Does temperature limit the invasion of Impatiens glandulifera and Heracleum mantegazzianum in the UK?

Abstract

1. Impatiens glandulifera Royle and Heracleum mantegazzianum Sommier et Levier are widespread, non-indigenous plant species in the UK. A variety of correlational analyses suggest that their spatial extent is limited by climate, although no experimental studies have tested this hypothesis. This paper reports the first detailed experimental examination of the impact of climate on the performance of the two species. 2. Seeds of each species were sown, in each of 2 years, in replicated plots along an elevational gradient (10-600 m a.s.l.) in north-east England. Both species germinated readily at all elevations, even in areas well above their current limits within the study area. The plants were, however, smaller at higher altitudes. Impatiens glandulifera also produced fewer seeds with increasing elevation. 3. Plant performance was assessed in relation to actual and interpolated climate data along the elevational transect. For H. mantegazzianum, the timing of germination was correlated most strongly with the pre-emergence heat sum; for I. glandulifera this relationship was significant in one year only. Maximum height of both species was correlated with increasing post-emergence heat sum, as was pod production by I. glandulifera. The biomass of second-year H. mantegazzianum plants varied nonlinearly with post-emergence heat sum. For both species, overwinter survival of seeds was not related to winter temperature or frost days. Overwinter survival of first-year H. mantegazzianum plants declined with increasing frost incidence. 4. The results suggest that, of the two species, only I. glandulifera is currently most limited by temperature, although this is not the only factor determining the distribution of the species.