Environmental severity and variation in the reproductive traits of Impatiens glandulifera

1. Seed mass is widely regarded as a key plant trait, yet is notoriously variable within an individual species. This study reports the first field experiments to disentangle the relative importance of four major correlates of seed mass variation: resource limitation; plant height; seed number; and seasonality. 2. Variation in reproductive traits in Impatiens glandulifera Royle was partitioned among six populations established across an elevation gradient, among plants at each elevation and within individual plants. 3. Seed mass was the least variable trait both at the individual and population levels, while the number of pods per plant varied most. Seed mass variation was greatest among individual plants (42%) and was least among populations (28%). 4. Taller plants produced proportionally more seed pods and more seeds per pod. Seed mass was not related to the number of pods per plant, or the number of seeds per pod. Thus with increasing resources, I. glandulifera allocated resources to produce more seeds rather than larger seeds. 5. Seed mass was negatively correlated with the heat sum over the period of seed maturation, explaining increased seed mass at higher elevations and increasing seed mass over time at most elevations. This may indicate that environmental condition pre- and post-fertilization determines the relationship between seed mass and elevation, thereby limiting the potential for evolutionary selection for seed-size optima. 6. The study warns against correlative studies of seed mass variation that select arbitrary populations or ignore the temporal dynamics in seed mass, as these may generate spurious correlations among life-history traits. 7. The success of I. glandulifera as an invasive species may reflect the extended period of seed release, considerable seed mass variation and large seed size under conditions of environmental severity. These traits will facilitate the exploitation of spatially and temporally heterogeneous natural environments commonly found in riparian ecosystems.