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Influence of planting depth on Potamogeton gramineus L.
Aquatic Botany, 36 (1990) 343-350 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
343
I n f l u e n c e of P l a n t i n g D e p t h on Potamogeton
gramineus L. DAVID F. SPENCER and GREGORY G. KSANDER
USDA-ARS Aquatic Weed Laboratory, Department of Botany, University of California, Davis, CA 95616 (U.S.A.) (Accepted for publication 19 October 1989)
ABSTRACT Spencer, D.F. and Ksander, G.G., 1990. Influence of planting depth on Potamogeton gramineus L. Aquat. Bot., 36: 343-350. The weight of Potamogeton gramineus L. winter buds was greatest between 6 and 10 cm deep in the sediment for samples collected from two northern California irrigation canals. The relationship between winter bud weight and depth was adequatelydescribedby a polynomialequation containing a quadratic term, but coefficientsin equations were different for two canals. For buds planted at various depths, after 10 weeksgrowth measurements in terms of plant dry weight, shoot length, rhizome length, number of flowers, ramets, leaves, and floating leaves per plant were all negativelyrelated to planting depth. Plants growingfrom greater depths had lower rates of shoot elongation,leafproduction, and ramet production. Allocationofbiomass to photosynthetic tissues and flowerswas also influencedby initial planting depth, as was the time of emergencefrom the sediment. These findings suggest that predicting macrophyte growth and phenologyfrom environmental changes alone may not be adequate.
INTRODUCTION M a n y aquatic plants rely on t he pr oduct i on of u n d e r g r o u n d vegetative propagules, such as tubers, turions, or winter buds to w i t hst and periods adverse to growth (Sculthorpe, 1967; H u t c h i n s o n , 1975). Accurately predicting the emergence an d growth of a pl a nt population depends in part on u n d e r s t a n d i n g the structure and dynamics of t he associated propagule b a n k (Harper, 1977). One likely i m p o r t a n t aspect of a propagule b a n k is the vertical distribution of the propagules in t he sediment ( T i t us a nd Hoover, 1990). Such data for submersed macr o p hyt es are limited, but i nf orm at i on about two species, VaUisneria americana Michx. (Rybicki and Carter, 1986) and Potamogeton pectinatus L. (Ogg et al., 1969; Spencer, 1987) is available. T hi s report describes the vertical distribution of Potamogeton gramineus L. winter buds in n o r t h e r n California irrigation canals, and presents experi m ent al results which illustrate the
344
D.F. SPENCER AND G.G. KSANDER
effects of planting depth on growth and phenology of this submersed aquatic macrophyte. METHODS The field studies were conducted in irrigation canals near Dixon, CA (Latitude 38.19 ° N; Longitude 121.49 ° W), which are part of the Solano Irrigation District. These canals contain no flowing water from mid-October through mid-April. We collected sediment cores (about 13 cm deep, i.e. the depth of the sediment) from the Byrnes Canal on 25 November (six cores ) and 6 December 1985 (three cores) using a 15-cm diameter corer constructed from PVC pipe. Nine additional cores were collected from the Weyend Canal on 15 December 1988. The intact cores were extruded into pails, returned to the laboratory, and sliced into sections 2.5 cm thick. Winter buds in each section were separated from the sediment by washing over a 3-mm mesh screen; they were then dried at 105 ° C for 48 h and weighed. Samples collected from 4 × 200 m (w × l) strips of the Byrnes and Weyend canals in March 1984 were analyzed for per cent organic matter and particle size. Individual samples were collected at 10-m intervals within the strips. Percent organic matter was estimated as the difference between oven-dry weight and ash-free dry weight following heating to 500°C for 8 h (Brower and Zar, 1984). Particle size analysis and assignment of soil fractions followed Brower and Zar (1984). Taxonomy follows Fassett (1957) and Munz (1970); voucher specimens were deposited in the University of California, Davis herbarium. For the growth experiment winter buds were collected from the Byrnes Canal. Winter buds weighing between 200 and 300 mg (fresh weight) were planted in 3.75-1 plastic containers at depths of 2.5, 5, 10, and 20 cm, with four replicates for each depth (a replicate being one pot with one plant). The planting medium was modified UC mix (Spencer and Anderson, 1987). Containers were placed in randomly assigned positions on the bottom of an outdoor concrete vault (1000 1) having a water depth of 0.52 m. The water in the vault was changed weekly, and the m i n i m u m / m a x i m u m water temperature was recorded every 2-3 days. The plants were checked at 2- to 3-day intervals for emergence from the sediment. For plants that had emerged, plant height, the number of leaves per plant, the number of ramets, the number of flowers, and floating leaves were measured. Ten weeks after planting the plants were harvested and separated into shoots, rhizomes, roots, and flowers. The dry weights of the various parts were determined (48 h at 105 °C). The entire experiment was repeated. During the second experiment the oxidation reduction potential (redox) was measured at 5 and 20 cm deep. A platinum electrode was inserted to a suitable depth in each of four pots (two at 5 cm and two at 20 cm) and an additional electrode was placed in the water column. Each electrode was connected to a recording data logger (Easy LoggerT M Recording System, Omnidata Interna-
INFLUENCEOFPLANTINGDEPTHONPOTAMOGETON
345
tional, Inc., Logan, UT, U.S.A. ) in conjunction with a calomel reference electrode The data logger recorded the redox from each electrode once each hour for the first 10 days following the start of the experiment. Data from the cores were examined statistically using PROC GLM in SAS (SAS Institute, 1988). We tried to answer two questions: (1) what was the best equation to describe the relationship between winter bud weight and depth; and(2 ) was the same equation applicable to both data sets (i.e. data from the Byrnes and Weyend canals)? To answer these questions we constructed a regression equation containing terms for canal, depth, depth 2, depth ~, and interactions; the study began with a complete model. Following the appropriate calculations, non-significant terms were deleted from the equation and the regression recalculated until only significant terms or terms involved in significant interactions remained. The results of the growth experiment were analyzed by linear regression of the various growth parameters vs. depth. Chew (1976) recommended this approach over analysis of variance followed by a means separation procedure, when treatment effects are continuous. RESULTS
Potamogeton gramineus winter buds were not uniformly distributed across depth (Fig. 1 ). Maximum winter bud weight occurred between 6 and 10 cm deep in the sediment. Winter bud weight was consistently less at depths above 6 cm or greater than 10 cm. The relationship between winter bud weight and depth was best described by an equation containing a quadratic term. However, o q|
z,z~aaz~ Byrnes Canal AAAAA Weyend Canal
6
~
\\
_
./
12i ~ o.o
o.1 o.2 o.~ o.'4 Winter Bud Weight
o.~ (g)
0?6
Fig. 1. Vertical distribution of P. gramineus winter bud weight in the sediment for the Byrnes and Weyend Canals. Symbols are the means of nine samples. The solid line represents the polynomial regression equation for Byrnes Canal data and the dashed line represents the equation for the Weyend Canal data (n = 45 ). For the Byrnes Canal the equation is winter bud (g) = - 0.158 + 0.116 (depth)-0.008 {depth2); R2=0.24. For the Weyend Canal the equation is winter bud (g) = -0.036+0.213 (depth) -0.014 (depth2); R2=0.39. Depths are in cm.
346
D.F. SPENCER AND G.G. KSANDER
t h e s a m e e q u a t i o n w a s n o t a p p l i c a b l e to d a t a f r o m b o t h c a n a l s (ANOVA;
F = 7.18; d f = 1,84; P < 0.01 ) b e c a u s e o f t h e g r e a t e r w e i g h t o f w i n t e r b u d s in t h e W e y e n d C a n a l s a m p l e s . I n t h e B y r n e s a n d W e y e n d canals, s e d i m e n t s are silty clay (clay f r a c t i o n = 57 to 62% ), b u t B y r n e s C a n a l s e d i m e n t s w e r e c h a r a c t e r ized b y a slightly g r e a t e r p r o p o r t i o n o f silt (35 vs. 29% ). T h e o r g a n i c c o n t e n t was low ( 7 - 8 % ) a n d did n o t differ b e t w e e n canals. T h e d i f f e r e n c e s in t h e v e r tical d i s t r i b u t i o n s o f w i n t e r b u d w e i g h t do n o t a p p e a r to be a t t r i b u t a b l e to t h e s e slight d i f f e r e n c e s in s e d i m e n t c h a r a c t e r i s t i c s b e t w e e n t h e t w o canals. P l a n t i n g d e p t h s i g n i f i c a n t l y i n f l u e n c e d b o t h t h e g r o w t h ( T a b l e s 1 a n d 2) a n d p h e n o l o g y (Figs. 2 a n d 3) o f P. gramineus. D r y w e i g h t for P. gramineus d e c r e a s e d w i t h i n c r e a s i n g p l a n t i n g d e p t h . W i n t e r b u d s originally p l a n t e d a t 20 c m did n o t p r o d u c e p l a n t s . R h i z o m e l e n g t h d e c r e a s e d as a f u n c t i o n o f p l a n t i n g d e p t h . P r o d u c t i o n o f f l o a t i n g leaves a n d flowers w a s i n v e r s e l y r e l a t e d to p l a n t ing d e p t h . P. gramineus a t 2.5 or 5 c m p r o d u c e d p l a n t s w h i c h a l l o c a t e d 2 or 3% TABLE1 Growth parameters for P. gramineus planted at four depths Parameter
Total weight Shoot weight Rhizome weight Root weight Flower weight No. of floating leaves No. of flowers Rhizome length
Planting depth (cm)
Sign.
2.5
5
10
20
5844± 279 3994 ± 161 917 ± 53 762 ± 75 171 ± 82 20 ± 3 5± 2 104 ± 5
5210 ± 211 3524 ± 152 819 ± 46 762 ± 70 106 ± 46 12 _+4 3± 1 121 _+15
2011 ±841 1443 ± 604 327 _+132 234 + 105 6± 3 1± 1 1 _+0 47 ± 19
23 _+13 0 23 ± 13 0 0 0 0 0
*** *** *** *** * *** ** ***
Values are the mean ± standard error; n=4. Dry weights are mg and lengths are cm. The column labeled 'Sign.' gives the statistical significance of linear regression relating the parameter to depth: * Denotes P < 0.05; ** denotes P < 0.01; *** denotes P < 0.001. TABLE2 Production rates for P. gramineus planted at four depths. Values are the linear regression coefficient _+standard error Parameter
L e n g t h m m d ~ -1 R a m e t s ~ y -1 Leavesday -~
Planting depth (cm) 2.5
5
10
20
6.7±0.3 0.5±0.03 2.6±0.2
7.1±0.2 0.4±0.04 2.5±0.3
5.5±0.2 0.2±0.03 1.2±0.1
0 0.1±0.01 0
INFLUENCE
OF PLANTING
DEPTH
ON
POTAMOGETON
347
30-
500-
2.5 ¢m
E E
~
20
cm cm
20-
-E 300++~ E~
E u
© 200-
10-
~- I00-
'
14
'
v2L8
'
~v~
~516v ~
v7b
Days After Start
Days After Start
Fig. 2. Plant length (A) and number of ramets per plant (B) over time for P. gramineus planted at four depths. Values are means; n=4.
200-
2.5
150-
~
•±-7 5 10 20
cm cm
cm cm
ii00-
50-
00
~~
4-
....
0,¢ CC,T, C,C, C 28
42
C ,C, 56
Ci 70
Days After Start Fig. 3. Number of leaves per plant for P. gramineus planted at four depths. Values are means; n = 4.
of the biomass to flowers while those planted at depths greater than 5 cm allocated no biomass to flowers. Plants in the 10-cm depth treatment had more biomass allocated to rhizome than those growing from shallower depths. Those growing from shallower depths allocated more biomass to photosynthetic organs and emerged from the soil about 10 days sooner than those planted at 10 cm depth. Plants that emerge earlier may be at an advantage over those that emerge later even though other growth characteristics are similar (Firbank and Watkinson, 1985). Individuals growing from greater than 5 cm depth were shorter, had fewer leaves per plant, and fewer ramets than those growing from 5 cm or less. The rate of leaf production, ramet production, and shoot elongation also reflects the influence of planting depth (Table 2). Floating leaves were produced 10 days before flowers and both were produced under a 14.5 h
348
D.F. SPENCER AND G.G. KSANDER
photoperiod. Water temperature varied considerably (8°-30°C) and did not appear to be related to either floating leaf or flower production. Redox of the sediment in the pots declined steadily for the first 10 days after the pots were placed under water. DISCUSSION The vertical distribution of winter buds in canal sediments was somewhat similar to that reported for Vallisneria americana, but different from that observed for P. pectinatus. Information about the vertical distribution of V. americana tubers in Potomac River sediments (U.S.A.) showed that most were found between 5 and 15 cm deep in sandy sediments and between 10 and 20 cm deep in silty clay sediments (Rybicki and Carter, 1986). When the number of tubers is plotted against depth (Rybicki and Carter, 1986; see their Figs 3 and 4) the resulting curves are similar in shape to those reported here for P. gramineus winter bud weight, even though the depth of the peak abundance is different. The vertical distribution of P. pectinatus propagules appears to be quite different from that for P. gramineus. Ogg et al. (1969) examined the abundance ofP. pectinatus tubers in 30-cm deep cores from an irrigation canal near Prosser, Washington (U.S.A.), and reported a marked decrease in the number of tubers present below 7.5 cm. In contrast the number ofP. pectinatus tubers in the Byrnes, Weyend, and Dally irrigation canals (Solano Irrigation District) was more or less independent of depth up to 23 cm (Spencer, 1987). While it is likely that sediment properties (Rybicki and Carter, 1986) influence the vertical distribution of vegetative propagules, the limited data support the notion that species-specific differences exist as well. The influence of planting depth on growth of P. gramineus is similar to that reported for two other macrophyte species. In growth experiments, survival of both Vallisneria and P. pectinatus was directly related to planting depth (Rybicki and Carter, 1986; Spencer, 1987). In addition, growth measured in terms of plant dry weight, length, and the number of ramets was inversely related to planting depth for P. pectinatus (Spencer, 1987). In the present study, winter buds planted at 20 cm did not emerge from the sediment and examination of these at harvest showed that they had begun to decay; this suggested that the conditions at this depth were detrimental to winter bud growth. Although the mechanisms leading to apparent winter bud death in this experiment are unknown, the redox potential of the sediment in the pots was low after 10 days ( - 75 mV) indicating low oxygen concentrations. Germinating winter buds that have not emerged from the sediment may be less able to withstand low sediment oxygen concentrations than mature plants (Mitsch and Gosselink, 1986; see pages 130-139). The observation that biomass allocation was directly related to winter bud planting depth has implications for attempts to predict macrophyte phenology.
INFLUENCE OF PLANTING DEPTH ON POTAMOGETON
349
In particular, the influence of planting depth on flower production implies that changes in environmental elements (i.e. photoperiod and temperature) alone may not be sufficient to predict phenology. Planting depth may be important because of its influence on plant size; for example, Titus and Hoover (1990) reported that V. americana plants grown in greenhouse cultures flowered if they were 0.75 g or greater, however, the relationship was not as strong for plants from natural populations. For P. gramineus, the weight of flowers per plant was strongly correlated with plant weight (r-- 0.62; P < 0.02). These findings are also consistent with results from work with herbaceous terrestrial species (Werner, 1975; Gross, 1981). P. gramineus grew well when planted at shallow depths in the growth experiments, but P. gramineus winter buds were less abundant near the surface in the Byrnes and Weyend canals. This may be partly explained by the lack of flowing water in these canals during the winter and early spring. Since water is absent, the surface sediments may become quite dry and this in turn may lead to lower winter bud survival. Unpublished data indicate that related Potamogeton species (P. nodosus Poir. and P. pectinatus) were quite susceptible to desiccation at soil moistures of 23% or less. In summary, the distribution of P. gramineus winter buds was not uniform across depth. More winter buds were found between 6 and 10 cm deep in the sediment than at other depths. Experimental results indicated that planting depth significantly influenced several aspects of growth, including the allocation of biomass to flowers and photosynthetic tissues; also influenced were important phenological aspects, including the time of emergence from the sediment. These results show that aquatic plant growth is not solely regulated by abiotic factors (Barko et al., 1986), and that considering the depth distribution of propagules in the sediment may improve capabilities (Collins and Wlosinski, 1989) for predicting macrophyte growth.
ACKNOWLEDGEMENTS
We thank L. Maharis for technical assistance. L.W.J. Anderson and N. Dechoretz provided the samples from the Byrnes and Weyend canals that were analyzed for sediment characteristics. K. McKee provided instructions for constructing and using the redox electrodes. We appreciate the comments of E. Rejmankova, L. Mitich, B. Rorslett, and F.J. Ryan who read an earlier version of the manuscript. L. Whitehand provided assistance with the statistical analyses. Mention of a manufacturer does not constitute a warranty or guarantee of the product by the U.S. Department of Agriculture nor an endorsement over other products not mentioned.
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REFERENCES Barko, J.W., Adams, M.S. and Clesceri, N.L, 1986. Environmental factors and their consideration in the management of submersed aquatic vegetation: a review. J. Aquat. Plant Manage., 24: 110. Brower, J.E. and Zar, J.H., 1984. Field and Laboratory Methods for General Ecology, 2nd edn. Wm. C. Brown, Dubuque, IA, 226 pp. Chew, V., 1976. Comparing treatment means: a compendium. HortScience, 11: 318-357. Collins, C.D. and Wlosinksi, J.H., 1989. A macrophyte submodel for aquatic ecosystems. Aquat. Bot., 33: 191-206. Fassett, N.C., 1957. A Manual of Aquatic Plants. University of Wisconsin Press, Madison, WI, 403 pp. Firbank, L.G. and Watkinson, A.R., 1985. On the analysis of competition within two-species mixtures of plants. J. Appl. Ecol., 22; 503-517. Gross, K.L., 1981. Prediction of fate from rosette size in four 'biennial' plant species Verbascum thapsus, Oenothera biennis, Daucus carota and Tragopogon dubius. Oecologia, 48: 209-213. Harper, J.L., 1977. Population Biology of Plants. Academic Press, New York, 892 pp. Hutchinson, G.E., 1975. A Treatise on Limnology, Vol. III. Limnological Botany. Wiley, New York, 660 pp. Mitsch, W.J. and Gosselink, J.G., 1986. Wetlands. Van Nostrand Reinhold, New York, 539 pp. Munz, P.A., 1970. A California Flora. University of California Press, Berkeley, CA, 1681 pp. Ogg, A.G. Jr., Bruns, V.F., and Kelly, A.D., 1969. Response of sago pondweed to periodic removal of top growth. Weed Sci., 17: 139-141. Rybicki, N.B. and Carter, V. 1986. Effect of sediment depth and sediment type on the survival of Vallisneria americana Michx. grown from tubers. Aquat. Bot., 24: 233-240. SAS Institute Inc., 1988. SAS/STAT User's Guide, Release 6.03 Edition. SAS Institute, Cary, NC, 1028 pp. Sculthorpe, C.D., 1967. The Biology of Aquatic Vascular Plants. St. Martins Press, New York, 610 pp. Spencer, D.F., 1987. Tuber size and planting depth influence growth of Potamogeton pectinatus L. Am. Midl. Nat., 118: 77-84. Spencer, D.F. and Anderson, L.W.J., 1987. Influence of photoperiod on growth, pigment composition and vegetative propagule formation for Potamogeton nodosus Poir. and Potamogeton pectinatus L. Aquat. Bot., 28: 103-112. Titus, J.E. and Hoover, D.T., 1990. Towards predicting reproductive success in submersed freshwater angiosperms. In: M. Adams and K. Sand-Jensen, (Editors), Ecology of Submersed Macrophytes., in press. Werner, P.A., 1975. Predictions of fate from rosette size in teasel (Dipsacus fullonum L. ). Oecologia, 20: 197-201.
343
I n f l u e n c e of P l a n t i n g D e p t h on Potamogeton
gramineus L. DAVID F. SPENCER and GREGORY G. KSANDER
USDA-ARS Aquatic Weed Laboratory, Department of Botany, University of California, Davis, CA 95616 (U.S.A.) (Accepted for publication 19 October 1989)
ABSTRACT Spencer, D.F. and Ksander, G.G., 1990. Influence of planting depth on Potamogeton gramineus L. Aquat. Bot., 36: 343-350. The weight of Potamogeton gramineus L. winter buds was greatest between 6 and 10 cm deep in the sediment for samples collected from two northern California irrigation canals. The relationship between winter bud weight and depth was adequatelydescribedby a polynomialequation containing a quadratic term, but coefficientsin equations were different for two canals. For buds planted at various depths, after 10 weeksgrowth measurements in terms of plant dry weight, shoot length, rhizome length, number of flowers, ramets, leaves, and floating leaves per plant were all negativelyrelated to planting depth. Plants growingfrom greater depths had lower rates of shoot elongation,leafproduction, and ramet production. Allocationofbiomass to photosynthetic tissues and flowerswas also influencedby initial planting depth, as was the time of emergencefrom the sediment. These findings suggest that predicting macrophyte growth and phenologyfrom environmental changes alone may not be adequate.
INTRODUCTION M a n y aquatic plants rely on t he pr oduct i on of u n d e r g r o u n d vegetative propagules, such as tubers, turions, or winter buds to w i t hst and periods adverse to growth (Sculthorpe, 1967; H u t c h i n s o n , 1975). Accurately predicting the emergence an d growth of a pl a nt population depends in part on u n d e r s t a n d i n g the structure and dynamics of t he associated propagule b a n k (Harper, 1977). One likely i m p o r t a n t aspect of a propagule b a n k is the vertical distribution of the propagules in t he sediment ( T i t us a nd Hoover, 1990). Such data for submersed macr o p hyt es are limited, but i nf orm at i on about two species, VaUisneria americana Michx. (Rybicki and Carter, 1986) and Potamogeton pectinatus L. (Ogg et al., 1969; Spencer, 1987) is available. T hi s report describes the vertical distribution of Potamogeton gramineus L. winter buds in n o r t h e r n California irrigation canals, and presents experi m ent al results which illustrate the
344
D.F. SPENCER AND G.G. KSANDER
effects of planting depth on growth and phenology of this submersed aquatic macrophyte. METHODS The field studies were conducted in irrigation canals near Dixon, CA (Latitude 38.19 ° N; Longitude 121.49 ° W), which are part of the Solano Irrigation District. These canals contain no flowing water from mid-October through mid-April. We collected sediment cores (about 13 cm deep, i.e. the depth of the sediment) from the Byrnes Canal on 25 November (six cores ) and 6 December 1985 (three cores) using a 15-cm diameter corer constructed from PVC pipe. Nine additional cores were collected from the Weyend Canal on 15 December 1988. The intact cores were extruded into pails, returned to the laboratory, and sliced into sections 2.5 cm thick. Winter buds in each section were separated from the sediment by washing over a 3-mm mesh screen; they were then dried at 105 ° C for 48 h and weighed. Samples collected from 4 × 200 m (w × l) strips of the Byrnes and Weyend canals in March 1984 were analyzed for per cent organic matter and particle size. Individual samples were collected at 10-m intervals within the strips. Percent organic matter was estimated as the difference between oven-dry weight and ash-free dry weight following heating to 500°C for 8 h (Brower and Zar, 1984). Particle size analysis and assignment of soil fractions followed Brower and Zar (1984). Taxonomy follows Fassett (1957) and Munz (1970); voucher specimens were deposited in the University of California, Davis herbarium. For the growth experiment winter buds were collected from the Byrnes Canal. Winter buds weighing between 200 and 300 mg (fresh weight) were planted in 3.75-1 plastic containers at depths of 2.5, 5, 10, and 20 cm, with four replicates for each depth (a replicate being one pot with one plant). The planting medium was modified UC mix (Spencer and Anderson, 1987). Containers were placed in randomly assigned positions on the bottom of an outdoor concrete vault (1000 1) having a water depth of 0.52 m. The water in the vault was changed weekly, and the m i n i m u m / m a x i m u m water temperature was recorded every 2-3 days. The plants were checked at 2- to 3-day intervals for emergence from the sediment. For plants that had emerged, plant height, the number of leaves per plant, the number of ramets, the number of flowers, and floating leaves were measured. Ten weeks after planting the plants were harvested and separated into shoots, rhizomes, roots, and flowers. The dry weights of the various parts were determined (48 h at 105 °C). The entire experiment was repeated. During the second experiment the oxidation reduction potential (redox) was measured at 5 and 20 cm deep. A platinum electrode was inserted to a suitable depth in each of four pots (two at 5 cm and two at 20 cm) and an additional electrode was placed in the water column. Each electrode was connected to a recording data logger (Easy LoggerT M Recording System, Omnidata Interna-
INFLUENCEOFPLANTINGDEPTHONPOTAMOGETON
345
tional, Inc., Logan, UT, U.S.A. ) in conjunction with a calomel reference electrode The data logger recorded the redox from each electrode once each hour for the first 10 days following the start of the experiment. Data from the cores were examined statistically using PROC GLM in SAS (SAS Institute, 1988). We tried to answer two questions: (1) what was the best equation to describe the relationship between winter bud weight and depth; and(2 ) was the same equation applicable to both data sets (i.e. data from the Byrnes and Weyend canals)? To answer these questions we constructed a regression equation containing terms for canal, depth, depth 2, depth ~, and interactions; the study began with a complete model. Following the appropriate calculations, non-significant terms were deleted from the equation and the regression recalculated until only significant terms or terms involved in significant interactions remained. The results of the growth experiment were analyzed by linear regression of the various growth parameters vs. depth. Chew (1976) recommended this approach over analysis of variance followed by a means separation procedure, when treatment effects are continuous. RESULTS
Potamogeton gramineus winter buds were not uniformly distributed across depth (Fig. 1 ). Maximum winter bud weight occurred between 6 and 10 cm deep in the sediment. Winter bud weight was consistently less at depths above 6 cm or greater than 10 cm. The relationship between winter bud weight and depth was best described by an equation containing a quadratic term. However, o q|
z,z~aaz~ Byrnes Canal AAAAA Weyend Canal
6
~
\\
_
./
12i ~ o.o
o.1 o.2 o.~ o.'4 Winter Bud Weight
o.~ (g)
0?6
Fig. 1. Vertical distribution of P. gramineus winter bud weight in the sediment for the Byrnes and Weyend Canals. Symbols are the means of nine samples. The solid line represents the polynomial regression equation for Byrnes Canal data and the dashed line represents the equation for the Weyend Canal data (n = 45 ). For the Byrnes Canal the equation is winter bud (g) = - 0.158 + 0.116 (depth)-0.008 {depth2); R2=0.24. For the Weyend Canal the equation is winter bud (g) = -0.036+0.213 (depth) -0.014 (depth2); R2=0.39. Depths are in cm.
346
D.F. SPENCER AND G.G. KSANDER
t h e s a m e e q u a t i o n w a s n o t a p p l i c a b l e to d a t a f r o m b o t h c a n a l s (ANOVA;
F = 7.18; d f = 1,84; P < 0.01 ) b e c a u s e o f t h e g r e a t e r w e i g h t o f w i n t e r b u d s in t h e W e y e n d C a n a l s a m p l e s . I n t h e B y r n e s a n d W e y e n d canals, s e d i m e n t s are silty clay (clay f r a c t i o n = 57 to 62% ), b u t B y r n e s C a n a l s e d i m e n t s w e r e c h a r a c t e r ized b y a slightly g r e a t e r p r o p o r t i o n o f silt (35 vs. 29% ). T h e o r g a n i c c o n t e n t was low ( 7 - 8 % ) a n d did n o t differ b e t w e e n canals. T h e d i f f e r e n c e s in t h e v e r tical d i s t r i b u t i o n s o f w i n t e r b u d w e i g h t do n o t a p p e a r to be a t t r i b u t a b l e to t h e s e slight d i f f e r e n c e s in s e d i m e n t c h a r a c t e r i s t i c s b e t w e e n t h e t w o canals. P l a n t i n g d e p t h s i g n i f i c a n t l y i n f l u e n c e d b o t h t h e g r o w t h ( T a b l e s 1 a n d 2) a n d p h e n o l o g y (Figs. 2 a n d 3) o f P. gramineus. D r y w e i g h t for P. gramineus d e c r e a s e d w i t h i n c r e a s i n g p l a n t i n g d e p t h . W i n t e r b u d s originally p l a n t e d a t 20 c m did n o t p r o d u c e p l a n t s . R h i z o m e l e n g t h d e c r e a s e d as a f u n c t i o n o f p l a n t i n g d e p t h . P r o d u c t i o n o f f l o a t i n g leaves a n d flowers w a s i n v e r s e l y r e l a t e d to p l a n t ing d e p t h . P. gramineus a t 2.5 or 5 c m p r o d u c e d p l a n t s w h i c h a l l o c a t e d 2 or 3% TABLE1 Growth parameters for P. gramineus planted at four depths Parameter
Total weight Shoot weight Rhizome weight Root weight Flower weight No. of floating leaves No. of flowers Rhizome length
Planting depth (cm)
Sign.
2.5
5
10
20
5844± 279 3994 ± 161 917 ± 53 762 ± 75 171 ± 82 20 ± 3 5± 2 104 ± 5
5210 ± 211 3524 ± 152 819 ± 46 762 ± 70 106 ± 46 12 _+4 3± 1 121 _+15
2011 ±841 1443 ± 604 327 _+132 234 + 105 6± 3 1± 1 1 _+0 47 ± 19
23 _+13 0 23 ± 13 0 0 0 0 0
*** *** *** *** * *** ** ***
Values are the mean ± standard error; n=4. Dry weights are mg and lengths are cm. The column labeled 'Sign.' gives the statistical significance of linear regression relating the parameter to depth: * Denotes P < 0.05; ** denotes P < 0.01; *** denotes P < 0.001. TABLE2 Production rates for P. gramineus planted at four depths. Values are the linear regression coefficient _+standard error Parameter
L e n g t h m m d ~ -1 R a m e t s ~ y -1 Leavesday -~
Planting depth (cm) 2.5
5
10
20
6.7±0.3 0.5±0.03 2.6±0.2
7.1±0.2 0.4±0.04 2.5±0.3
5.5±0.2 0.2±0.03 1.2±0.1
0 0.1±0.01 0
INFLUENCE
OF PLANTING
DEPTH
ON
POTAMOGETON
347
30-
500-
2.5 ¢m
E E
~
20
cm cm
20-
-E 300++~ E~
E u
© 200-
10-
~- I00-
'
14
'
v2L8
'
~v~
~516v ~
v7b
Days After Start
Days After Start
Fig. 2. Plant length (A) and number of ramets per plant (B) over time for P. gramineus planted at four depths. Values are means; n=4.
200-
2.5
150-
~
•±-7 5 10 20
cm cm
cm cm
ii00-
50-
00
~~
4-
....
0,¢ CC,T, C,C, C 28
42
C ,C, 56
Ci 70
Days After Start Fig. 3. Number of leaves per plant for P. gramineus planted at four depths. Values are means; n = 4.
of the biomass to flowers while those planted at depths greater than 5 cm allocated no biomass to flowers. Plants in the 10-cm depth treatment had more biomass allocated to rhizome than those growing from shallower depths. Those growing from shallower depths allocated more biomass to photosynthetic organs and emerged from the soil about 10 days sooner than those planted at 10 cm depth. Plants that emerge earlier may be at an advantage over those that emerge later even though other growth characteristics are similar (Firbank and Watkinson, 1985). Individuals growing from greater than 5 cm depth were shorter, had fewer leaves per plant, and fewer ramets than those growing from 5 cm or less. The rate of leaf production, ramet production, and shoot elongation also reflects the influence of planting depth (Table 2). Floating leaves were produced 10 days before flowers and both were produced under a 14.5 h
348
D.F. SPENCER AND G.G. KSANDER
photoperiod. Water temperature varied considerably (8°-30°C) and did not appear to be related to either floating leaf or flower production. Redox of the sediment in the pots declined steadily for the first 10 days after the pots were placed under water. DISCUSSION The vertical distribution of winter buds in canal sediments was somewhat similar to that reported for Vallisneria americana, but different from that observed for P. pectinatus. Information about the vertical distribution of V. americana tubers in Potomac River sediments (U.S.A.) showed that most were found between 5 and 15 cm deep in sandy sediments and between 10 and 20 cm deep in silty clay sediments (Rybicki and Carter, 1986). When the number of tubers is plotted against depth (Rybicki and Carter, 1986; see their Figs 3 and 4) the resulting curves are similar in shape to those reported here for P. gramineus winter bud weight, even though the depth of the peak abundance is different. The vertical distribution of P. pectinatus propagules appears to be quite different from that for P. gramineus. Ogg et al. (1969) examined the abundance ofP. pectinatus tubers in 30-cm deep cores from an irrigation canal near Prosser, Washington (U.S.A.), and reported a marked decrease in the number of tubers present below 7.5 cm. In contrast the number ofP. pectinatus tubers in the Byrnes, Weyend, and Dally irrigation canals (Solano Irrigation District) was more or less independent of depth up to 23 cm (Spencer, 1987). While it is likely that sediment properties (Rybicki and Carter, 1986) influence the vertical distribution of vegetative propagules, the limited data support the notion that species-specific differences exist as well. The influence of planting depth on growth of P. gramineus is similar to that reported for two other macrophyte species. In growth experiments, survival of both Vallisneria and P. pectinatus was directly related to planting depth (Rybicki and Carter, 1986; Spencer, 1987). In addition, growth measured in terms of plant dry weight, length, and the number of ramets was inversely related to planting depth for P. pectinatus (Spencer, 1987). In the present study, winter buds planted at 20 cm did not emerge from the sediment and examination of these at harvest showed that they had begun to decay; this suggested that the conditions at this depth were detrimental to winter bud growth. Although the mechanisms leading to apparent winter bud death in this experiment are unknown, the redox potential of the sediment in the pots was low after 10 days ( - 75 mV) indicating low oxygen concentrations. Germinating winter buds that have not emerged from the sediment may be less able to withstand low sediment oxygen concentrations than mature plants (Mitsch and Gosselink, 1986; see pages 130-139). The observation that biomass allocation was directly related to winter bud planting depth has implications for attempts to predict macrophyte phenology.
INFLUENCE OF PLANTING DEPTH ON POTAMOGETON
349
In particular, the influence of planting depth on flower production implies that changes in environmental elements (i.e. photoperiod and temperature) alone may not be sufficient to predict phenology. Planting depth may be important because of its influence on plant size; for example, Titus and Hoover (1990) reported that V. americana plants grown in greenhouse cultures flowered if they were 0.75 g or greater, however, the relationship was not as strong for plants from natural populations. For P. gramineus, the weight of flowers per plant was strongly correlated with plant weight (r-- 0.62; P < 0.02). These findings are also consistent with results from work with herbaceous terrestrial species (Werner, 1975; Gross, 1981). P. gramineus grew well when planted at shallow depths in the growth experiments, but P. gramineus winter buds were less abundant near the surface in the Byrnes and Weyend canals. This may be partly explained by the lack of flowing water in these canals during the winter and early spring. Since water is absent, the surface sediments may become quite dry and this in turn may lead to lower winter bud survival. Unpublished data indicate that related Potamogeton species (P. nodosus Poir. and P. pectinatus) were quite susceptible to desiccation at soil moistures of 23% or less. In summary, the distribution of P. gramineus winter buds was not uniform across depth. More winter buds were found between 6 and 10 cm deep in the sediment than at other depths. Experimental results indicated that planting depth significantly influenced several aspects of growth, including the allocation of biomass to flowers and photosynthetic tissues; also influenced were important phenological aspects, including the time of emergence from the sediment. These results show that aquatic plant growth is not solely regulated by abiotic factors (Barko et al., 1986), and that considering the depth distribution of propagules in the sediment may improve capabilities (Collins and Wlosinski, 1989) for predicting macrophyte growth.
ACKNOWLEDGEMENTS
We thank L. Maharis for technical assistance. L.W.J. Anderson and N. Dechoretz provided the samples from the Byrnes and Weyend canals that were analyzed for sediment characteristics. K. McKee provided instructions for constructing and using the redox electrodes. We appreciate the comments of E. Rejmankova, L. Mitich, B. Rorslett, and F.J. Ryan who read an earlier version of the manuscript. L. Whitehand provided assistance with the statistical analyses. Mention of a manufacturer does not constitute a warranty or guarantee of the product by the U.S. Department of Agriculture nor an endorsement over other products not mentioned.
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D.F. SPENCERANDG.G.KSANDER
REFERENCES Barko, J.W., Adams, M.S. and Clesceri, N.L, 1986. Environmental factors and their consideration in the management of submersed aquatic vegetation: a review. J. Aquat. Plant Manage., 24: 110. Brower, J.E. and Zar, J.H., 1984. Field and Laboratory Methods for General Ecology, 2nd edn. Wm. C. Brown, Dubuque, IA, 226 pp. Chew, V., 1976. Comparing treatment means: a compendium. HortScience, 11: 318-357. Collins, C.D. and Wlosinksi, J.H., 1989. A macrophyte submodel for aquatic ecosystems. Aquat. Bot., 33: 191-206. Fassett, N.C., 1957. A Manual of Aquatic Plants. University of Wisconsin Press, Madison, WI, 403 pp. Firbank, L.G. and Watkinson, A.R., 1985. On the analysis of competition within two-species mixtures of plants. J. Appl. Ecol., 22; 503-517. Gross, K.L., 1981. Prediction of fate from rosette size in four 'biennial' plant species Verbascum thapsus, Oenothera biennis, Daucus carota and Tragopogon dubius. Oecologia, 48: 209-213. Harper, J.L., 1977. Population Biology of Plants. Academic Press, New York, 892 pp. Hutchinson, G.E., 1975. A Treatise on Limnology, Vol. III. Limnological Botany. Wiley, New York, 660 pp. Mitsch, W.J. and Gosselink, J.G., 1986. Wetlands. Van Nostrand Reinhold, New York, 539 pp. Munz, P.A., 1970. A California Flora. University of California Press, Berkeley, CA, 1681 pp. Ogg, A.G. Jr., Bruns, V.F., and Kelly, A.D., 1969. Response of sago pondweed to periodic removal of top growth. Weed Sci., 17: 139-141. Rybicki, N.B. and Carter, V. 1986. Effect of sediment depth and sediment type on the survival of Vallisneria americana Michx. grown from tubers. Aquat. Bot., 24: 233-240. SAS Institute Inc., 1988. SAS/STAT User's Guide, Release 6.03 Edition. SAS Institute, Cary, NC, 1028 pp. Sculthorpe, C.D., 1967. The Biology of Aquatic Vascular Plants. St. Martins Press, New York, 610 pp. Spencer, D.F., 1987. Tuber size and planting depth influence growth of Potamogeton pectinatus L. Am. Midl. Nat., 118: 77-84. Spencer, D.F. and Anderson, L.W.J., 1987. Influence of photoperiod on growth, pigment composition and vegetative propagule formation for Potamogeton nodosus Poir. and Potamogeton pectinatus L. Aquat. Bot., 28: 103-112. Titus, J.E. and Hoover, D.T., 1990. Towards predicting reproductive success in submersed freshwater angiosperms. In: M. Adams and K. Sand-Jensen, (Editors), Ecology of Submersed Macrophytes., in press. Werner, P.A., 1975. Predictions of fate from rosette size in teasel (Dipsacus fullonum L. ). Oecologia, 20: 197-201.