Germination and seedling frost tolerance differ between the native and invasive range in common ragweed

Abstract

Germination characteristics and frost tolerance of seedlings are crucial parameters for establishment and invasion success of plants. The characterization of differences between populations in native and invasive ranges may improve our understanding of range expansion and adaptation. Here, we investigated germination characteristics of Ambrosia artemisiifolia L., a successful invader in Europe, under a temperature gradient between 5 and 25 °C. Besides rate and speed of germination we determined optimal, minimal and maximal temperature for germination of ten North American and 17 European populations that were sampled along major latitudinal and longitudinal gradients. We furthermore investigated the frost tolerance of seedlings. Germination rate was highest at 15 °C and germination speed was highest at 25 °C. Germination rate, germination speed, frost tolerance of seedlings, and the temperature niche width for germination were significantly higher and broader, respectively, for European populations. This was partly due to a higher seed mass of these populations. Germination traits lacked evidence for adaptation to climatic variables at the point of origin for both provenances. Instead, in the native range, seedling frost tolerance was positively correlated with the risk of frosts which supports the assumption of local adaptation. The increased frost tolerance of European populations may allow germination earlier in the year which may subsequently lead to higher biomass allocation--due to a longer growing period--and result in higher pollen and seed production. The increase in germination rates, germination speed and seedling frost tolerance might result in a higher fitness of the European populations which may facilitate further successful invasion and enhance the existing public health problems associated with this species.

Fig. 1

Box and whisker plots (whiskers with maximum 1.5 interquartile range) showing effects of temperature on germination of North American and European Ambrosia artemisiifolia populations. a Cumulative germination rates after 60 days and b number of days required to achieve 50 % of final germination (T50) of North American (NA; n = 10) and European (E; n = 17) populations

Fig. 2

Relationship between a germination rate (G _max) with seed mass (native range, Spearman ρ = 0.624, P = 0.054, n = 10; invasive range, ρ = 0.340, P = 0.181, n = 17) and b frost tolerance with seed mass (native range, ρ = 0.378, P = 0.225, n = 12; invasive range, ρ = 0.636, P = 0.035, n = 11) for A. artemisiifolia populations from native and invasive ranges

Fig. 3

Frost tolerance of seedlings for North American A. artemisiifolia populations in relation to the risk of spring frosts at the respective point of origin, n = 12