Growth Rate, Seed Production, and Assessing the Spatial Risk of Lythrum salicaria using Growing Degree-Days

Abstract

Purple loosestrife (Lythrum salicaria) is an introduced wetland plant that is of both environmental and economic concern. Although it has been in Prairie Canada for many years, basic biological and spatial data is lacking. In this study we determined growth rates and seed production using season long field data, statistically modelled season long growth, and developed an interpolated growing degree-day (GDD’s) risk surface. We found that stems emerged from perennial root stocks in late May to early June after about 45 GDD’s. On average, stems reached 50 % of their final height at about 338 GDD’s, and final stem heights were reached at about 1007 GDD’s. The mean number of seed capsules produced per stem was 276.4 while the mean number of seeds produced by a seed capsule was 79.4. We found that a three parameter logistic function was a robust predictor of growth (R2 = 0.86). Our interpolated risk surface indicated that GDD’s may be a constraining parameter for the distribution of purple loosestrife in Prairie Canada. The results of this study contribute new biological and spatial data that can be used to support management plans, inform policy, and develop enhanced spatial predictive models.