.D .. L.W. Weldon, R.R. Yeo, and T.l\J. Taylor. ] !1m. Tdentification and distribution of similar appearing aquatic • Vallisneria weeds in FI()rida. Hyacinth COlltr. .J. H:ll-21. ~ . Fosberg, C. 1~):i9, Qllantitati\c salllplillg of suh-aquatic \ cgcta • Hydrilla tioll. Oikos 10:2:rl-:2W. 1.66 :1. Halln, W.T. 1~)7 .. L I'ho(o,,'n(huic characteristics of the suh· mersed aqllatic plants hHlrilla. southern naiad, ami \allisllcria. Doctoral Diss. lJlli\', of Florida, Cainesville. HH pp, "I. Hestand, R.S ... H.E. l\[(l\', D.P. Shultz, and C.R. Walker. 197:1. Ecological illlplicat ions 'of water lc\cls 011 plant growth ill a shallow watci rcscJ\oir. Hyacinth COlltL J. 11 ::"j..J:-!lH. 2.00L....--L.20----40---6-0-----.-8"':"'0--~100 ;"). Tknsima, T. F)(i/i, Ecological studies em tile productivity of aquatic plant COlllnlllllities II. Seasonal changes in standing Percent of Surface Radiation crop and plOdllcti\it\ of a natllral submerged COl1l1l11ll1ity of ,'a/fisllcria (/eIlS('I'IIi/(I/a. Bot. ]\lag. Tokyo 79:7-1~). Figure :2. Light pClletl:atioll throll~h walel: ,Ill!! plant ~'()Illillulliti('s .in ii. .I oi1l1Son, B. and J. ]\iallning. 1()71. Hvdrill;l-the llloSt serious Rodman RcserYOIr LIght llleasurelllclIts 111 the hvdnlla COllllllUlIlt\ threat to LOllisiana's water rC-;()1II«(,S. LOllisiana COllscnatiollist were made through sl;lall (~O on~) opellings ill 'the callopy. :!()::::(i-2!), 50