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Spread, Impact, and Control of Purple Loosestrife (Lythrum salicaria) in North American Wetlands

Growth Requirements

Shamsi and Whitehead (1974a, 1974b, 1977a, 1977b) carried out a series of laboratory and field experiments in their investigation of the factors determining the ecological and geographical distributions of Epilobium hirsutum (great hairy willow-herb) and L. salicaria. Both of these erect, perennial herbs are pioneers in marsh and fen communities in the British Isles. Optimum substrates for the growth of L. salicaria are moist soils of neutral to slightly acid pH. Nevertheless, we have found the plant growing in such a wide range of soil textures and types as to suggest that moisture is the most important factor for growth and reproduction. Parent materials invaded by purple loosestrife in North America varied from rock crevasses to gravel, sand, clay, and organic soils. Some of the least vigorous plants (50 cm tall) were growing on crushed-rock ballast on a railroad bridge near Lowell, Massachusetts. Some of the tallest specimens (300 cm) were growing on alluvial sand near Elm Creek, Nebraska; these are among the largest plants that we have encountered in the North American range.

Light Intensity and Photoperiod

Our field observations indicated that purple loosestrife can survive in 50% of full sun, but declines in vigor and fails to reproduce at lower light levels. Shamsi and Whitehead (1974b) reported that both light intensity and photoperiod had marked effects on the growth of L. salicaria. Under three levels of light (100, 70, and 40%) over a 16-h day, they observed decreased fruit and seed production with each reduction in illumination. L. salicaria's response to reduced illumination was a progressive increase in leaf size and a decrease in amount of tomentum and leaf thickness. These authors also noted that L. salicaria responded to reduced light by increasing the dry weight production of chlorophyll per unit leaf area. This compensation was seemingly enough to sustain total dry-weight production through roughly half of the experimental harvests. Reduction to 70% of full light caused very little change in growth or function; however, at 40% there was a significant reduction in mean dry weight of seed produced.

Shamsi and Whitehead (1974b) reported very different responses between Epilobium hirsutum and L. salicaria to a range of photoperiods of 9-16 h. E. hirsutum responded to all increments of reduced day length with extensions in growth. Its critical day length for flowering was 14 h. L. salicaria showed no growth responses in 10-, 11-, and 12-h photoperiods, but made an abrupt elongation of its shoots in the 13-h treatment when the critical point for flowering was reached.


The wide range of soils colonized by L. salicaria in North America is reflected in Shamsi and Whitehead's (1977a) experiments with comparative mineral nutrition of L. salicaria and E. hirsutum. L. salicaria's growth in a standard nutrient solution was much less reduced with successive dilutions than E. hirsutum's. The former responded to each level of dilution with an increase in root to top ratio. These authors also ran a similar experiment that tested L. salicaria and E. hirsutum responses to deficiencies in phosphorus (P), nitrogen (N), and potassium (K). As in their previous experiment, standard nutrient solutions were used, but the amounts of P, N, and K were reduced while all other nutrients were held at normal level. With successive dilutions, the growth of L. salicaria was more reduced by N deficiency than by P or K deficiencies, whereas E. hirsutum was sensitive to reductions of each of these elements. L. salicaria responded (as did E. hirsutum) to reductions in P and N, with an increased root to shoot ratio. In flower and fruit production, E. hirsutum was sensitive to reduction in all elements, whereas L. salicaria produced flowers at all levels of P deficiencies.

Temperature and Interspecific Competition

Shamsi and Whitehead (1977b) also studied the effects of temperature and interspecific competition on the growth of L. salicaria and E. hirsutum. They found that although both species produced similar dry weights and leaf areas under controlled growth conditions at 18°C, L. salicaria's growth was far more restricted by low temperatures (8°C). In greenhouse experiments at 25°C, they reported that L. salicaria was a more successful competitor than E. hirsutum in standard and diluted nutrient solutions. Similarly, under field conditions, L. salicaria seedlings planted in early summer in soils of low fertility were superior competitors against E. hirsutum. However, in a field experiment on fertile fen soil that included the low temperatures of both autumn and spring, E. hirsutum was the more successful species in the following growing season.

In summary, Shamsi and Whitehead (1977b) observed that "Given higher temperatures, Lythrum salicaria survives and grows faster than Epilobium hirsutum on nutrient-deficient media and soils. "They also noted that the confinement of E. hirsutum to base-rich fens and its complete exclusion from less fertile sites (in British Isles) can be related to its high nutrient demands. Although these authors did not attempt to compare the ecological distribution of E. hirsutum and L. salicaria in North America, their conclusions on mineral nutrition may in part explain the strikingly different behavior of these two aliens on this continent. Both species arrived from Europe more than 150 years ago and became naturalized in the Northeast. Subsequently, L. salicaria spread across the continent and is now established in all of our major river basins except the Arkansas, Colorado, and Rio Grande. Meanwhile, E. hirsutum continues to occupy a somewhat limited range in the Northeast and has gradually spread westward (Stuckey 1970); northern Illinois was its farthest westward spread into the Great Lakes region by the late 1960's. Stuckey considered the appearance of E. hirsutum in the State of Washington in the late 1960's to be the result of accidental rather than natural spread. Temperature may be a limiting factor in the northern distribution of L. salicaria in North America. Within the average annual minimum temperature map displayed by Bailey and Bailey (1976), L. salicaria has been a vigorous colonizer as far north as the −29 to −23°C zone; it has been less vigorous in the −34.5 to −29°C zone.

Shamsi and Whitehead (1974a, 1974b, 1977a, and 1977b) did not include soil moisture in their comparative eco-physiology studies of E. hirsutum and L. salicaria. Although soil moisture may not affect the wide geographic limits of these species, it can be an important factor in the position of each plant in local habitats. The senior author (D. Q. Thompson, 1984 unpublished field notes) observed that in central Italy and eastern Austria, L. salicaria grew on the hydric side of a soil moisture gradient. On the gently sloping shores of Lake Trasimeno (Italy) this difference was enough to place the species in separate but adjoining vegetation zones along the north side of the lake. Similarly, in a fallow field 0.5 km west of Trevi (Italy) numerous E. hirsutum and L. salicaria were growing together along the edge of a narrow ditch; however, great hairy willow-herbs were rooted in the topsoil on the edge of the ditch, whereas purple loosestrifes were rooted at the mean waterline in the ditch, about 30 cm below the position of E. hirsutum.

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