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Durham University

Department of Biosciences


Publication details for Professor Stephen G Willis

Willis, SG & Hulme, PE (2004). Environmental severity and variation in the reproductive traits of Impatiens glandulifera. Functional Ecology 18(6): 887-898.

Author(s) from Durham


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