Distinguishing Impatiens capensis from Impatiens pallida (Balsaminaceae) using leaf traits

Impatiens capensis (orange jewelweed) and Impatiens pallida (yellow
jewelweed) are annual species with similar phenotypes that grow in similar
environments throughout the eastern United States. This makes them
extremely difficult to distinguish when (chasmogamous) flowers are absent.
We use morphometric analyses to identify leaf characters that distinguish
these species. After collecting and scanning 342 leaves from plants of
each species growing in co-occurring populations in Madison, WI, we
quantified: leaf size, shape (using elliptical Fourier analysis),
serratedness, and color. Using leaf size and shape traits, a linear
discriminate analysis assigned up to 100% of leaves to the correct
species. The uppermost fully expanded leaf yielded the most accurate
species assignments based on size and shape traits. This leaf was on
average, smaller, less deeply serrated, with a more acute base, apex, and
elliptical shape in I. capensis as compared to I. pallida. Impatiens
pallida leaves had more color contrast (lighter veins and margins) than I.
capensis, which were solid green throughout. Morphometric analysis is a
promising technique to identify species-distinguishing characters in the
absence of binary traits or molecular genetic analyses. Leaves from across
these species’ ranges should be analyzed to test the robustness of the
species-distinguishing characters we present.