Individual study: The effects of nutrient pulsing on the threatened, floodplain species, claspingleaf doll's daisy Boltonia decurrens, Southern Illinois University, Edwardsville (SIUE), Illinois, USA
Mettler P.A., Smith M. & Victory K. (2001) The effects of nutrient pulsing on the threatened, floodplain species, Boltonia decurrens. Plant Ecology, 155, 91-98
The floodplains of the Illinois River (USA) formerly experienced moderate seasonal flooding, but major human alteration to the river channel have greatly reduced flood events. This has resulted in a decline in populations of claspingleaf doll's daisy Boltonia decurrens, a native, herbaceous floodplain species of the Illinois River Valley. Historically, it occurred in a contiguous population along a 400 km stretch of the Illinois River to its confluence with the Mississippi, but is now isolated in disjunct populations. One effect of the flooding is the input of nutrients into floodplain soils. To assess if this might be a factor in declining B.decurrens populations, the effects of increased nutrient availability on growth and reproduction of B.decurrens and awl aster Aster pilosus (a species replacing B.decurrens in the floodplain) were investigated.
In February 1995, 200 rosettes B.decurrens and A.pilosus were collected, under licence, at Horseshoe Lake, Madison County, Illinois, east central USA. These were stored at 4 ◦C for 14 days and then planted in 10 cm × 10 cm pots (Pro-Mix commercial potting medium) and placed in the greenhouse at Southern Illinois University, Edwardsville (SIUE).
Three soil types (sandy loam, silty loam and clay) were collected at the end of March 1995 from sites with extant B.decurrens populations, and placed in 20 cm × 21.5 cm pots (90 for each soil type). On April 1, 135 rosettes of each species in the size class 6-9 cm were selected. Roots were rinsed and planted such that each species had 45 rosettes growing in each soil type, with soil types divided into three nutrient treatments: a control receiving no extra nutrients; additional nutrients in the spring; and additional nutrients in autumn just prior to flower production. The plants were grown outdoors until seed set in autumn.
Spring treatment plants each received 200 ml of 25% Hoagland's nutrient solution (196 ppm NO3, 31 ppm P and 279 ppm K in a complete solution) and control plants received 200 ml of deionized water once a week for four weeks (May 23 through June 14). Autumn treatment plants received the same amount of nutrient solution within a week of detection of inflorescence production (21 August to 18 September). B.decurrens was harvested 10 October and A.pilosus on 27 October (A. pilosus set seed later than B.decurrens). Roots and shoots were separated and dried. Total biomass and root to shoot ratios were calculated for each plant; inflorescences were counted on all B.decurrens plants and on five randomly selected A.pilosus individuals in each treatment.
Boltonia decurrens: Plants grown in sandy loam exhibited the greatest increase in growth and fecundity in response to nutrient addition (both in spring and autumn). Total biomass (sandy loam: spring - 41g; autumn - 36g; control - 28g) and inflorescences per plant (sandy loam: spring - 96; autumn - 97; control - c.74. Note, values read from graphs) were significantly greater than the control regardless of the timing of nutrient addition.
A.pilosus: Plants exhibited no clear pattern of increased growth and fecundity in a specific soil type or in response nutrient treatment. There was a slight trend towards spring nutrient addition enhancing growth across the range of soil types compared with autumn nutrient addition and controls.
Conclusions: A pulse of nutrients (especially in spring as might occur in spring floods during snow melt) benefited B.decurrens more than A.pilosus, particularly in the sandy loam soil loam (the predominant soil type of the Illinois River Valley). The increased vigor and fecundity of B.decurrens with periodic seasonal nutrient pulses (now mostly lost due to flood control) could be one factor that influences population survival.
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