Transpiration rates of the understory annual Impatiens capensis (Balsaminaceae) in response to the autumnal changes in canopy leaf area

Abstract

Summary Leaf transpiration rates of Impatiens capensis were measured beneath a broadleaved deciduous forest canopy over successive growing seasons using a steady-state porometer. The transpiration measurements, which continued into early autumn, provided a framework for assessing whether I. capensis exhibits stomatal opening in response to the autumnal increase in available direct-beam radiation reaching the forest floor. The deciduous canopy LAI (leaf area index) decreased from a growing season maximum of 3.94 m 2 m −2 , while the understory I. capensis population located along a stream channel maintained LAI values ranging from 0.58 to 1.05 m 2 m −2 late into the growing season. Late morning and early afternoon leaf transpiration rates during the months of June and July averaged about 8 μg cm −2 s −1 , with a mean stomatal conductance of 0.5 cm s −1 . In August, leaf transpiration averaged almost 12 μg cm −2 s −1 , with stomatal conductance exceeding 1.5 cm s −1 . However, beginning in early to mid-September, before canopy leaf-fall, the persistent green leaves of I. capensis exhibited a sharp decline in transpiration, possibly a result of decreasing vapor pressure deficits or non-lethal physiological damage induced by cold stress. This physiological decline offsets any advantage that could have been gained by the increased exposure to direct-beam radiation after canopy leaf-fall in mid-October. Although green leaf area and seed-bearing capsules may persist until the first frost in October or early November, there is no evidence of stomatal opening suggestive of carbon assimilation for enhanced seed development during this early autumn period. We conclude that the persistent green leaf area of I. capensis fails to exploit the increase in available direct-beam radiation in the final stage of its life cycle.