Hydrobiologia (2006) 570:237–242 Ó Springer 2006 J.M. Caffrey, A. Dutartre, J. Haury, K.J. Murphy & P.M. Wade (eds), Macrophytes in Aquatic Ecosystems: From Biology to Management DOI 10.1007/s10750-006-0186-0
Plasticity of Lythrum salicaria and Phragmites australis growth characteristics across a European geographical gradient D. Bastlova´1,*, M. Bastl1, H. Cˇı´ zˇkova´3 & J. Kveˇt1,2 1 Faculty of Biological Sciences, University of South Bohemia, Branisˇovka´ 31, CZ-370 05, Cˇeske´ Budeˇjovice, Czech Republic 2 Institute of Botany, Academy of Sciences of the Czech Republic, Dukelska´ 135, CZ-379 01 Trˇebonˇ, Czech Republic 3 Institute of Landscape Ecology, Academy of Sciences of the Czech Republic, Dukelska´ 145, CZ-379 82 Trˇebonˇ, Czech Republic (*Author for correspondence: E-mail:
[email protected])
Key words: life history, flowering time, competitive ability, plant invasions, geographic variation, clinal variation
Abstract Plants of Lythrum salicaria and Phragmites australis originating from localities across the European north– south geographical gradient were cultivated in parallel in an outdoor tub experiment. A strong correlation was found between growth and morphometric characteristics related to plant size (plant height, basal diameter, aboveground- and belowground plant biomass, etc.) and the position of the respective populations along the north–south gradient. Plants of both L. salicaria and P. australis from the southern localities grew taller and more vigorously and flowered later than plants from relatively more northern localities. From this point of view, the plants originating from south European populations were comparable to invasive North American plants. Our study indicates that explanation of the competitive success of populations invading new geographical areas may involve the role of geographic gradients within the species native range.
Introduction Lythrum salicaria L. and Phragmites australis Cav. (Trin. ex Steud.) both represent species that are common components of wetland communities in Europe (Hejny´, 1960), which behave in an invasive manner in North American wetland habitats. L. salicaria, a Eurasian native species, is a broadleaved dicotyledonous plant, which spreads mostly by sexual reproduction. Introduced accidentally to North America at the end of the 18th century, L. salicaria spread rapidly from the 1930s along rivers and into marshes in the northeastern USA and neighbouring areas of Canada (Stuckey, 1980). It is now found in all states and provinces between 35° and 51° N latitudes. In both, native and invasive ranges of occurrence, L. salicaria grows in a many plant communities and habitats
(Dubyna et al., 1993; Bastlova´-Hanze´lyova´, 2001). While in native areas L. salicaria became a dominant species of plant communities only exceptionally, it was documented as important plant species of invaded wetland habitats in North America (Bastlova´-Hanze´lyova´, 2001). Phragmites australis is a cosmopolitan perennial grass that relies on both generative reproduction and vegetative spreading of its clones. P. australis is an important dominant species in Eurasian wetlands, especially in littoral zones of lakes and ponds (Haslam, 1972, 1973; RodewaldRudescu, 1974). The species is apparently native not only to Eurasia, but also to North America, where many populations are non-invasive and do not form monospecific stands (Marks et al., 1994; Havens et al., 1997; Saltonstall, 2002). At many localities in the United States and Canada,
238 however, populations of P. australis behave invasively, forming very dense, monospecific stands. The aggressively invasive P. australis populations in North America are widely believed to have been introduced from Eurasia (Marks et al., 1994). The general purpose of our investigation is to elucidate some of the biological mechanisms behind the competitive ability of selected European populations of L. salicaria and P. australis in relation to their potential invasiveness in North America. This particular study is focused on the variability in life history characteristics of native populations of the two species along the north– south geographical gradient in Europe.
Materials and methods Lythrum salicaria and P. australis plants were cultivated in a parallel outdoor tub experiment in Trˇ ebonˇ, Czech Republic. Four populations (represented by four parental plants per each population) of L. salicaria originating from four countries (Sweden, Poland, Slovenia and Israel) and six populations of P. australis (each represented by six clones) originating from six countries (Sweden, The Netherlands, Czech Republic, Hungary, Romania and Spain) were used for the experiment. Offspring of each parental plant was planted in four replicates in 2.5 l pots in L. salicaria. Rhizome cuttings of each clone were planted in six replicates in 5 l buckets in P. australis. Full nutrition was added to the pots or buckets in the form of a slowly diluting granulate fertilizer, Osmocote Plus, in a dose of 6 g l)1 of substrate, mixed with the sand medium at the time of plantation. The containers were placed in tubs situated outdoors and kept flooded throughout cultivation to one half of their height and to the substrate surface for plants of L. salicaria and P. australis, respectively. The cultivation started in May and ended in September 2000, when all the plant material was harvested. At the time of harvest the shoot height, total number of primary lateral branches (growing directly from the main stem) and basal stem diameter were recorded for each L. salicaria plant sampled. The specific leaf area (SLA), dry weight of roots, stems and lateral branches (without leaves and inflorescences), leaves and reproductive parts (inflorescences on
main shoot and on lateral shoots + flower-bearing parts of the stems) were then determined. In P. australis the total number of shoots and that of flowering shoots, the length, basal stem diameter and number of nodes in the longest shoot, SLA and the ratio of aboveground/belowground total dry weight per bucket, were determined at the time of harvest. Statistical treatment The dependence of plant growth characteristics on latitude of original geographical location was fitted to generalised linear models (GLM) (McCullagh & Nelder, 1989) using S-plus software package (Statistical Sciences, 1995a, b). GLM was also used for hierarchical analyses of data to explain partitioning of total variability between the variability at geographical location level, between populations and within population levels. In L. salicaria three levels of variability were distinguished: (1) within population, i.e. between offspring of different mother plants from the population, (2) between populations within geographical location, and (3) between geographical locations. In P. australis two levels of variability were tested: (1) between clones within geographical location, and (2) between geographical locations. Significance of the GLM was tested using the F-test (Zar, 1984).
Results Plants of both L. salicaria and P. australis originating from localities across the geographical gradient differed in their morphological traits, with the following growth characteristics being negatively correlated with latitude in both plant species (Fig. 1): plant height, aboveground and belowground dry weight, stem diameter and dry weight. In contrast, the SLA was positively correlated with latitude in both plant species. Plants of L. salicaria originating from Sweden were semi-prostrate with small leaves and their lateral branches were almost as thick as the main stem. On the other hand, plants from Israel were tall, with vigorous, erect main stem and lateral branches shorter and thinner than the main stem.
40
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Latitude
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Latitude
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Stem diameter (cm)
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SLA (cm2.g-1)
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Belowground dw (g)
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Aboveground dw (g)
100 80 60
Plant height (cm)
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Latitude
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Belowground dw (g)
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Aboveground dw (g)
150 100
Plant height (cm)
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239
30 35 40 45 50 55 60
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30 35 40 45 50 55 60
30 35 40 45 50 55 60
30 35 40 45 50 55 60
30 35 40 45 50 55 60
Latitude
Latitude
Latitude
Latitude
Latitude
Latitude
Figure 1. Dependence of P. australis (above) and L. salicaria (below) plant growth characteristics on the latitude of the plant populations original geographical location. The regression curves were fitted using GLM. (dw = dry weight, SLA = specific leaf area).
A similar trend was observed for P. australis. Plants of northern (Swedish and Dutch) populations had fairly thin semi-prostrate shoots with small leaves while plants of southern (Romanian and Spanish) populations formed tall erect shoots. In L. salicaria plants from Sweden and Israel did not flower, or flowered only rarely (Sweden). Flowering plants from the southern locality (Slovenia), flowered later and had a lower dry weight of inflorescences than plants from northern localities (Sweden, Poland). In P. australis, only Czech and Hungarian populations flowered by the end of experiment. Based on the hierarchical analysis of variance, a highly significant proportion of total variability was ascribed to the effect of geographical location on the life history characteristics related to plant size (Table 1). A much smaller proportion of variability in these growth characteristics can be attributed to either within or between population differences in both species.
Discussion The observed significant dependence of most of the plant morphological and growth characteristics on the latitude of the plants’ geographical location suggests that a natural gradient exists in the variability of both studied plant species. The variability in phenology and life history characteristics across the north–south gradient may result from longterm adaptation to prevailing geographical conditions (Peacock & McMillan, 1968; Kudoh et al., 1995; Li et al., 1998; Pollard et al., 200l). Photoperiod is one factor of great importance that is highly variable with latitude and influenced plant life history characteristics. For example, populations of two Solidago species originating from northern locations flowered earlier and reached a smaller size at maturity than plants from southern locations (Weber & Schmid, 1998). The same applies to plants of P. australis from different
240 Table 1. Partitioning of variation among geographical locations, between populations within one geographical location (between population – in L. salicaria only) and between parental plants (in L. salicaria) or clones (in P. australis) originating from one population (within population) L. salicaria
P. australis
Explained variability
Explained variability
%
F
p
F
%
p
107.80
64
***
Plant height Geographical location
169.70
91
***
Between populations
3.61
2
***
Within population
1.48
2
*
10.09
21
***
18.65 3.86
49 10
*** ***
71.79
63
***
1.14
10
NS
3.97
13
***
8.68
20
*
Shoot basal diameter Geographical location Between population Within population Number of internodes Geographical location
0.09
0
NS
Between population
3.69
24
**
Within population
1.07
25
NS
2.81
24
***
147.51 2.28
83 2
*** ***
257.88
82
***
1.50
4
*
8.96
10
***
Geographical location
64.06
79
***
110.71
53
**
Between populations
3.14
4
**
Within population
1.81
6
**
19.76
34
***
Single shoot dry weight Geographical location Between population Within population Aboveground dry weight
Belowground dry weight Geographical location Between population
5.59 2.32
21 14
*** ***
89.51
54
**
Within population
2.14
24
***
14.84
32
***
Geographical location
49.80
55
***
18.67
39
***
Between population
1.10
4
*
Within population
1.70
15
*
2.11
16
**
SLA
***p < 0.001, **p < 0.01, *p < 0.05, F-test of GLM.
latitudes in Europe (Ve´ber, 1978; Clevering et al., 2001). The differences between L. salicaria plants from northern and southern populations found in the present experiment corresponded with those distinguishing short-day from long-day plants according to Shamsi & Whitehead (1974). Similarly as in the above studies, the variability in phenology and growth characteristics found in our study for both plant species may be strongly correlated with physiological requirements for floral initiation and is probably genetically based (Shamsi & Whitehead, 1974; Jouve et al., 1998).
Implications for invasiveness of Lythrum salicaria and Phragmites australis In comparative studies on native and invasive populations of L. salicaria from areas with similar climatic conditions, the invasive populations showed more vigorous growth (Edwards et al., 1998, 1999; Bastlova´ & Kveˇt, 2002). This is in accordance with the view that invasive populations of L. salicaria grow generally taller and show higher competitive ability in comparison with native European populations (Blossey & No¨tzold,
241 1995; Willis & Blossey, 1999). However, when comparing native and invasive populations of L. salicaria from wider geographical areas, some of the native populations studied were more similar to the invasive ones than the others, e.g. native plant from more eutrophic sites (Edwards et al., 1998, 1999) or southern locations (Bastlova´ et al., 2004) were similar to invasive populations. In our present study, plants of both L. salicaria and P. australis from the southern localities grew taller and more vigorously and flowered later than plants from relatively more northern localities. From this point of view, the plants of both species originating from south European populations were comparable to invasive North American plants in terms of their vigorous vegetative growth and reduced proportion of dry-mass allocated to generative reproduction. It has been suggested that species that successfully invade new geographic areas tend to allocate more biomass to vegetative growth, and less to reproduction or herbivore defence in a enemy-free space of secondary area (EICA hypothesis) (Blossey & No¨tzold, 1995), or come from more competitive genotypes of the species in its native area (Mooney & Drake, 1986; di Castri et al., 1990). Our study indicates that explanation of the competitive success of populations invading new geographical areas may involve the role of geographic gradients within the species native range. Acknowledgement This study was part of Projects No. 206/00/1113 of the Grant Agency of the Czech Republic and No. 614 1403 of Grant Agency of Academy of Sciences. Technical help of E. Zabilkova´ and H. Brabcova´ is gratefully acknowledged.
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