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Interactive effects of lateral shade and wind on stem allometry, biomass allocation, and mechanical stability in Abutilon theophrasti (Malvaceae)¹

²Department of Botany, University of Toronto, Toronto, Ontario, Canada, M5S 3B2; ³Faculty of Forestry, University of Toronto, Toronto, Ontario, Canada, M5S 3B2

The effects of lateral shade and wind on stem allometry, whole-plant biomass allocation, and mechanical stability were examined for Abutilon theophrasti in a fully factorial glasshouse experiment. Lateral shade from neighboring plants increased stem height by 33% relative to control plants grown individually, despite a decrease in plant dry mass. Intermittent wind decreased stem height by 18% in unshaded plants, but by only 3% in shaded plants. Surprisingly, both lateral shade and wind caused decreases in stem diameter, even with diameter controlled for height, resulting in low diameter : height ratios in wind-treated plants relative to untreated plants. Under shade, wind-treated plants had higher root allocation than untreated plants, which allowed wind-treated shade plants to compensate for a low diameter : height ratio. This did not occur in the absence of shade, where stem tissue density and root allocation of wind-treated plants did not exceed that of untreated plants. Nevertheless, wind-treated plants experienced low drag relative to untreated plants due to a lower leaf area. Consequently, stem deflections of wind-treated plants did not exceed those of untreated plants at any given windspeed. Our results document a complex interaction between shade and wind on plant morphology and suggest that the nature of this interaction is generally that lateral shade acts to reduce or eliminate thigmomorphogenic responses.

Key Words: Abutilon theophrasti • biomass allocation • biomechanical stability • drag • flexural stiffness • lateral shade • Malvaceae • stem allometry • thigmomorphogenesis • wind

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